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Phylogenomics of Sterol Synthesis: Insights into the Origin, Evolution, and Diversity of a Key Eukaryotic Feature

Phylogenomics of Sterol Synthesis: Insights into the Origin, Evolution, and Diversity of a Key... RESEARCH ARTICLE Phylogenomics of Sterol Synthesis: Insights into the Origin, Evolution, and Diversity of a Key Eukaryotic Feature Elie Desmond and Simonetta Gribaldo Institut Pasteur, Unite ´ de Biologie Mole ´ culaire du ge ` ne chez les Extre ˆ mophiles, Paris, France The availability of complete genomes from a wide sampling of eukaryotic diversity has allowed the application of phylogenomics approaches to study the origin and evolution of unique eukaryotic cellular structures, but these are still poorly applied to study unique eukaryotic metabolic pathways. Sterols are a good example because they are an essential feature of eukaryotic membranes. The sterol pathway has been well dissected in vertebrates, fungi, and land plants. However, although different types of sterols have been identified in other eukaryotic lineages, their pathways have not been fully characterized. We have carried out an extensive analysis of the taxonomic distribution and phylogeny of the enzymes of the sterol pathway in a large sampling of eukaryotic lineages. This allowed us to tentatively indicate features of the sterol pathway in organisms where this has not been characterized and to point out a number of steps for which yet-to-discover enzymes may be at work. We also inferred that the last eukaryotic common ancestor already harbored a large panel of enzymes for sterol synthesis and that subsequent evolution over the eukaryotic tree occurred by tinkering, mainly by gene losses. We highlight a high capacity of sterol synthesis in the myxobacterium Plesiocystis pacifica, and we support the hypothesis that the few bacteria that harbor homologs of the sterol pathway have likely acquired these via horizontal gene transfer from eukaryotes. Finally, we propose a potential candidate for the elusive enzyme performing C-3 ketoreduction (ERG27 equivalent) in land plants and probably in other eukaryotic phyla. Introduction oxygen. On the other hand, in Eukaryotes a first step (squa- lene monooxygenation, ERG1) adds an oxygen to squalene Triterpenoids are a large family of lipids that are leading to squalene epoxide, which is then cyclized in a sec- widely distributed in Bacteria (hopanoids) and Eukaryotes ond step either to lanosterol or to cycloartenol by enzymes (sterols). Sterols are present in all eukaryotes, where they homologous to SHC (ERG7). Because sterol synthesis is are essential and are involved in both intra- and intercellular widely distributed in eukaryotes, it can be assumed that signaling and in the organization of membranes. In mem- the last eukaryotic common ancestor (LECA) already syn- branes they affect fluidity and permeability (London 2002; thesized sterols. However, the origin of this pathway is not Tyler et al. 2009) and are major players in the formation of clear. The cyclization of squalene and the following steps lipid rafts, which are regions of reduced fluidity formed by are very demanding in oxygen. For example, 11 molecules the close association of sterols with sphingolipids (Jacobson of oxygen are required for the synthesis of one molecule of and Dietrich 1999; Brown and London 2000; Anderson and cholesterol (Summons et al. 2006). Organisms that live un- Jacobson 2002). Proteins involved in concerted functions der anaerobic conditions therefore resort to external sources such as cell signaling can be associated through their selec- of sterol by ingesting other eukaryotes. Moreover, some eu- tive incorporation into lipid rafts (Melkonian et al. 1999; karyotes that are able to synthesize sterols in the presence of Simons and Toomre 2000). Moreover, sterols play a key role oxygen have to acquire them from food when under anaer- in typical eukaryotic features such as phagocytosis. For ex- obic conditions, as it has been shown in yeast (Schneiter ample, genes involved in sterol biosynthesis have been 2007). A commonly accepted hypothesis is thus that the shown to be selectively upregulated in the amoebozoan pathway of sterol biosynthesis appeared after the emer- Dictyostelium discoideum during phagocytosis (Sillo et al. gence of oxygenic photosynthesis and the oxygenation 2008). Eukaryotes that are not able to synthesize sterols have of the atmosphere and oceans, thought to have occurred be- to obtain them from food, such as is the case, for example, of tween 2.7 and 2.4 Ga (Summons et al. 2006). It has been insects and of most marine invertebrates. recently put forward that the initial role of sterols in eukar- Bacterial and eukaryotic triterpenoids belong to the yotes may have been that of protection against oxidative class of isoprenoids. Isoprenoids are all derived from their stress when oxygen levels rose (Galea and Brown 2009). universal precursor isopentenyl diphosphate (IPP). IPP syn- Moreover, it has been proposed that the ancestral pathway thesis can follow two different routes, either via the meval- made cycloartenol as a final product and that the rising con- onate pathway in Archaea and some Eukaryotes or via the centrations of oxygen in the atmosphere would have led to 2-C-methyl-D-erythritol 4-phosphate pathway in Bacteria an evolution of the pathway beyond cycloartenol toward and other Eukaryotes (Rohmer et al. 1993; Boucher more stable sterol compounds (Ourisson and Nakatani et al. 2004; Volkman 2005). IPP is the precursor of squalene 1994). and its cyclization products. In Bacteria, hopanoids are syn- Intriguingly, although archaea are not known to be thesized from the cyclization of squalene by a squalene- able to synthesize sterols, a few bacteria have this ability. hopene cyclase (SHC) in a process that does not require Methylococcus capsulatus (gamma-Proteobacteria) has been shown to produce different sterols (Bouvier et al. Key words: sterols, phylogenomics, eukaryotes, evolution. 1976), and Gemmata obscuriglobus (Planctomycetales) E-mail: simonetta.gribaldo@pasteur.fr. produces lanosterol and its isomer parkeol (Pearson et al. Genome. Biol. Evol. 1:364–381. 2003). According to their sterol-synthesizing abilities, ho- doi:10.1093/gbe/evp036 Advance Access publication September 10, 2009 mologous of the first two enzymes of the pathway (ERG1 The Author(s) 2009. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.5), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Phylogenomics of Sterol Synthesis 365 and ERG7) have been found in M. capsulatus and G. may be at work. Moreover, we reconstructed the potential obscuriglobus (Pearson et al. 2003; Lamb et al. 2007). A ancestral abilities of sterol production in the LECA and variety of more or less elaborated sterols have been isolated the subsequent evolution of the pathway over the eukary- from a number of Myxobacteria (delta-Proteobacteria), otic tree, which appears to have occurred by tinkering, such as cycloartenol in Stigmatella aurantiaca, and 7-cho- mainly by gene losses. We highlight a high capacity of lesten-3beta-ol synthesized by some strains of Nannocystis sterol synthesis in the Myxobacterium P. pacifica, and sp. (Bode et al. 2003). The elaborated sterols synthesized by we support the hypothesis that the few bacteria that harbor Myxobacteria imply that these bacteria not only have the homologs of enzymes of the sterol pathway have likely first two enzymes of the pathway but also up to four or five acquired these via ancient HGT from eukaryotes. Finally, more downstream. A gene coding for a homolog of the sec- an analysis of phyletic patterns allowed us to propose a po- ond enzyme of the pathway (ERG7) has been sequenced tential candidate for the elusive enzyme performing C-3 from S. aurantiaca (Bode et al. 2003). However, mutants ketoreduction (ERG27 equivalent) in land plants and other where the gene was inactivated showed no noticeable phe- eukaryotic phyla, highlighting the power of phylogenom- notype, indicating that sterol production is not essential in ics approaches to the study of biochemical pathways. this bacterium (Bode et al. 2003). The function of these lip- ids in bacteria remains indeed unknown (Bode et al. 2003). Previous phylogenetic analyses have shown that ERG1 ho- Materials and Methods mologs from Methylococcus and Gemmata are specifically related to their eukaryotic counterparts (Pearson et al. Homologs of the enzymes characterized in fungi, ver- 2003). Concerning ERG7, it has been shown that Methyl- tebrates, and land plants were identified using the BlastP ococcus, Gemmata, and Stigmatella homologs are also program version 2.2.1.8 (Altschul et al. 1997) and the closely related to their eukaryotic counterparts (Pearson HMMER tools (http://hmmer.janelia.org/) on a local data- et al. 2003; Chen et al. 2007). The closeness of ERG1 bank of complete genomes (38 eukaryotes: Opisthokonta/ and ERG7 among these sterol-producing bacteria and eu- metazoans: Drosophila melanogaster, Tribolium casta- karyotes has been explained by a possible ancient horizon- neum, Caenorhabditis elegans, Homo sapiens, Mus muscu- tal gene transfer (HGT) (Pearson et al. 2003; Chen et al. lus, Danio rerio; Opisthokonta/choanoflagellates: 2007). However, it remains unclear whether these two en- Monosiga brevicollis; Opisthokonta/fungi: Aspergillus fu- zymes originated in bacteria or in eukaryotes. It is likely migatus, Neurospora crassa, Saccharomyces cerevisiae, that the assembly of the pathway started with these two ini- Schizosaccharomyces pombe, Ustilago maydis, Cryptococ- tial steps. Subsequently, other enzymatic steps would have cus neoformans, Encephalitozoon cuniculi; Ameobozoa: added to produce more elaborated sterols. However, it is not D. discoideum, Entamoeba histolytica; Plantae/land known when this would have happened and which type of plants: Arabidopsis thaliana, Oryza sativa; Plantae/green sterols the LECA would have been potentially able to algae: Chlamydomonas reinhardtii, Ostreococcus tauri; produce. Plantae/red algae: Cyanidioschyzon merolae; Alveolata/ The diversity and nature of sterols and the pathways ciliates: Tetrahymena thermophila, Paramecium tetraure- leading to these compounds have been thoroughly studied lia; Alveolata/apicomplexans: Cryptosporidium parvum, in vertebrates, fungi, and land plants. Characterizations of Plasmodium falciparum, Plasmodium yoelii, Theileria sterols from other organisms have unveiled a wide variety annulata; Heterokonta/oomycetes: Phytophthora ramo- of molecules, among which not only the same sterols rum; Heterokonta/diatoms: Thalassiosira pseudonana: known in animals, fungi, and land plants but also other Heterokonta/brown algae: Aureococcus anophagefferens; types of sterols with insaturations at various positions of Excavata/kinetoplastids: Trypanosoma brucei, Trypanoso- the cycle or in the side chain, and possibly alkylations or ma cruzi, Leishmania infantum, Leishmania major, Leish- inclusion of a cyclopropane ring mostly in C-24 or C-22 mania braziliensis; Excavata/heterolobozoans: Naegleria (Volkman 2003). Based on these characterizations, fossil gruberi; Excavata/diplomonads: Giardia lamblia; Excava- steranes found in sediments are used as biomarkers for past ta/parabasalids: Trichomonas vaginalis; 586 bacteria, 48 ar- eukaryotic life (Kodner, Pearson, et al. 2008). However, chaea). Most of the complete genomes were retrieved from relatively little is known about the structure of the pathway the Refseq database at the National Center for Biotechnol- in these organisms. A large number of complete genomes ogy Information (http://www.ncbi.nlm.nih.gov), except for are now available from representatives of major eukaryotic N. gruberi, O. tauri, A. anophagefferens, P. ramorum, and phyla, as well as from representatives of sterol-producing T. pseudonana genomes which were retrieved from the JGI bacteria, notably the two Myxobacteria S. aurantiaca database (http://genome.jgi-psf.org/euk_home.html) and and Plesiocystis pacifica. We have thus carried out an ex- for C. merolae genome which was retrieved from http:// tensive analysis of the taxonomic distribution and phylog- merolae.biol.s.u-tokyo.ac.jp. eny of the enzymes of the sterol pathway in a large sampling When a particular gene was not found in some species, of organisms representing eukaryotic diversity for which we searched the nucleotide sequences of these complete ge- complete genomic sequences are available. The use of com- nomes using TBlastN. The retrieved proteins were aligned plete genomes is in fact essential to infer presence or ab- using MUSCLE 3.6 (Edgar 2004), the alignments were edi- sence of homologs of the different enzymes. We tentatively ted and refined manually using the ED program from the reconstructed the potential structure of the sterol pathway in MUST package (Philippe 1993). diverse organisms where it has not been characterized and Phylogenies were reconstructed for each protein. Re- point out a number of steps for which yet-to-discover enzymes gions where homology was doubtful were manually 366 Desmond and Gribaldo removed from the data sets before phylogenetic analysis us- enzyme may be dispensable. The fungi pathway is very ing the NET program of the MUST package (Philippe similar to that of vertebrates. However, the ergosterol pro- 1993), and phylogenetic trees were computed with PHYML duced contains 28 carbons contrary to cholesterol, which (Guindon and Gascuel 2003) and MrBayes (Ronquist and contains only 27 carbons (fig. 1A). The additional carbon Huelsenbeck 2003; Huelsenbeck and Ronquist 2001). present in fungi ergosterol is added by the C-24 methylase PHYML trees were calculated using the Whelan and ERG6, which is in fact absent in vertebrates (fig. 1C). On Goldman model, a gamma correction to take into account the other hand, the sitosterol and stigmasterol produced by the heterogeneity of evolutionary rates across sites (four dis- land plants contain 29 carbons (fig. 1A). These two addi- crete classes of sites, an estimated alpha parameter, and an tional carbons are added in two steps catalyzed by two estimated proportion of invariable sites), and subtree prun- copies of ERG6: SMT1 performing C-24 methylation ing and regrafting topology searches. The robustness of each and SMT2 performing C-28 methylation (fig. 1A and C). branch was estimated by a nonparametric bootstrap proce- dure implemented in PHYML (100 replicates of the original data set and the same parameters). MrBayes trees were cal- Phylogenomics of the Sterol Pathway culated with an estimated fixed rate model and a gamma cor- rection as for the PHYML trees. The Markov Chain Monte Starting from the characterized genes in fungi, verte- Carlo runs were performed with four chains on 1,000,000 brates, and land plants, we investigated their taxonomic dis- generations sampled every 100 generation, and the final tree tribution and phylogeny in complete genomes from a broad was summarized discarding the first 1,000 generations. taxonomic sample of eukaryotic diversity (38 genomes con- taining representatives of main eukaryotic supergroups: one choanoflagellate, six animals, seven fungi, two amoebozo- ans, two land plants, two green algae, one red algae, two Results ciliates, four apicomplexans, one brown algae, one diatom, Canonical Pathways one oomycete, five kinetoplastids, one heterolobosean, one The pathway of sterol synthesis has been thoroughly diplomonad, one parabasalid, for a complete list see Mate- studied in vertebrates, fungi, and land plants (fig. 1). A rials and Methods) as well as from complete genomes from number of enzymatic reactions are found in the three path- 586 bacteria and 48 archaea. Phylogenetic analysis allowed ways (fig. 1A). In these three ‘‘canonical pathways,’’ squa- us assigning each of these homologs to an orthology group lene is oxygenated in a first step and cyclized in a second and thus to a potential function (table 1 and Supplementary step either to lanosterol in vertebrates and fungi or to cyclo- Material 1, Supplementary Material online). Consistently artenol in land plants (fig. 1A). Cycloartenol includes a cyclo- with the oxygen demand of the pathway, orthologs are to- propane ring between carbon C-9 and C-10 (fig. 1A and B). tally absent in strict anaerobic organisms (Encephalitozoon, This cycle is cleaved at a later step of the pathway by a spe- Giardia, Entamoeba, Cryptosporidium, and Trichomonas) cific land plants enzyme (CPI1) leading to typical sterols but also in aerobic ones (Plasmodium, Theileria, Drosoph- with four-carbon cycles (fig. 1A and B). Lanosterol or cyclo- ila, and Tribolium) (not shown). We found only a partial artenol is then converted to cholesterol in vertebrates, er- number of orthologs in the oomycete P. ramorum gosterol in fungi, and to campesterol, sitosterol, and (ERG3 and DHCR7), and the two ciliates (ERG3 and stigmasterol in land plants by a succession of oxidations, FK) (not shown), consistent with the notion that these reductions, and demethylations (fig. 1A). Most of these organisms do not synthesize sterols (Trigos et al. 2005). steps are shared in the three pathways, although they do These orthologs may be in fact used to process sterols not occur in the same order. C-4 demethylation occurs twice obtained from diet. in the three canonical pathways and is performed both times In fungi, vertebrates, and land plants, the same enzy- via three concerted steps (C-4 methyl oxidation, C-3 matic steps are generally performed by homologous en- dehydrogenation/C-4 decarboxylation, and C-3 ketoreduc- zymes (table 1). Two exceptions are the delta-8, delta-7 tion; fig. 1C). In vertebrates and fungi, the two C-4 deme- isomerization step performed by the nonhomologous thylations occur one after the other (fig. 1A). Conversely, in ERG2 in fungi and EBP/HYD1 in vertebrates/land plants land plants the two C-4 demethylations do not occur one and the delta-24 reduction performed by the nonhomolo- after the other, and two C-4 methyl oxidases (SMO1 and gous ERG4 in fungi and DHCR24/DWF1 in vertebrates/ SMO2) perform these steps (fig. 1C), each being specific land plants (table 1). A few enzymes performing different for a particular substrate (SMO1 acting on 24-methylene- steps in fungi, vertebrates, and land plants are evolution- cycloartenol, and SMO2 acting several steps later on 24- arily related (table 1). For example, ERG4-(DHCR7/ ethylenelophenol or 24-methylenelophenol; fig. 1A). In land DWF5)-(ERG24/TM7SF2/FK) all belong to a large protein plants, the enzyme responsible for C-3 ketoreduction is un- family. In a global phylogeny of this family, members with known (fig. 1C and Bouvier et al. 2005). It has been shown different functions segregate into different orthology in Saccharomyces that C-4 demethylation takes place at the groups (Supplementary Material 1, Supplementary Mate- endoplasmic reticulum membrane through the anchoring of rial online) indicating that these functions arose by gene ERG26 and ERG27 by ERG28 (Gachotte et al. 2001). It is duplication. It has to be noted that ERG24/TM7SF2 and likely that this anchoring also occurs in vertebrates and land FK have the same function but belong to paralogous clus- plants because they have a copy of ERG28 (fig. 1C). How- ters (Supplementary Material 1, Supplementary Material ever, it has been shown that ERG28 is not essential in Sac- online). Finally, ERG3/ERG25 are paralogs within a large charomyces (Gachotte et al. 2001), indicating that this family of oxidases (not shown), as well as ERG5/ERG11 Phylogenomics of Sterol Synthesis 367 FIG. 1.—(A) Canonical pathways of sterol synthesis leading to land plants, fungi, and vertebrate sterols. Upstream of squalene, the mevalonate (MVA) and 2-C-methyl-D-erythrol 4-phosphate (MEP) ways leading to IPP are shown. Downstream of squalene, the bacterial pathway of hopanoid synthesis via SHC is also indicated. (B) Numbering of carbons and cycles of steroids. (C) Table indicating the names and EC numbers of fungi, vertebrates, and land plants genes corresponding to each step of the pathway shown in (A). The red box indicates that the gene performing C-3 ketoreduction is still unknown in land plants. 368 Desmond and Gribaldo Table 1 Distribution of Orthologs of Enzymes of the Sterol Pathway in the Eukaryotic Genomes of Our Data Set The same color indicates homology. Presence is indicated by an X symbol. A double or triple X indicates presence of two or three copies. In the case of the paralogous families ERG24/TM7SF2-FK, an X with asterisk indicates the presence of orthologues of TM7SF2-FK, an X without asterisk indicates the presence of orthologues of ERG24, and a double X with asterisk indicates the presence of both paralogues (i.e. in Monosiga). In the case of the non-homologous families ERG4/DHCR24-DWF1 and ERG2/EBP-HYD1, the presence of one or the other copy is indicated by green and violet boxes, respectively, whereas the presence of both copies is indicated by a dark yellow box. Absences consistent with experimental characterizations are indicated by empty boxes, while absences not consistent with experimental characterizations are indicated by red boxes with exclamation points (see text for discussion). within the large family of cytochrome P450 (not shown). been largely studied (Summons et al. 2006). In particular, the Interestingly, we found that the fungi Aspergillus and Neu- production of lanosterol or cycloartenol in fungi, verte- rospora (Pezizomycotina) harbor additional homologs of brates, and land plants has been linked to the presence enzymes not belonging to the canonical fungi pathway of particular residues in the active site of ERG7: Three po- (EBP and DHCR24), as well as an additional SMT copy sitions are in fact differentially conserved between lanoster- (Supplementary Material 1, Supplementary Material on- ol synthases and cycloartenol synthases (Summons et al. line). The first two enzymes bring redundant functions to 2006; fig. 3). In fungi and vertebrates, lanosterol synthase these fungi, whereas the additional SMT copy may produce is characterized by T381, C/Q449, and V453, whereas cy- C-29 sterols, differently from the classical fungi ergosterol cloartenol synthase is characterized by Y381, H449, and (C-28 sterol). I453 (Summons et al. 2006). However, as shown for kinet- A number of sterols have been characterized from or- oplastids, position 381 can be variable (they have an Y but ganisms present in our data set other than fungi, vertebrates, make lanosterol; Buckner et al. 2000), and position 449 can and land plants, to the exception of the green algae O. tauri also be variable because Methylococcus and Gemmata have and the red algae C. merolae (fig. 2) (Raederstorff and an H while they make lanosterol (Summons et al. 2006) Rohmer 1987; Nes et al. 1990; Giner and Boyer 1998; (fig. 3). Thus, only position 453 is indicative of lanosterol Veron et al. 1998; Salimova et al. 1999; Roberts et al. or cycloartenol production. An alignment of ERG7 homo- 2003; Kodner, Summons, et al. 2008). With the combined logs from our data set (fig. 3) shows that position 453 is knowledge of these sterols and our phylogenomic data, we consistent with the experimental characterizations available sought to tentatively predict some additional features of the for a few additional phyla, such as in the amoebozoan sterol pathways in these organisms. One first general infor- Dictyostelium (I453, cycloartenol route; Nes et al. 1990), mation on the nature of the pathway can be obtained from the heterolobosean Naegleria (I453, cycloartenol route; the characteristics of ERG7 orthologs, which catalyze the Raederstorff and Rohmer 1987), and the kinetoplastids cyclization of squalene epoxide either to lanosterol or to (V453, lanosterol route; Roberts et al. 2003). No data are cycloartenol (fig. 1A and C), one of the most complex re- available on the other phyla, although based on this position actions catalyzed by a single enzyme (Summons et al. it can be predicted that the choanoflagellate Monosiga makes 2006). Among all enzymes of the sterol pathway, ERG7 lanosterol (V453), consistent with their affiliation with is one of the most conserved at the sequence level and or- fungi and animals into the phylum Opisthokonta, whereas thologs are indeed present in all species capable of de novo the brown algae Aureococcus, the diatom Thalassiosira, synthesis (table 1). The active sites are conserved and have the red algae Cyanidioschyzon, and the green algae Phylogenomics of Sterol Synthesis 369 FIG. 2.—Sterols characterized from organisms other than fungi, vertebrates, and land plants present in our data set. For sterol modifications discussed in the text, refer to figure 1B for numbering. Chlamydomonas and Ostreococcus all make cycloartenol group in position 14 (C-14 demethylation þ C-14 reduc- (I453) (fig. 3). tion); ii loss of both methyl groups at position 4 (C-4 methyl Among the variety of the sterols shown in figure 2, oxidation, C-3 dehydrogenation/C-4 decarboxylation, C-3 a number of features are in common with the products ketoreduction); iii) reduction of the delta-8 double bond of canonical pathways (fig. 1A and B): i) they are all derived (delta-8, delta-7 isomerization); and iv) formation of a dou- from cyclization of squalene into lanosterol or cycloartenol; ble bond between C-5 and C-6 (C-5 desaturation). Enzymes ii) final products have no methyl group in position 4 and catalyzing two of these steps are present in all these sterol- position 14 compared with the first products of cyclization synthesizing organisms (table 1): oxidosqualene cyclization of squalene; iii) they are free of double bonds between car- (ERG7) and C-14 demethylation þ C-14 reduction (ERG11 bon C-8 and C-9; and iv) they present a double bond þ ERG24/FK). A particular case concerns C-3 dehydroge- between carbon C-5 and C-6, except for dictyosterol nation/C-4 decarboxylation (ERG26): In a phylogeny of (fig. 2). These characteristics imply that the following steps ERG26 (Supplementary Material 1, Supplementary Mate- must necessarily take place in the synthesis of all these ster- rial online) we identify a clear cluster of orthologs ols (fig. 1C): i) squalene cyclization (squalene monooxyge- including the known enzymes of plants, fungi, and verte- nation þ oxidosqualene cyclization); ii) loss of the methyl brates. However, this cluster lacks M. brevicollis, 370 Desmond and Gribaldo FIG. 3.—Alignment of ERG7 homologs from representative taxa. The catalytic residue D455 is highlighted in red. Positions 381, 449, and 453 are differentially conserved between lanosterol synthases (yellow) and cycloartenol synthases (green). The numbering refers to the H. sapiens ERG7 homolog (NP_002331). O. tauri, T. pseudonana, A. anophagefferens, and kineto- clamation points in table 1). Finally, to our knowledge, two plastids. Interestingly, these genomes harbor homologs that species of our data set (O. tauri and C. merolae) have not form a separate group (Supplementary Material 1, Supple- yet been studied for their sterol composition. Assuming that mentary Material online). The perfect matching of this phy- the enzymatic steps that take place in all other organisms letic pattern and the requirement for C-3 dehydrogenation/ analyzed also take place in these organisms, and given C-4 decarboxylation indicates that these distant homologs our phylogenomic data, we tentatively predicted the kind are good candidates to perform an ERG26 function. of sterols they may produce (Supplementary Material 2, Intriguingly, several genomes lack clear homologs for Supplementary Material online). the remaining steps that we infer to be necessary to produce To sum up, our phylogenomic data indicate a wide va- the sterols that have been characterized in these organisms riety of pathways in different eukaryotic organisms. Inter- (table 1): lack of squalene monooxygenase (ERG1) in estingly, there is no clear separation between the pathway of M. brevicollis, C. reinhardtii, T. pseudonana, and A. photosynthetic organisms and nonphotosynthetic ones, dif- anophagefferens; lack of C-4 methyl oxidase (ERG25) ferently to what commonly assumed. For example, the in M. brevicollis, C. reinhardtii, T. pseudonana, A. anopha- green algae O. tauri and C. reinhardtii do not appear to gefferens, and kinetoplastids; lack of C-3 ketoreductase have the typical land plants pathway because C-14 reduc- (ERG27) in all analyzed genomes apart from vertebrates tion is performed by an ERG24 ortholog and not by a typical and fungi; lack of ER-anchoring protein (ERG28) in land plants FK ortholog (table 1). Interestingly, the early- M. brevicollis, O. tauri, T. pseudonana, A. anophageffe- emerging land plant lineage Physcomitrella patens harbors rens, and kinetoplastids; lack of delta-8, delta-7 isomerase a set of enzymes that is identical to that of O. sativa and A. (ERG2 or EBP) in T. pseudonana and A. anophagefferens; thaliana (data not shown), indicating that what is known as and lack of C-5 desaturase (ERG3) in T. pseudonana and the typical land plant pathway can be generalized to the A. anophagefferens. Because these steps are essential, yet whole lineage. No orthologues of land plants SMO1 and unknown enzymes must exist that fulfill these missing func- SMO2 are present in the diatom T. pseudonana, the brown tions (to the possible exception of ERG28 that may be dis- algae A.anophagefferens, and the red algae C. reinhardtii, pensable; Gachotte et al. 2001). In particular, it is possible suggesting that these organisms may harbor a pathway that the enzyme performing C-3 ketoreduction in these or- whose order of steps is somehow different from that of ganisms will turn out to be the same of that acting in land land plants. On the other hand, the nonphotosynthetic pro- plants, which, as mentioned before, is still unknown. Im- tists N. gruberi and D. discoideum synthesize sterol via cy- portantly, we found orthologs of CPI1 in the organisms cloartenol and perform a C-14 reduction by an ortholog of where we predicted a pathway following a cycloartenol typical land plants FK (table 1). Thus, phylogenomic anal- route based on the active sites of their ERG7 (fig. 3 and ysis indicates that these different organisms harbor path- table 1). Other steps are not universally present and are dis- ways that display a mixture of features of the three cussed in detail in Supplementary Material 2 (Supplemen- canonical pathways from fungi, vertebrates, and land tary Material online). In general, our analysis allowed us plants. This leads to the question on what may have been to infer the presence/absence of orthologs and the corre- the ancestral pathway in eukaryotes. sponding enzymatic steps in all these organisms (table 1). However, the precise nature of the corresponding pathways is difficult to predict because, as we have seen in the canon- Did the LECA Already Make Sterols? ical pathways, the order of steps can be arranged in various ways. Nevertheless, we could point out a number of missing From the distribution of orthologs in table 1, we can steps that are not compatible with the sterols known to tentatively infer the set of enzymes that were already pres- be produced by a given organism, these steps being thus ent in LECA. Although the phylogenies of these enzymes performed by yet-to-discover enzymes (red cases with ex- are generally not fully resolved due to lack of signal, major Phylogenomics of Sterol Synthesis 371 FIG. 4.—Inference of ancestral sets of sterol enzymes. These are inferred based on a consensus phylogeny of eukaryotes included in our data set according to the most recent data (Burki et al. 2008; Hampl et al. 2009) rooted in between unikonts and bikonts. Lineages where the ability to synthesize sterols has been lost are indicated by red crosses. We inferred maximal and minimal ancestral enzyme contents at the three most ancient nodes (LECA, Unikonts, and Bikonts). Additional proteins in the maximal content with respect to the minimal content are indicated by question marks (see text for details). Putative duplications inferred in the lineage leading to the LECA are indicated by a star. Losses and gains of proteins are indicated only for the most basal branches by purple and blue arrows, respectively. 372 Desmond and Gribaldo eukaryotic groups are generally recovered. This allows in- homologous equivalent of opisthokonts ERG27 that has yet ferring ancestral sets of proteins by mapping their presence to be discovered in land plants as well as in all other lin- on a consensus eukaryotic phylogeny (Burki et al. 2008; eages (table 1). The place of the root of the eukaryotic tree Hampl et al. 2009) rooted in between unikonts and bikonts in between unikonts and bikonts has been recently ques- (fig. 4). For example, an enzyme whose orthologs are pres- tioned based on the fact that a member of Amoebozoa (uni- ent in fungi, amoeobozoans, plants, and kinetoplastids and konts) harbors bikont characters (Minge et al. 2009; Roger are well separated in the corresponding phylogeny (see, and Simpson 2009). If we consider that the root of the eu- e.g., ERG5 in Supplementary Material 1, Supplementary karyotic tree is presently unknown and we replace the con- Material online) lead us to infer the presence of this enzyme sensus phylogeny by a polytomy, we are forced to infer in the LECA and that its current absence in some phyla is a maximal set of enzymes back to the LECA in order to due to subsequent gene loss. We inferred the ancestral sets minimize convergent protein gains. This leads to the same of enzymes in the three most ancient nodes of the eukary- ancestral sets inferred with the unikont/bikont rooting. otic phylogeny (LECA, unikonts, and bikonts) (fig. 4) by Phylogenomic inference suggests that the LECA had minimizing HGT events among eukaryotic lineages. It a large potential of enzymatic functions and may have been has been put forward that HGT among eukaryotes may capable of synthesizing a diverse panel of sterols. In fact, be more frequent than generally assumed (Andersson among the set of enzymes inferred to have been already 2009). However, we could not observe clear cases of such present in the LECA, a few are nonorthologous, but per- HGT in our phylogenetic reconstructions, to one potential form redundant functions, such as C-14 reductases exception discussed below. (ERG24/FK), delta-8 delta-7 isomerases (ERG2/EBP), Phylogenetic analysis indicates that the duplications and delta-24 reductases (ERG4/DHCR24) (table 1 and that gave rise to the gene families corresponding to fig. 4). Our analysis does not allow inferring how the path- ERG4-ERG24-FK-DHCR7 likely occurred before the di- way was progressively assembled in the lineage leading to vergence of major eukaryotic phyla (Supplementary Mate- LECA. However, if LECA possessed a CPI1, this implies rial 1, Supplementary Material online), indicating that these that it used a cycloartenol route rather than a lanosterol enzymes were already present in the LECA (fig. 4). The route because no known organisms that possess a CPI1 syn- same may be said for the gene duplications giving rise thesize lanosterol. Consequently, the switch from a cycloar- to SMO1-SMO2, and to SMT1-SMT2, because the unikont tenol to a lanosterol route would have occurred at least D. discoideum has two gene copies of each family, similar twice independently in eukaryotic evolution (i.e., in opis- to land plants and some other bikonts, which cluster within thokonts and in kinetoplastids). An ancestral cycloartenol different orthologous groups (Supplementary Material 1, route is indeed consistent with previous hypotheses (Bloch Supplementary Material online) (fig. 4). However, phylo- 1991; Ourisson and Nakatani 1994). genetic reconstruction does not allow excluding that D. dis- Our analysis indicates that organisms that do not syn- coideum obtained its additional SMO1 and SMT2 via HGT thesize sterols have lost the pathway (fig. 4). Interestingly, from a bikont lineage (Supplementary Material 1, Supple- the loss of sterol synthesis in insects and nematodes appears mentary Material online). In this case, these two duplica- to be specific to these lineages because the placozoan Tri- tions would have occurred after the unikont/bikont choplax adhaerens has a full set of typical animal enzymes, bifurcation. Therefore, we indicated the occurrence of these except for DHCR7 (data not shown). Sometimes, a few en- duplications and the presence of the corresponding proteins zymes have been retained, possibly for the processing of in the LECA by question marks (fig. 4). The presence of sterols harvested from the environment, such as, for exam- a CPI1 ortholog in the unikont D. discoideum (table 1) ple, is the case of DHRC7, where a copy is present in the may indicate that the LECA had this enzyme, although non–sterol-producing oomycete P. ramorum (Supplemen- the poor resolution of the corresponding phylogenetic tree tary Material 1, Supplementary Material online). Our and the proximity with the ortholog from P. pacifica (Sup- analysis also indicates that the present-day distribution of plementary Material 1, Supplementary Material online) sterol enzymes in the different eukaryotic organisms ana- cannot exclude that it was acquired by HGT (therefore lyzed is the consequence of differential losses that probably a question mark in fig. 4). Interestingly, this analysis sug- accompanied specialization of the pathways along with gests that the LECA may have already harbored at least 19 eukaryotic diversification. However, appearance of novel enzymes (fig. 4). Even if D. discoideum obtained its addi- enzymes that are not yet known may have also taken place. tional SMO1, SMT2, and CPI1 copies via HGT from a bi- This prompts to explore further the sterol-synthesizing abil- kont lineage, a minimum set of 16 enzymes can still be ities of a wider sampling of eukaryotic diversity, all the inferred in the LECA (fig. 4). Among all enzymes, more so that the presence of a particular pathway in a given ERG27 only has specifically appeared in the Opisthokonta organism may not be generalized to the whole phylum it lineage. The opisthokont-specific ERG27 belongs to a gene belongs to. family including bacterial short-chain dehydrogenases as well as other distant eukaryotic hypothetical proteins (not shown) and may have thus been recruited in the ances- A Bacterial Origin? tor of Opisthokonta (fig. 4). However, because C-3 ketor- eduction would have been essential in the pathway inferred Consistent with previous findings, we found orthologs in the LECA, this opisthokont-specific ERG27 may have of the first two genes of the pathway (ERG1 and ERG7) in replaced an ancestral enzyme performing the same function G. obscuriglobus (Pearson et al. 2003), M. capsulatus in the LECA. It is thus possible that the LECA had the non- (Lamb et al. 2007), and S. aurantiaca (Bode et al. Phylogenomics of Sterol Synthesis 373 FIG. 5.—Genomic context of ERG1 and ERG7 genes in the genomes of the four sterol-synthesizing bacteria G. obscuriglobus, M. capsulatus, S. aurantiaca, and P. pacifica. 2003), and we report the presence of these two genes in et al. 2003). In general, previously published trees have P. pacifica.In G. obscuriglobus and M. capsulatus, the geno- been presented with only representatives of eukaryotes mic context of ERG1 and ERG7 is known to be conserved and these bacteria (Pearson et al. 2003). However, ERG1 and this has been taken as further indication for a common homologs are not only present in the four sterol-producing HGT involving these two genes (Pearson et al. 2003). How- bacteria but also have homologs in other bacteria within ever, the genome contexts in S. aurantiaca and P. pacifica a large family of monooxygenases. In a tree including are different (fig. 5). In P. pacifica, ERG1 is not close on the the whole family (fig. 6), the ERG1 homologues of the four genome to ERG7, but to the bacterial ERG7 homolog SHC. sterol-producing bacteria branch basally, but do not appear This is puzzling because SHC is not involved in the syn- to be more closely related to eukaryotic ERG1 than they are thesis of sterols and ERG1 is not involved in the synthesis to the other bacterial monooxygenases, and they share no of hopanoids. Interestingly, in S. aurantiaca, a gene coding specific sequence signature with eukaryotic ERG1 (fig. 7). for the enzyme responsible for the last step of the pathway Therefore, the hypothesis of an ancient HGT involving the leading from IPP to squalene (squalene synthase) is found ERG1 homologues of the four sterol-producing bacteria in between ERG1 and ERG7 (fig. 5), leading to a conserved and eukaryotic ERG1 remains open. Conversely, ERG7 context for three enzymes acting consecutively. and their bacterial SHC counterparts clearly separate into The presence of the first two enzymes of the pathway two clusters in the corresponding phylogeny (fig. 8). The in the few sterol-producing bacteria has been previously four bacterial ERG7 appear to be more closely related to discussed as resulting from an ancient HGT (Pearson their eukaryotic homologs (fig. 8), indicating a specific 374 Desmond and Gribaldo FIG. 6.—Maximum likelihood tree of ERG1 homologs. Only a subset of the most closely related bacterial monooxygenases are included. Following removal of ambiguously aligned regions, the final data set included 130 conserved positions. The tree was obtained by PHYML (Guindon and Gascuel 2003) as detailed in Materials and Methods. For clarity, only bootstrap values .50% are shown. evolutionary relationship, as previously put forward G. obscuriglobus, and P. pacifica all have also a typical (Pearson et al. 2003; Chen et al. 2007). Three bacterial bacterial SHC ortholog, whereas this is not the case for ERG7 branch basally with respect to their eukaryotic coun- S. aurantiaca (fig. 8). Thus, the possibility remains that terparts, and this may be interpreted in favor of a hypothesis the three bacterial ERG7 were obtained from eukaryotes, where eukaryotic ERG7 originated from bacteria. How- either via an ancient HGT before eukaryotic diversification ever, the ERG7 of S. aurantiaca emerges from within eu- or that fast evolution following transfer leads to an artificial karyotes (fig. 8), suggesting that it was obtained via HGT. grouping of the three bacterial ERG7 at the base of eukar- Moreover, along with their ERG7 copies, M. capsulatus, yotes. The SHC sequences of G. obscuriglobus and Phylogenomics of Sterol Synthesis 375 FIG. 7.—Alignment of ERG1 homologs including sequences of the four sterol-synthesizing bacteria (G. obscuriglobus, M. capsulatus, S. aurantiaca, and P. pacifica), of two other bacteria (Saccharopolyspora erythraea and Frankia alni), and of five representatives of eukaryotes (T. brucei, S. cerevisiae, H. sapiens, D. discoideum, and A. thaliana). P. pacifica are very divergent, in particular that of Gemma- relaxed the functional constraints of native SHC sequences. ta, which is split into three genes. It is possible that the pres- It would nevertheless be interesting to verify the activities ence of an ERG7 and therefore the synthesis of sterols have of these SHC orthologs. 376 Desmond and Gribaldo FIG. 8.—Maximum likelihood tree of ERG7 homologs. Following removal of ambiguously aligned regions, the final data set included 386 conserved positions. The tree was obtained by PHYML with the same criteria as detailed in Materials and Methods. Gemmata obscuriglobus and S. aurantiaca have no DHCR24 responsible for delta-24 reduction (fig. 1) is con- other homologs of the pathway, whereas M. capsulatus sistent with the 4alpha-methyl-5alpha-cholest-8(14)-en- has a homolog of ERG11, consistent with the fact that this 3beta-ol and 4,4-dimethyl-5alpha-cholest-8(14)-en-3beta-ol bacterium is able to produce more complex sterols (Lamb produced by M. capsulatus (Bouvier et al. 1976). Similar et al. 2007). Interestingly, we found that M. capsulatus also to M. capsulatus, P. pacifica also has a homolog of has a homolog of DHCR24 that branches robustly with Met- ERG11 (Supplementary Material 1, Supplementary Material azoans and choanoflagellates (Supplementary Material 1, online). ERG11 homologs are also present in Mycobacteria Supplementary Material online) and thus has likely origi- (Supplementary Material 1, Supplementary Material online). nated via HGT from these eukaryotes. The presence of It has been proposed that mycobacterial and M. capsulatus Phylogenomics of Sterol Synthesis 377 ERG11 originated via HGT from plants (Rez ˇen et al. 2004). vious action of ERG25 and ERG26. Interestingly, these ar- However, the ERG11 of M. capsulatus and mycobacteria guments also imply the presence of an ERG27. However, group with the ERG11 homolog of P. pacifica. Thus, the because P. pacifica has no homologs of fungi and verte- eventual HGT would have occurred in one of these bacteria brates ERG27, it is possible that it harbors a homolog of followed by HGT among them. Moreover, phylogenetic the still-missing enzyme providing an ERG27 function analysis does not indicate a closer proximity of plants in land plants and other eukaryotic lineages (table 1). If this ERG11 with these bacterial homologs (Supplementary Ma- is so, this putative ERG27 equivalent may display a closer terial 1, Supplementary Material online). By congruence relationship to eukaryotes (and be preferentially absent in with ERG7, it is possible that ERG11 was also acquired Opisthokonta and non–sterol-producing lineages) than via an ancient HGT from an ancestor of Eukaryotes or a spe- other bacteria. We extracted from our databank the ge- cific eukaryotic source that remains undetermined due to the nomes of eukaryotes lacking ERG27 plus 52 bacteria from high divergence of bacterial ERG11 sequences. all phyla, and we searched the whole genome of P. pacifica against it. Then, we filtered for those genes that had all these eukaryotes as first Blast hits within an e value ,1  10 . The Case of P. pacifica We found eight genes harboring such a phyletic pattern. In addition to ERG1, ERG7, and ERG11, P. pacifica Four of these correspond to ERG11, ERG7, and the two has six homologs of enzymes of the pathway catalyzing copies of FK, what makes a good positive control. The re- C-14 reduction (two copies of FK), C-4 demethylation maining four genes are a member of the P450 family (dis- (ERG25 and ERG26), delta-8–delta-7 isomerization tant relative of ERG 11), a hypothetical serine/threonine (ERG2), and cyclopropylsterol isomerization (CpI1), con- protein phosphatase (which is, however, also present in eu- sistent with the identification of very elaborated sterols in its karyotes that do not make sterol), an aspartatecarbamoyl close relative Nannocystis (Bode et al. 2003). The sterols transferase catalytic subunit (but phylogenetic analysis in- produced by P. pacifica have not yet been characterized. dicates that it is not closer to eukaryotes than to other bac- However, the combination of these enzymes leads us to in- teria), and a succinate-semialdehyde dehydrogenase fer production of 7,24-cholestadien-3beta-ol in P. pacifica, (NAD(P)þ) (which has only distant bacterial homologues). similar to some characterized sterols from Nannocystis The increase of the e value cut-off gave no other good can- (Bode et al. 2003). The presence of a CPI1 homolog in didates. The last protein (succinate-semialdehyde dehydro- P. pacifica is interesting because this is the first reported genase [NAD(P)þ]) is particularly interesting because its case of a bacterial homolog of this enzyme, which would annotated activity is congruent with an ERG27 function indicate a cycloartenol route. It is thus possible that P. pa- (C-3 ketoreduction), and further analysis on all eukaryotic cifica produces cycloartenol, although no cycloartenol has genomic data reveals that it is absent in eukaryotes that do been identified in its close relative Nannocystis (Bode et al. not make sterols. Moreover, this protein is also specifically 2003). Interestingly, we found that the ERG7 homolog of absent in eukaryotes that harbor an ERG27 copy, to the ex- P. pacifica has a V453, which is characteristic of a lanosterol ception of fungi (fig. 9). If this were the real ancestral route (fig. 3). It will be interesting to characterize the ERG27, it would mean that fungi have retained it and P. pacifica CPI1 homolog in order to assess its precise func- use it for a different function. In fact, S. cerevisiae mutants tion, as well as the types of sterols produced by P. pacifica. for ERG27 are sterol auxotrophs (Goffeau et al. 1996), in- The presence of CPI1 uniquely in P. pacifica among bac- dicating that this potential ERG27 analog (NP_011904) teria strongly points toward a recent acquisition via HGT cannot rescue the fungi ERG27 function. Interestingly, this from eukaryotes, although its sequence does not appear par- protein appears to function in the endoplasmic reticulum in ticularly close to any precise eukaryotic source (Supple- S. cerevisiae, precisely like ERG27, and mutants are defec- mentary Material 1, Supplementary Material online). tive in directing meiotic recombination events to homolo- Plesiocystis pacifica has two clear orthologs of FK and gous chromatids (Goffeau et al. 1996). It would be one ortholog of ERG2, which cluster within eukaryotic se- extremely interesting to verify the involvement of this en- quences and thus likely originated via HGT from eukar- zyme in the sterol pathway of nonfungal lineages lacking yotes (Supplementary Material 1, Supplementary a bona fide ERG27. Material online). Concerning the step of C-4 demethylation, P. pacifica has homologs of ERG25 and ERG26. P. pacif- ica ERG25 emerges from within the large protein family Discussion and Conclusions that includes eukaryotic ERG3/ERG25 (not shown) and therefore also very likely originated via HGT from eukar- The emergence of specific features in the lineage lead- yotes. As for ERG26, phylogenetic analysis does not show ing to eukaryotes is one of the most intriguing issues in the a clear orthology relationship between P. pacifica and its domain of early evolution. Phylogenomics is a recent and eukaryotic counterparts, although this may be due to its powerful approach to dissect the emergence and subsequent high divergence (Supplementary Material 1, Supplemen- evolution of cellular processes. However, its application to tary Material online). The ERG25 and ERG26 homologs eukaryotes is just becoming to emerge thanks to the recent from P. pacifica likely perform ERG25 and ERG26 func- availability of a sufficient sampling of complete genomes. tions, for two reasons: 1) the possibility that P. pacifica Recent analyses have investigated the emergence and evo- produces sterols as elaborated as Nannocystis, which would lution of different unique eukaryotic cellular structures require the action of an ERG25 and an ERG26, and 2) the (Bapteste et al. 2005; Eme et al. 2009; Field and Dacks presence of an ERG2 which in principle requires the pre- 2009). However, the emergence and evolution of 378 Desmond and Gribaldo FIG. 9.—Maximum likelihood tree of putative ERG27 analogues. Following removal of ambiguously aligned regions, the final data set included 483 conserved positions. The tree was obtained by PHYML with the same criteria as in Materials and Methods. eukaryotic-specific metabolic pathways is still poorly ex- (oxidases), ERG26 (NADP-dependent dehydrogenases), plored. Here we have reported a phylogenomics analysis and DHCR24 (bacterial FAD/FMN-containing dehydro- of one such eukaryotic-specific metabolic pathway. The genases). These have been likely recruited from preexisting presence of sterols is an essential characteristic of all eu- enzymes in parallel to the emergence of the sterol pathway karyotic membranes and thus presumably accompanied in the lineage leading to the LECA. By contrast, four en- the very emergence of this domain of life. In particular, zymes (ERG28 and EBP, ERG24, and ERG6) do not have the appearance of sterols may have allowed an increase flu- any bacterial homologue and have thus presumably arisen idity of eukaryotic membranes and thus possibly repre- in the eukaryotic lineage. The same is likely true for five sented an important step toward increasing cell additional enzymes: Three (ERG4, DHCR7, and ERG2) size. Additionally, the emergence of sterols may have pro- have very few bacterial homologs other than P. pacifica vided protection against oxidative stress (Galea and Brown and these clearly derive from HGT from eukaryotes 2009). (ERG4, Coxiella) and DHCR7 (Coxiella and Protochlamy- We have shown that eight enzymes of the sterol path- dia), ERG2 (Mycobacteria), and two (CPI1 and FK) have way belong to large gene families that include distant bac- only P. pacifica as bacterial homolog (Supplementary Ma- terial homologs (if the few sterol-making bacteria are not terial 1, Supplementary Material online). considered). These are ERG1 (monoxygenases), ERG7 We have highlighted that P. pacifica harbors the largest (SHCs), ERG5-ERG11 (cytochrome P450), ERG3-ERG25 reported set of homologs of eukaryotic sterol-synthesizing Phylogenomics of Sterol Synthesis 379 enzymes. It may be put forward that this myxobacterium is at ERG1, we think that the emergence of ERG7 in an ancestor the origin of these eukaryotic enzymes, which may lend sup- of eukaryotes was likely a key event in the assembly of the port to the hypothesis that Myxobacteria might have been the eukaryotic sterol pathway. Moreover, we tentatively infer potential partners of a syntrophic association at the origin of that the LECA harbored a cycloartenol route rather than eukaryotes (Lopez-Garcia and Moreira 1999). Indeed, P. pa- a lanosterol route, consistent with previous hypotheses cifica sequences appear to branch basally to eukaryotes, (Bloch 1991; Ourisson and Nakatani 1994). Importantly, which may be consistent with this bacterium being the source because the action of ERG7 and the subsequent steps in of eukaryotic homologs. However, for all enzymes previ- the sterol pathway are highly linked to oxygen and because ously mentioned that belong to large gene families, P. pacif- we inferred the presence of a complex sterol pathway in the ica sequences do not branch with their typical bacterial LECA, this suggests that this ancestor lived in a fully ox- homologs, indicating that they may not have arisen from ygenated environment. gene duplication of native genes followed by functional Our study allowed reconstructing the potential capaci- switch. Nevertheless, this may also be due to an acceleration ties for making specific sterols in the eukaryotic organisms of evolutionary rates that prevent assessing the real evolu- where the pathways have not yet been characterized. The tionary relationships of these P. pacifica enzymes. However, emerging enzymatic landscape highlighted in this study in- we have shown that it is possible that P. pacifica has acquired dicates that the differences in the characterized pathways its pathway for sterol synthesis via HGT from eukaryotes from fungi, land plants, and vertebrates, as well as those (either ancient or not). By the same reasoning, as proposed inferred in the noncharacterized organisms, arose by differ- previously (Pearson et al. 2003) it is possible that ERG7 has ent organization of a common set of conserved enzymes, been transferred to one of the few sterol-producing bacteria loss of specific enzymes, and the addition of a few novel and then among them. However, we put forward the possi- enzymes. In particular, the different order of steps does bility that ERG1 in these bacteria does not derive from HGT not seem to be linked to a cycloartenol or lanosterol route from eukaryotes. Consequently, it is possible that the ERG1 because, for example, kinetoplastids have a set of enzymes function was recruited into the sterol pathway of these bac- that is very similar to that of fungi, whereas they use a lan- teria from a native enzyme only after HGT from eukaryotes osterol route with an order of steps very similar to that of bringing along the ERG7 copy. If this were so, ERG1 homo- land plants (e.g., C-4 demethylation occurring through two logs would have been recruited independently in these bac- nonconsecutive steps; Lepesheva et al. 2004). Thus, the re- teria and eukaryotes to function with ERG7. Interestingly, organization of a similar order of steps observed in fungi phylogenetic analysis of the eukaryotic enzymes involved and animals may be specific to these two lineages and pos- in the pathway leading from IPP to squalene do not show sibly linked to the recruitment of their unique ERG27, any particular evolutionary relationships with the sterol- which replaced an ancestral analogous enzyme. Interest- producing bacteria (data not shown), supporting the hypoth- ingly, this would have occurred twice independently in esis that the capacity of making sterols in these bacteria arose fungi and animals because choanoflagellates appear to have via HGT from eukaryotes and that the eukaryotic sterol retained the ancestral C-3 ketoreductase and possibly an or- pathway originated in the LECA from the branching off der of steps similar to that of land plants. on a preexisting native pathway leading to squalene. In this Importantly, we showed that phylogenomics ap- regard, the evolutionary proximity of P. pacifica and eukary- proaches are a powerful tool to identify still-missing en- otic sterol enzymes should not be taken as support for an or- zymes, such is the case of the elusive ERG27 equivalent igin of eukaryotes from a symbiosis between myxobacteria in land plants and probably in other eukaryotic phyla. In this and Archaea because in this case the previous steps should respect, more experimental data on a wider sampling of eu- also show a similar pattern. Similarly, the proposal that karyotic diversity are surely necessary to test our predictions. actinobacteria are at the origin of the eukaryotic pathway Finally, because sterols are key markers for indicating (Cavalier-Smith 2006) is clearly weakened by the fact that the presence of specific eukaryotic lineages in the fossil re- the few homologs present in mycobacteria very likely derive cord, our study may be a useful reference for palaeogeo- from HGT from eukaryotes. More importantly, no actino- chemistry studies. bacteria have so far been shown to synthesize sterols (Rez ˇen et al. 2004) and these enzymes in mycobacteria are likely Supplementary Material used to process sterols from their eukaryotic hosts. Our analysis also indicates that the LECA had the po- Supplementary Material is available at Genome Biol- tential to make a wide array of different sterols and that sub- ogy and Evolution online (http://www.oxfordjurnals.org/ sequent evolution occurred through tinkering via our_journals/gbe/). differential enzyme losses and specializations in the various eukaryotic lineages in parallel with their divergence from Acknowledgments the LECA. This is consistent with the idea that the ability of synthesizing elaborated sterols would have paved the The authors declare that they have no competing in- way toward specific eukaryotic characters such as cell sig- terest. S.G. conceived the study. E.D. performed all anal- naling and multicellularity (Bloch 1991). Our analysis does yses. E.D. and S.G. wrote the manuscript, which was not allow us to predict the order of steps leading to the as- read, edited, and approved by both authors. 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Phylogenomics of Sterol Synthesis: Insights into the Origin, Evolution, and Diversity of a Key Eukaryotic Feature

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Abstract

RESEARCH ARTICLE Phylogenomics of Sterol Synthesis: Insights into the Origin, Evolution, and Diversity of a Key Eukaryotic Feature Elie Desmond and Simonetta Gribaldo Institut Pasteur, Unite ´ de Biologie Mole ´ culaire du ge ` ne chez les Extre ˆ mophiles, Paris, France The availability of complete genomes from a wide sampling of eukaryotic diversity has allowed the application of phylogenomics approaches to study the origin and evolution of unique eukaryotic cellular structures, but these are still poorly applied to study unique eukaryotic metabolic pathways. Sterols are a good example because they are an essential feature of eukaryotic membranes. The sterol pathway has been well dissected in vertebrates, fungi, and land plants. However, although different types of sterols have been identified in other eukaryotic lineages, their pathways have not been fully characterized. We have carried out an extensive analysis of the taxonomic distribution and phylogeny of the enzymes of the sterol pathway in a large sampling of eukaryotic lineages. This allowed us to tentatively indicate features of the sterol pathway in organisms where this has not been characterized and to point out a number of steps for which yet-to-discover enzymes may be at work. We also inferred that the last eukaryotic common ancestor already harbored a large panel of enzymes for sterol synthesis and that subsequent evolution over the eukaryotic tree occurred by tinkering, mainly by gene losses. We highlight a high capacity of sterol synthesis in the myxobacterium Plesiocystis pacifica, and we support the hypothesis that the few bacteria that harbor homologs of the sterol pathway have likely acquired these via horizontal gene transfer from eukaryotes. Finally, we propose a potential candidate for the elusive enzyme performing C-3 ketoreduction (ERG27 equivalent) in land plants and probably in other eukaryotic phyla. Introduction oxygen. On the other hand, in Eukaryotes a first step (squa- lene monooxygenation, ERG1) adds an oxygen to squalene Triterpenoids are a large family of lipids that are leading to squalene epoxide, which is then cyclized in a sec- widely distributed in Bacteria (hopanoids) and Eukaryotes ond step either to lanosterol or to cycloartenol by enzymes (sterols). Sterols are present in all eukaryotes, where they homologous to SHC (ERG7). Because sterol synthesis is are essential and are involved in both intra- and intercellular widely distributed in eukaryotes, it can be assumed that signaling and in the organization of membranes. In mem- the last eukaryotic common ancestor (LECA) already syn- branes they affect fluidity and permeability (London 2002; thesized sterols. However, the origin of this pathway is not Tyler et al. 2009) and are major players in the formation of clear. The cyclization of squalene and the following steps lipid rafts, which are regions of reduced fluidity formed by are very demanding in oxygen. For example, 11 molecules the close association of sterols with sphingolipids (Jacobson of oxygen are required for the synthesis of one molecule of and Dietrich 1999; Brown and London 2000; Anderson and cholesterol (Summons et al. 2006). Organisms that live un- Jacobson 2002). Proteins involved in concerted functions der anaerobic conditions therefore resort to external sources such as cell signaling can be associated through their selec- of sterol by ingesting other eukaryotes. Moreover, some eu- tive incorporation into lipid rafts (Melkonian et al. 1999; karyotes that are able to synthesize sterols in the presence of Simons and Toomre 2000). Moreover, sterols play a key role oxygen have to acquire them from food when under anaer- in typical eukaryotic features such as phagocytosis. For ex- obic conditions, as it has been shown in yeast (Schneiter ample, genes involved in sterol biosynthesis have been 2007). A commonly accepted hypothesis is thus that the shown to be selectively upregulated in the amoebozoan pathway of sterol biosynthesis appeared after the emer- Dictyostelium discoideum during phagocytosis (Sillo et al. gence of oxygenic photosynthesis and the oxygenation 2008). Eukaryotes that are not able to synthesize sterols have of the atmosphere and oceans, thought to have occurred be- to obtain them from food, such as is the case, for example, of tween 2.7 and 2.4 Ga (Summons et al. 2006). It has been insects and of most marine invertebrates. recently put forward that the initial role of sterols in eukar- Bacterial and eukaryotic triterpenoids belong to the yotes may have been that of protection against oxidative class of isoprenoids. Isoprenoids are all derived from their stress when oxygen levels rose (Galea and Brown 2009). universal precursor isopentenyl diphosphate (IPP). IPP syn- Moreover, it has been proposed that the ancestral pathway thesis can follow two different routes, either via the meval- made cycloartenol as a final product and that the rising con- onate pathway in Archaea and some Eukaryotes or via the centrations of oxygen in the atmosphere would have led to 2-C-methyl-D-erythritol 4-phosphate pathway in Bacteria an evolution of the pathway beyond cycloartenol toward and other Eukaryotes (Rohmer et al. 1993; Boucher more stable sterol compounds (Ourisson and Nakatani et al. 2004; Volkman 2005). IPP is the precursor of squalene 1994). and its cyclization products. In Bacteria, hopanoids are syn- Intriguingly, although archaea are not known to be thesized from the cyclization of squalene by a squalene- able to synthesize sterols, a few bacteria have this ability. hopene cyclase (SHC) in a process that does not require Methylococcus capsulatus (gamma-Proteobacteria) has been shown to produce different sterols (Bouvier et al. Key words: sterols, phylogenomics, eukaryotes, evolution. 1976), and Gemmata obscuriglobus (Planctomycetales) E-mail: simonetta.gribaldo@pasteur.fr. produces lanosterol and its isomer parkeol (Pearson et al. Genome. Biol. Evol. 1:364–381. 2003). According to their sterol-synthesizing abilities, ho- doi:10.1093/gbe/evp036 Advance Access publication September 10, 2009 mologous of the first two enzymes of the pathway (ERG1 The Author(s) 2009. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.5), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Phylogenomics of Sterol Synthesis 365 and ERG7) have been found in M. capsulatus and G. may be at work. Moreover, we reconstructed the potential obscuriglobus (Pearson et al. 2003; Lamb et al. 2007). A ancestral abilities of sterol production in the LECA and variety of more or less elaborated sterols have been isolated the subsequent evolution of the pathway over the eukary- from a number of Myxobacteria (delta-Proteobacteria), otic tree, which appears to have occurred by tinkering, such as cycloartenol in Stigmatella aurantiaca, and 7-cho- mainly by gene losses. We highlight a high capacity of lesten-3beta-ol synthesized by some strains of Nannocystis sterol synthesis in the Myxobacterium P. pacifica, and sp. (Bode et al. 2003). The elaborated sterols synthesized by we support the hypothesis that the few bacteria that harbor Myxobacteria imply that these bacteria not only have the homologs of enzymes of the sterol pathway have likely first two enzymes of the pathway but also up to four or five acquired these via ancient HGT from eukaryotes. Finally, more downstream. A gene coding for a homolog of the sec- an analysis of phyletic patterns allowed us to propose a po- ond enzyme of the pathway (ERG7) has been sequenced tential candidate for the elusive enzyme performing C-3 from S. aurantiaca (Bode et al. 2003). However, mutants ketoreduction (ERG27 equivalent) in land plants and other where the gene was inactivated showed no noticeable phe- eukaryotic phyla, highlighting the power of phylogenom- notype, indicating that sterol production is not essential in ics approaches to the study of biochemical pathways. this bacterium (Bode et al. 2003). The function of these lip- ids in bacteria remains indeed unknown (Bode et al. 2003). Previous phylogenetic analyses have shown that ERG1 ho- Materials and Methods mologs from Methylococcus and Gemmata are specifically related to their eukaryotic counterparts (Pearson et al. Homologs of the enzymes characterized in fungi, ver- 2003). Concerning ERG7, it has been shown that Methyl- tebrates, and land plants were identified using the BlastP ococcus, Gemmata, and Stigmatella homologs are also program version 2.2.1.8 (Altschul et al. 1997) and the closely related to their eukaryotic counterparts (Pearson HMMER tools (http://hmmer.janelia.org/) on a local data- et al. 2003; Chen et al. 2007). The closeness of ERG1 bank of complete genomes (38 eukaryotes: Opisthokonta/ and ERG7 among these sterol-producing bacteria and eu- metazoans: Drosophila melanogaster, Tribolium casta- karyotes has been explained by a possible ancient horizon- neum, Caenorhabditis elegans, Homo sapiens, Mus muscu- tal gene transfer (HGT) (Pearson et al. 2003; Chen et al. lus, Danio rerio; Opisthokonta/choanoflagellates: 2007). However, it remains unclear whether these two en- Monosiga brevicollis; Opisthokonta/fungi: Aspergillus fu- zymes originated in bacteria or in eukaryotes. It is likely migatus, Neurospora crassa, Saccharomyces cerevisiae, that the assembly of the pathway started with these two ini- Schizosaccharomyces pombe, Ustilago maydis, Cryptococ- tial steps. Subsequently, other enzymatic steps would have cus neoformans, Encephalitozoon cuniculi; Ameobozoa: added to produce more elaborated sterols. However, it is not D. discoideum, Entamoeba histolytica; Plantae/land known when this would have happened and which type of plants: Arabidopsis thaliana, Oryza sativa; Plantae/green sterols the LECA would have been potentially able to algae: Chlamydomonas reinhardtii, Ostreococcus tauri; produce. Plantae/red algae: Cyanidioschyzon merolae; Alveolata/ The diversity and nature of sterols and the pathways ciliates: Tetrahymena thermophila, Paramecium tetraure- leading to these compounds have been thoroughly studied lia; Alveolata/apicomplexans: Cryptosporidium parvum, in vertebrates, fungi, and land plants. Characterizations of Plasmodium falciparum, Plasmodium yoelii, Theileria sterols from other organisms have unveiled a wide variety annulata; Heterokonta/oomycetes: Phytophthora ramo- of molecules, among which not only the same sterols rum; Heterokonta/diatoms: Thalassiosira pseudonana: known in animals, fungi, and land plants but also other Heterokonta/brown algae: Aureococcus anophagefferens; types of sterols with insaturations at various positions of Excavata/kinetoplastids: Trypanosoma brucei, Trypanoso- the cycle or in the side chain, and possibly alkylations or ma cruzi, Leishmania infantum, Leishmania major, Leish- inclusion of a cyclopropane ring mostly in C-24 or C-22 mania braziliensis; Excavata/heterolobozoans: Naegleria (Volkman 2003). Based on these characterizations, fossil gruberi; Excavata/diplomonads: Giardia lamblia; Excava- steranes found in sediments are used as biomarkers for past ta/parabasalids: Trichomonas vaginalis; 586 bacteria, 48 ar- eukaryotic life (Kodner, Pearson, et al. 2008). However, chaea). Most of the complete genomes were retrieved from relatively little is known about the structure of the pathway the Refseq database at the National Center for Biotechnol- in these organisms. A large number of complete genomes ogy Information (http://www.ncbi.nlm.nih.gov), except for are now available from representatives of major eukaryotic N. gruberi, O. tauri, A. anophagefferens, P. ramorum, and phyla, as well as from representatives of sterol-producing T. pseudonana genomes which were retrieved from the JGI bacteria, notably the two Myxobacteria S. aurantiaca database (http://genome.jgi-psf.org/euk_home.html) and and Plesiocystis pacifica. We have thus carried out an ex- for C. merolae genome which was retrieved from http:// tensive analysis of the taxonomic distribution and phylog- merolae.biol.s.u-tokyo.ac.jp. eny of the enzymes of the sterol pathway in a large sampling When a particular gene was not found in some species, of organisms representing eukaryotic diversity for which we searched the nucleotide sequences of these complete ge- complete genomic sequences are available. The use of com- nomes using TBlastN. The retrieved proteins were aligned plete genomes is in fact essential to infer presence or ab- using MUSCLE 3.6 (Edgar 2004), the alignments were edi- sence of homologs of the different enzymes. We tentatively ted and refined manually using the ED program from the reconstructed the potential structure of the sterol pathway in MUST package (Philippe 1993). diverse organisms where it has not been characterized and Phylogenies were reconstructed for each protein. Re- point out a number of steps for which yet-to-discover enzymes gions where homology was doubtful were manually 366 Desmond and Gribaldo removed from the data sets before phylogenetic analysis us- enzyme may be dispensable. The fungi pathway is very ing the NET program of the MUST package (Philippe similar to that of vertebrates. However, the ergosterol pro- 1993), and phylogenetic trees were computed with PHYML duced contains 28 carbons contrary to cholesterol, which (Guindon and Gascuel 2003) and MrBayes (Ronquist and contains only 27 carbons (fig. 1A). The additional carbon Huelsenbeck 2003; Huelsenbeck and Ronquist 2001). present in fungi ergosterol is added by the C-24 methylase PHYML trees were calculated using the Whelan and ERG6, which is in fact absent in vertebrates (fig. 1C). On Goldman model, a gamma correction to take into account the other hand, the sitosterol and stigmasterol produced by the heterogeneity of evolutionary rates across sites (four dis- land plants contain 29 carbons (fig. 1A). These two addi- crete classes of sites, an estimated alpha parameter, and an tional carbons are added in two steps catalyzed by two estimated proportion of invariable sites), and subtree prun- copies of ERG6: SMT1 performing C-24 methylation ing and regrafting topology searches. The robustness of each and SMT2 performing C-28 methylation (fig. 1A and C). branch was estimated by a nonparametric bootstrap proce- dure implemented in PHYML (100 replicates of the original data set and the same parameters). MrBayes trees were cal- Phylogenomics of the Sterol Pathway culated with an estimated fixed rate model and a gamma cor- rection as for the PHYML trees. The Markov Chain Monte Starting from the characterized genes in fungi, verte- Carlo runs were performed with four chains on 1,000,000 brates, and land plants, we investigated their taxonomic dis- generations sampled every 100 generation, and the final tree tribution and phylogeny in complete genomes from a broad was summarized discarding the first 1,000 generations. taxonomic sample of eukaryotic diversity (38 genomes con- taining representatives of main eukaryotic supergroups: one choanoflagellate, six animals, seven fungi, two amoebozo- ans, two land plants, two green algae, one red algae, two Results ciliates, four apicomplexans, one brown algae, one diatom, Canonical Pathways one oomycete, five kinetoplastids, one heterolobosean, one The pathway of sterol synthesis has been thoroughly diplomonad, one parabasalid, for a complete list see Mate- studied in vertebrates, fungi, and land plants (fig. 1). A rials and Methods) as well as from complete genomes from number of enzymatic reactions are found in the three path- 586 bacteria and 48 archaea. Phylogenetic analysis allowed ways (fig. 1A). In these three ‘‘canonical pathways,’’ squa- us assigning each of these homologs to an orthology group lene is oxygenated in a first step and cyclized in a second and thus to a potential function (table 1 and Supplementary step either to lanosterol in vertebrates and fungi or to cyclo- Material 1, Supplementary Material online). Consistently artenol in land plants (fig. 1A). Cycloartenol includes a cyclo- with the oxygen demand of the pathway, orthologs are to- propane ring between carbon C-9 and C-10 (fig. 1A and B). tally absent in strict anaerobic organisms (Encephalitozoon, This cycle is cleaved at a later step of the pathway by a spe- Giardia, Entamoeba, Cryptosporidium, and Trichomonas) cific land plants enzyme (CPI1) leading to typical sterols but also in aerobic ones (Plasmodium, Theileria, Drosoph- with four-carbon cycles (fig. 1A and B). Lanosterol or cyclo- ila, and Tribolium) (not shown). We found only a partial artenol is then converted to cholesterol in vertebrates, er- number of orthologs in the oomycete P. ramorum gosterol in fungi, and to campesterol, sitosterol, and (ERG3 and DHCR7), and the two ciliates (ERG3 and stigmasterol in land plants by a succession of oxidations, FK) (not shown), consistent with the notion that these reductions, and demethylations (fig. 1A). Most of these organisms do not synthesize sterols (Trigos et al. 2005). steps are shared in the three pathways, although they do These orthologs may be in fact used to process sterols not occur in the same order. C-4 demethylation occurs twice obtained from diet. in the three canonical pathways and is performed both times In fungi, vertebrates, and land plants, the same enzy- via three concerted steps (C-4 methyl oxidation, C-3 matic steps are generally performed by homologous en- dehydrogenation/C-4 decarboxylation, and C-3 ketoreduc- zymes (table 1). Two exceptions are the delta-8, delta-7 tion; fig. 1C). In vertebrates and fungi, the two C-4 deme- isomerization step performed by the nonhomologous thylations occur one after the other (fig. 1A). Conversely, in ERG2 in fungi and EBP/HYD1 in vertebrates/land plants land plants the two C-4 demethylations do not occur one and the delta-24 reduction performed by the nonhomolo- after the other, and two C-4 methyl oxidases (SMO1 and gous ERG4 in fungi and DHCR24/DWF1 in vertebrates/ SMO2) perform these steps (fig. 1C), each being specific land plants (table 1). A few enzymes performing different for a particular substrate (SMO1 acting on 24-methylene- steps in fungi, vertebrates, and land plants are evolution- cycloartenol, and SMO2 acting several steps later on 24- arily related (table 1). For example, ERG4-(DHCR7/ ethylenelophenol or 24-methylenelophenol; fig. 1A). In land DWF5)-(ERG24/TM7SF2/FK) all belong to a large protein plants, the enzyme responsible for C-3 ketoreduction is un- family. In a global phylogeny of this family, members with known (fig. 1C and Bouvier et al. 2005). It has been shown different functions segregate into different orthology in Saccharomyces that C-4 demethylation takes place at the groups (Supplementary Material 1, Supplementary Mate- endoplasmic reticulum membrane through the anchoring of rial online) indicating that these functions arose by gene ERG26 and ERG27 by ERG28 (Gachotte et al. 2001). It is duplication. It has to be noted that ERG24/TM7SF2 and likely that this anchoring also occurs in vertebrates and land FK have the same function but belong to paralogous clus- plants because they have a copy of ERG28 (fig. 1C). How- ters (Supplementary Material 1, Supplementary Material ever, it has been shown that ERG28 is not essential in Sac- online). Finally, ERG3/ERG25 are paralogs within a large charomyces (Gachotte et al. 2001), indicating that this family of oxidases (not shown), as well as ERG5/ERG11 Phylogenomics of Sterol Synthesis 367 FIG. 1.—(A) Canonical pathways of sterol synthesis leading to land plants, fungi, and vertebrate sterols. Upstream of squalene, the mevalonate (MVA) and 2-C-methyl-D-erythrol 4-phosphate (MEP) ways leading to IPP are shown. Downstream of squalene, the bacterial pathway of hopanoid synthesis via SHC is also indicated. (B) Numbering of carbons and cycles of steroids. (C) Table indicating the names and EC numbers of fungi, vertebrates, and land plants genes corresponding to each step of the pathway shown in (A). The red box indicates that the gene performing C-3 ketoreduction is still unknown in land plants. 368 Desmond and Gribaldo Table 1 Distribution of Orthologs of Enzymes of the Sterol Pathway in the Eukaryotic Genomes of Our Data Set The same color indicates homology. Presence is indicated by an X symbol. A double or triple X indicates presence of two or three copies. In the case of the paralogous families ERG24/TM7SF2-FK, an X with asterisk indicates the presence of orthologues of TM7SF2-FK, an X without asterisk indicates the presence of orthologues of ERG24, and a double X with asterisk indicates the presence of both paralogues (i.e. in Monosiga). In the case of the non-homologous families ERG4/DHCR24-DWF1 and ERG2/EBP-HYD1, the presence of one or the other copy is indicated by green and violet boxes, respectively, whereas the presence of both copies is indicated by a dark yellow box. Absences consistent with experimental characterizations are indicated by empty boxes, while absences not consistent with experimental characterizations are indicated by red boxes with exclamation points (see text for discussion). within the large family of cytochrome P450 (not shown). been largely studied (Summons et al. 2006). In particular, the Interestingly, we found that the fungi Aspergillus and Neu- production of lanosterol or cycloartenol in fungi, verte- rospora (Pezizomycotina) harbor additional homologs of brates, and land plants has been linked to the presence enzymes not belonging to the canonical fungi pathway of particular residues in the active site of ERG7: Three po- (EBP and DHCR24), as well as an additional SMT copy sitions are in fact differentially conserved between lanoster- (Supplementary Material 1, Supplementary Material on- ol synthases and cycloartenol synthases (Summons et al. line). The first two enzymes bring redundant functions to 2006; fig. 3). In fungi and vertebrates, lanosterol synthase these fungi, whereas the additional SMT copy may produce is characterized by T381, C/Q449, and V453, whereas cy- C-29 sterols, differently from the classical fungi ergosterol cloartenol synthase is characterized by Y381, H449, and (C-28 sterol). I453 (Summons et al. 2006). However, as shown for kinet- A number of sterols have been characterized from or- oplastids, position 381 can be variable (they have an Y but ganisms present in our data set other than fungi, vertebrates, make lanosterol; Buckner et al. 2000), and position 449 can and land plants, to the exception of the green algae O. tauri also be variable because Methylococcus and Gemmata have and the red algae C. merolae (fig. 2) (Raederstorff and an H while they make lanosterol (Summons et al. 2006) Rohmer 1987; Nes et al. 1990; Giner and Boyer 1998; (fig. 3). Thus, only position 453 is indicative of lanosterol Veron et al. 1998; Salimova et al. 1999; Roberts et al. or cycloartenol production. An alignment of ERG7 homo- 2003; Kodner, Summons, et al. 2008). With the combined logs from our data set (fig. 3) shows that position 453 is knowledge of these sterols and our phylogenomic data, we consistent with the experimental characterizations available sought to tentatively predict some additional features of the for a few additional phyla, such as in the amoebozoan sterol pathways in these organisms. One first general infor- Dictyostelium (I453, cycloartenol route; Nes et al. 1990), mation on the nature of the pathway can be obtained from the heterolobosean Naegleria (I453, cycloartenol route; the characteristics of ERG7 orthologs, which catalyze the Raederstorff and Rohmer 1987), and the kinetoplastids cyclization of squalene epoxide either to lanosterol or to (V453, lanosterol route; Roberts et al. 2003). No data are cycloartenol (fig. 1A and C), one of the most complex re- available on the other phyla, although based on this position actions catalyzed by a single enzyme (Summons et al. it can be predicted that the choanoflagellate Monosiga makes 2006). Among all enzymes of the sterol pathway, ERG7 lanosterol (V453), consistent with their affiliation with is one of the most conserved at the sequence level and or- fungi and animals into the phylum Opisthokonta, whereas thologs are indeed present in all species capable of de novo the brown algae Aureococcus, the diatom Thalassiosira, synthesis (table 1). The active sites are conserved and have the red algae Cyanidioschyzon, and the green algae Phylogenomics of Sterol Synthesis 369 FIG. 2.—Sterols characterized from organisms other than fungi, vertebrates, and land plants present in our data set. For sterol modifications discussed in the text, refer to figure 1B for numbering. Chlamydomonas and Ostreococcus all make cycloartenol group in position 14 (C-14 demethylation þ C-14 reduc- (I453) (fig. 3). tion); ii loss of both methyl groups at position 4 (C-4 methyl Among the variety of the sterols shown in figure 2, oxidation, C-3 dehydrogenation/C-4 decarboxylation, C-3 a number of features are in common with the products ketoreduction); iii) reduction of the delta-8 double bond of canonical pathways (fig. 1A and B): i) they are all derived (delta-8, delta-7 isomerization); and iv) formation of a dou- from cyclization of squalene into lanosterol or cycloartenol; ble bond between C-5 and C-6 (C-5 desaturation). Enzymes ii) final products have no methyl group in position 4 and catalyzing two of these steps are present in all these sterol- position 14 compared with the first products of cyclization synthesizing organisms (table 1): oxidosqualene cyclization of squalene; iii) they are free of double bonds between car- (ERG7) and C-14 demethylation þ C-14 reduction (ERG11 bon C-8 and C-9; and iv) they present a double bond þ ERG24/FK). A particular case concerns C-3 dehydroge- between carbon C-5 and C-6, except for dictyosterol nation/C-4 decarboxylation (ERG26): In a phylogeny of (fig. 2). These characteristics imply that the following steps ERG26 (Supplementary Material 1, Supplementary Mate- must necessarily take place in the synthesis of all these ster- rial online) we identify a clear cluster of orthologs ols (fig. 1C): i) squalene cyclization (squalene monooxyge- including the known enzymes of plants, fungi, and verte- nation þ oxidosqualene cyclization); ii) loss of the methyl brates. However, this cluster lacks M. brevicollis, 370 Desmond and Gribaldo FIG. 3.—Alignment of ERG7 homologs from representative taxa. The catalytic residue D455 is highlighted in red. Positions 381, 449, and 453 are differentially conserved between lanosterol synthases (yellow) and cycloartenol synthases (green). The numbering refers to the H. sapiens ERG7 homolog (NP_002331). O. tauri, T. pseudonana, A. anophagefferens, and kineto- clamation points in table 1). Finally, to our knowledge, two plastids. Interestingly, these genomes harbor homologs that species of our data set (O. tauri and C. merolae) have not form a separate group (Supplementary Material 1, Supple- yet been studied for their sterol composition. Assuming that mentary Material online). The perfect matching of this phy- the enzymatic steps that take place in all other organisms letic pattern and the requirement for C-3 dehydrogenation/ analyzed also take place in these organisms, and given C-4 decarboxylation indicates that these distant homologs our phylogenomic data, we tentatively predicted the kind are good candidates to perform an ERG26 function. of sterols they may produce (Supplementary Material 2, Intriguingly, several genomes lack clear homologs for Supplementary Material online). the remaining steps that we infer to be necessary to produce To sum up, our phylogenomic data indicate a wide va- the sterols that have been characterized in these organisms riety of pathways in different eukaryotic organisms. Inter- (table 1): lack of squalene monooxygenase (ERG1) in estingly, there is no clear separation between the pathway of M. brevicollis, C. reinhardtii, T. pseudonana, and A. photosynthetic organisms and nonphotosynthetic ones, dif- anophagefferens; lack of C-4 methyl oxidase (ERG25) ferently to what commonly assumed. For example, the in M. brevicollis, C. reinhardtii, T. pseudonana, A. anopha- green algae O. tauri and C. reinhardtii do not appear to gefferens, and kinetoplastids; lack of C-3 ketoreductase have the typical land plants pathway because C-14 reduc- (ERG27) in all analyzed genomes apart from vertebrates tion is performed by an ERG24 ortholog and not by a typical and fungi; lack of ER-anchoring protein (ERG28) in land plants FK ortholog (table 1). Interestingly, the early- M. brevicollis, O. tauri, T. pseudonana, A. anophageffe- emerging land plant lineage Physcomitrella patens harbors rens, and kinetoplastids; lack of delta-8, delta-7 isomerase a set of enzymes that is identical to that of O. sativa and A. (ERG2 or EBP) in T. pseudonana and A. anophagefferens; thaliana (data not shown), indicating that what is known as and lack of C-5 desaturase (ERG3) in T. pseudonana and the typical land plant pathway can be generalized to the A. anophagefferens. Because these steps are essential, yet whole lineage. No orthologues of land plants SMO1 and unknown enzymes must exist that fulfill these missing func- SMO2 are present in the diatom T. pseudonana, the brown tions (to the possible exception of ERG28 that may be dis- algae A.anophagefferens, and the red algae C. reinhardtii, pensable; Gachotte et al. 2001). In particular, it is possible suggesting that these organisms may harbor a pathway that the enzyme performing C-3 ketoreduction in these or- whose order of steps is somehow different from that of ganisms will turn out to be the same of that acting in land land plants. On the other hand, the nonphotosynthetic pro- plants, which, as mentioned before, is still unknown. Im- tists N. gruberi and D. discoideum synthesize sterol via cy- portantly, we found orthologs of CPI1 in the organisms cloartenol and perform a C-14 reduction by an ortholog of where we predicted a pathway following a cycloartenol typical land plants FK (table 1). Thus, phylogenomic anal- route based on the active sites of their ERG7 (fig. 3 and ysis indicates that these different organisms harbor path- table 1). Other steps are not universally present and are dis- ways that display a mixture of features of the three cussed in detail in Supplementary Material 2 (Supplemen- canonical pathways from fungi, vertebrates, and land tary Material online). In general, our analysis allowed us plants. This leads to the question on what may have been to infer the presence/absence of orthologs and the corre- the ancestral pathway in eukaryotes. sponding enzymatic steps in all these organisms (table 1). However, the precise nature of the corresponding pathways is difficult to predict because, as we have seen in the canon- Did the LECA Already Make Sterols? ical pathways, the order of steps can be arranged in various ways. Nevertheless, we could point out a number of missing From the distribution of orthologs in table 1, we can steps that are not compatible with the sterols known to tentatively infer the set of enzymes that were already pres- be produced by a given organism, these steps being thus ent in LECA. Although the phylogenies of these enzymes performed by yet-to-discover enzymes (red cases with ex- are generally not fully resolved due to lack of signal, major Phylogenomics of Sterol Synthesis 371 FIG. 4.—Inference of ancestral sets of sterol enzymes. These are inferred based on a consensus phylogeny of eukaryotes included in our data set according to the most recent data (Burki et al. 2008; Hampl et al. 2009) rooted in between unikonts and bikonts. Lineages where the ability to synthesize sterols has been lost are indicated by red crosses. We inferred maximal and minimal ancestral enzyme contents at the three most ancient nodes (LECA, Unikonts, and Bikonts). Additional proteins in the maximal content with respect to the minimal content are indicated by question marks (see text for details). Putative duplications inferred in the lineage leading to the LECA are indicated by a star. Losses and gains of proteins are indicated only for the most basal branches by purple and blue arrows, respectively. 372 Desmond and Gribaldo eukaryotic groups are generally recovered. This allows in- homologous equivalent of opisthokonts ERG27 that has yet ferring ancestral sets of proteins by mapping their presence to be discovered in land plants as well as in all other lin- on a consensus eukaryotic phylogeny (Burki et al. 2008; eages (table 1). The place of the root of the eukaryotic tree Hampl et al. 2009) rooted in between unikonts and bikonts in between unikonts and bikonts has been recently ques- (fig. 4). For example, an enzyme whose orthologs are pres- tioned based on the fact that a member of Amoebozoa (uni- ent in fungi, amoeobozoans, plants, and kinetoplastids and konts) harbors bikont characters (Minge et al. 2009; Roger are well separated in the corresponding phylogeny (see, and Simpson 2009). If we consider that the root of the eu- e.g., ERG5 in Supplementary Material 1, Supplementary karyotic tree is presently unknown and we replace the con- Material online) lead us to infer the presence of this enzyme sensus phylogeny by a polytomy, we are forced to infer in the LECA and that its current absence in some phyla is a maximal set of enzymes back to the LECA in order to due to subsequent gene loss. We inferred the ancestral sets minimize convergent protein gains. This leads to the same of enzymes in the three most ancient nodes of the eukary- ancestral sets inferred with the unikont/bikont rooting. otic phylogeny (LECA, unikonts, and bikonts) (fig. 4) by Phylogenomic inference suggests that the LECA had minimizing HGT events among eukaryotic lineages. It a large potential of enzymatic functions and may have been has been put forward that HGT among eukaryotes may capable of synthesizing a diverse panel of sterols. In fact, be more frequent than generally assumed (Andersson among the set of enzymes inferred to have been already 2009). However, we could not observe clear cases of such present in the LECA, a few are nonorthologous, but per- HGT in our phylogenetic reconstructions, to one potential form redundant functions, such as C-14 reductases exception discussed below. (ERG24/FK), delta-8 delta-7 isomerases (ERG2/EBP), Phylogenetic analysis indicates that the duplications and delta-24 reductases (ERG4/DHCR24) (table 1 and that gave rise to the gene families corresponding to fig. 4). Our analysis does not allow inferring how the path- ERG4-ERG24-FK-DHCR7 likely occurred before the di- way was progressively assembled in the lineage leading to vergence of major eukaryotic phyla (Supplementary Mate- LECA. However, if LECA possessed a CPI1, this implies rial 1, Supplementary Material online), indicating that these that it used a cycloartenol route rather than a lanosterol enzymes were already present in the LECA (fig. 4). The route because no known organisms that possess a CPI1 syn- same may be said for the gene duplications giving rise thesize lanosterol. Consequently, the switch from a cycloar- to SMO1-SMO2, and to SMT1-SMT2, because the unikont tenol to a lanosterol route would have occurred at least D. discoideum has two gene copies of each family, similar twice independently in eukaryotic evolution (i.e., in opis- to land plants and some other bikonts, which cluster within thokonts and in kinetoplastids). An ancestral cycloartenol different orthologous groups (Supplementary Material 1, route is indeed consistent with previous hypotheses (Bloch Supplementary Material online) (fig. 4). However, phylo- 1991; Ourisson and Nakatani 1994). genetic reconstruction does not allow excluding that D. dis- Our analysis indicates that organisms that do not syn- coideum obtained its additional SMO1 and SMT2 via HGT thesize sterols have lost the pathway (fig. 4). Interestingly, from a bikont lineage (Supplementary Material 1, Supple- the loss of sterol synthesis in insects and nematodes appears mentary Material online). In this case, these two duplica- to be specific to these lineages because the placozoan Tri- tions would have occurred after the unikont/bikont choplax adhaerens has a full set of typical animal enzymes, bifurcation. Therefore, we indicated the occurrence of these except for DHCR7 (data not shown). Sometimes, a few en- duplications and the presence of the corresponding proteins zymes have been retained, possibly for the processing of in the LECA by question marks (fig. 4). The presence of sterols harvested from the environment, such as, for exam- a CPI1 ortholog in the unikont D. discoideum (table 1) ple, is the case of DHRC7, where a copy is present in the may indicate that the LECA had this enzyme, although non–sterol-producing oomycete P. ramorum (Supplemen- the poor resolution of the corresponding phylogenetic tree tary Material 1, Supplementary Material online). Our and the proximity with the ortholog from P. pacifica (Sup- analysis also indicates that the present-day distribution of plementary Material 1, Supplementary Material online) sterol enzymes in the different eukaryotic organisms ana- cannot exclude that it was acquired by HGT (therefore lyzed is the consequence of differential losses that probably a question mark in fig. 4). Interestingly, this analysis sug- accompanied specialization of the pathways along with gests that the LECA may have already harbored at least 19 eukaryotic diversification. However, appearance of novel enzymes (fig. 4). Even if D. discoideum obtained its addi- enzymes that are not yet known may have also taken place. tional SMO1, SMT2, and CPI1 copies via HGT from a bi- This prompts to explore further the sterol-synthesizing abil- kont lineage, a minimum set of 16 enzymes can still be ities of a wider sampling of eukaryotic diversity, all the inferred in the LECA (fig. 4). Among all enzymes, more so that the presence of a particular pathway in a given ERG27 only has specifically appeared in the Opisthokonta organism may not be generalized to the whole phylum it lineage. The opisthokont-specific ERG27 belongs to a gene belongs to. family including bacterial short-chain dehydrogenases as well as other distant eukaryotic hypothetical proteins (not shown) and may have thus been recruited in the ances- A Bacterial Origin? tor of Opisthokonta (fig. 4). However, because C-3 ketor- eduction would have been essential in the pathway inferred Consistent with previous findings, we found orthologs in the LECA, this opisthokont-specific ERG27 may have of the first two genes of the pathway (ERG1 and ERG7) in replaced an ancestral enzyme performing the same function G. obscuriglobus (Pearson et al. 2003), M. capsulatus in the LECA. It is thus possible that the LECA had the non- (Lamb et al. 2007), and S. aurantiaca (Bode et al. Phylogenomics of Sterol Synthesis 373 FIG. 5.—Genomic context of ERG1 and ERG7 genes in the genomes of the four sterol-synthesizing bacteria G. obscuriglobus, M. capsulatus, S. aurantiaca, and P. pacifica. 2003), and we report the presence of these two genes in et al. 2003). In general, previously published trees have P. pacifica.In G. obscuriglobus and M. capsulatus, the geno- been presented with only representatives of eukaryotes mic context of ERG1 and ERG7 is known to be conserved and these bacteria (Pearson et al. 2003). However, ERG1 and this has been taken as further indication for a common homologs are not only present in the four sterol-producing HGT involving these two genes (Pearson et al. 2003). How- bacteria but also have homologs in other bacteria within ever, the genome contexts in S. aurantiaca and P. pacifica a large family of monooxygenases. In a tree including are different (fig. 5). In P. pacifica, ERG1 is not close on the the whole family (fig. 6), the ERG1 homologues of the four genome to ERG7, but to the bacterial ERG7 homolog SHC. sterol-producing bacteria branch basally, but do not appear This is puzzling because SHC is not involved in the syn- to be more closely related to eukaryotic ERG1 than they are thesis of sterols and ERG1 is not involved in the synthesis to the other bacterial monooxygenases, and they share no of hopanoids. Interestingly, in S. aurantiaca, a gene coding specific sequence signature with eukaryotic ERG1 (fig. 7). for the enzyme responsible for the last step of the pathway Therefore, the hypothesis of an ancient HGT involving the leading from IPP to squalene (squalene synthase) is found ERG1 homologues of the four sterol-producing bacteria in between ERG1 and ERG7 (fig. 5), leading to a conserved and eukaryotic ERG1 remains open. Conversely, ERG7 context for three enzymes acting consecutively. and their bacterial SHC counterparts clearly separate into The presence of the first two enzymes of the pathway two clusters in the corresponding phylogeny (fig. 8). The in the few sterol-producing bacteria has been previously four bacterial ERG7 appear to be more closely related to discussed as resulting from an ancient HGT (Pearson their eukaryotic homologs (fig. 8), indicating a specific 374 Desmond and Gribaldo FIG. 6.—Maximum likelihood tree of ERG1 homologs. Only a subset of the most closely related bacterial monooxygenases are included. Following removal of ambiguously aligned regions, the final data set included 130 conserved positions. The tree was obtained by PHYML (Guindon and Gascuel 2003) as detailed in Materials and Methods. For clarity, only bootstrap values .50% are shown. evolutionary relationship, as previously put forward G. obscuriglobus, and P. pacifica all have also a typical (Pearson et al. 2003; Chen et al. 2007). Three bacterial bacterial SHC ortholog, whereas this is not the case for ERG7 branch basally with respect to their eukaryotic coun- S. aurantiaca (fig. 8). Thus, the possibility remains that terparts, and this may be interpreted in favor of a hypothesis the three bacterial ERG7 were obtained from eukaryotes, where eukaryotic ERG7 originated from bacteria. How- either via an ancient HGT before eukaryotic diversification ever, the ERG7 of S. aurantiaca emerges from within eu- or that fast evolution following transfer leads to an artificial karyotes (fig. 8), suggesting that it was obtained via HGT. grouping of the three bacterial ERG7 at the base of eukar- Moreover, along with their ERG7 copies, M. capsulatus, yotes. The SHC sequences of G. obscuriglobus and Phylogenomics of Sterol Synthesis 375 FIG. 7.—Alignment of ERG1 homologs including sequences of the four sterol-synthesizing bacteria (G. obscuriglobus, M. capsulatus, S. aurantiaca, and P. pacifica), of two other bacteria (Saccharopolyspora erythraea and Frankia alni), and of five representatives of eukaryotes (T. brucei, S. cerevisiae, H. sapiens, D. discoideum, and A. thaliana). P. pacifica are very divergent, in particular that of Gemma- relaxed the functional constraints of native SHC sequences. ta, which is split into three genes. It is possible that the pres- It would nevertheless be interesting to verify the activities ence of an ERG7 and therefore the synthesis of sterols have of these SHC orthologs. 376 Desmond and Gribaldo FIG. 8.—Maximum likelihood tree of ERG7 homologs. Following removal of ambiguously aligned regions, the final data set included 386 conserved positions. The tree was obtained by PHYML with the same criteria as detailed in Materials and Methods. Gemmata obscuriglobus and S. aurantiaca have no DHCR24 responsible for delta-24 reduction (fig. 1) is con- other homologs of the pathway, whereas M. capsulatus sistent with the 4alpha-methyl-5alpha-cholest-8(14)-en- has a homolog of ERG11, consistent with the fact that this 3beta-ol and 4,4-dimethyl-5alpha-cholest-8(14)-en-3beta-ol bacterium is able to produce more complex sterols (Lamb produced by M. capsulatus (Bouvier et al. 1976). Similar et al. 2007). Interestingly, we found that M. capsulatus also to M. capsulatus, P. pacifica also has a homolog of has a homolog of DHCR24 that branches robustly with Met- ERG11 (Supplementary Material 1, Supplementary Material azoans and choanoflagellates (Supplementary Material 1, online). ERG11 homologs are also present in Mycobacteria Supplementary Material online) and thus has likely origi- (Supplementary Material 1, Supplementary Material online). nated via HGT from these eukaryotes. The presence of It has been proposed that mycobacterial and M. capsulatus Phylogenomics of Sterol Synthesis 377 ERG11 originated via HGT from plants (Rez ˇen et al. 2004). vious action of ERG25 and ERG26. Interestingly, these ar- However, the ERG11 of M. capsulatus and mycobacteria guments also imply the presence of an ERG27. However, group with the ERG11 homolog of P. pacifica. Thus, the because P. pacifica has no homologs of fungi and verte- eventual HGT would have occurred in one of these bacteria brates ERG27, it is possible that it harbors a homolog of followed by HGT among them. Moreover, phylogenetic the still-missing enzyme providing an ERG27 function analysis does not indicate a closer proximity of plants in land plants and other eukaryotic lineages (table 1). If this ERG11 with these bacterial homologs (Supplementary Ma- is so, this putative ERG27 equivalent may display a closer terial 1, Supplementary Material online). By congruence relationship to eukaryotes (and be preferentially absent in with ERG7, it is possible that ERG11 was also acquired Opisthokonta and non–sterol-producing lineages) than via an ancient HGT from an ancestor of Eukaryotes or a spe- other bacteria. We extracted from our databank the ge- cific eukaryotic source that remains undetermined due to the nomes of eukaryotes lacking ERG27 plus 52 bacteria from high divergence of bacterial ERG11 sequences. all phyla, and we searched the whole genome of P. pacifica against it. Then, we filtered for those genes that had all these eukaryotes as first Blast hits within an e value ,1  10 . The Case of P. pacifica We found eight genes harboring such a phyletic pattern. In addition to ERG1, ERG7, and ERG11, P. pacifica Four of these correspond to ERG11, ERG7, and the two has six homologs of enzymes of the pathway catalyzing copies of FK, what makes a good positive control. The re- C-14 reduction (two copies of FK), C-4 demethylation maining four genes are a member of the P450 family (dis- (ERG25 and ERG26), delta-8–delta-7 isomerization tant relative of ERG 11), a hypothetical serine/threonine (ERG2), and cyclopropylsterol isomerization (CpI1), con- protein phosphatase (which is, however, also present in eu- sistent with the identification of very elaborated sterols in its karyotes that do not make sterol), an aspartatecarbamoyl close relative Nannocystis (Bode et al. 2003). The sterols transferase catalytic subunit (but phylogenetic analysis in- produced by P. pacifica have not yet been characterized. dicates that it is not closer to eukaryotes than to other bac- However, the combination of these enzymes leads us to in- teria), and a succinate-semialdehyde dehydrogenase fer production of 7,24-cholestadien-3beta-ol in P. pacifica, (NAD(P)þ) (which has only distant bacterial homologues). similar to some characterized sterols from Nannocystis The increase of the e value cut-off gave no other good can- (Bode et al. 2003). The presence of a CPI1 homolog in didates. The last protein (succinate-semialdehyde dehydro- P. pacifica is interesting because this is the first reported genase [NAD(P)þ]) is particularly interesting because its case of a bacterial homolog of this enzyme, which would annotated activity is congruent with an ERG27 function indicate a cycloartenol route. It is thus possible that P. pa- (C-3 ketoreduction), and further analysis on all eukaryotic cifica produces cycloartenol, although no cycloartenol has genomic data reveals that it is absent in eukaryotes that do been identified in its close relative Nannocystis (Bode et al. not make sterols. Moreover, this protein is also specifically 2003). Interestingly, we found that the ERG7 homolog of absent in eukaryotes that harbor an ERG27 copy, to the ex- P. pacifica has a V453, which is characteristic of a lanosterol ception of fungi (fig. 9). If this were the real ancestral route (fig. 3). It will be interesting to characterize the ERG27, it would mean that fungi have retained it and P. pacifica CPI1 homolog in order to assess its precise func- use it for a different function. In fact, S. cerevisiae mutants tion, as well as the types of sterols produced by P. pacifica. for ERG27 are sterol auxotrophs (Goffeau et al. 1996), in- The presence of CPI1 uniquely in P. pacifica among bac- dicating that this potential ERG27 analog (NP_011904) teria strongly points toward a recent acquisition via HGT cannot rescue the fungi ERG27 function. Interestingly, this from eukaryotes, although its sequence does not appear par- protein appears to function in the endoplasmic reticulum in ticularly close to any precise eukaryotic source (Supple- S. cerevisiae, precisely like ERG27, and mutants are defec- mentary Material 1, Supplementary Material online). tive in directing meiotic recombination events to homolo- Plesiocystis pacifica has two clear orthologs of FK and gous chromatids (Goffeau et al. 1996). It would be one ortholog of ERG2, which cluster within eukaryotic se- extremely interesting to verify the involvement of this en- quences and thus likely originated via HGT from eukar- zyme in the sterol pathway of nonfungal lineages lacking yotes (Supplementary Material 1, Supplementary a bona fide ERG27. Material online). Concerning the step of C-4 demethylation, P. pacifica has homologs of ERG25 and ERG26. P. pacif- ica ERG25 emerges from within the large protein family Discussion and Conclusions that includes eukaryotic ERG3/ERG25 (not shown) and therefore also very likely originated via HGT from eukar- The emergence of specific features in the lineage lead- yotes. As for ERG26, phylogenetic analysis does not show ing to eukaryotes is one of the most intriguing issues in the a clear orthology relationship between P. pacifica and its domain of early evolution. Phylogenomics is a recent and eukaryotic counterparts, although this may be due to its powerful approach to dissect the emergence and subsequent high divergence (Supplementary Material 1, Supplemen- evolution of cellular processes. However, its application to tary Material online). The ERG25 and ERG26 homologs eukaryotes is just becoming to emerge thanks to the recent from P. pacifica likely perform ERG25 and ERG26 func- availability of a sufficient sampling of complete genomes. tions, for two reasons: 1) the possibility that P. pacifica Recent analyses have investigated the emergence and evo- produces sterols as elaborated as Nannocystis, which would lution of different unique eukaryotic cellular structures require the action of an ERG25 and an ERG26, and 2) the (Bapteste et al. 2005; Eme et al. 2009; Field and Dacks presence of an ERG2 which in principle requires the pre- 2009). However, the emergence and evolution of 378 Desmond and Gribaldo FIG. 9.—Maximum likelihood tree of putative ERG27 analogues. Following removal of ambiguously aligned regions, the final data set included 483 conserved positions. The tree was obtained by PHYML with the same criteria as in Materials and Methods. eukaryotic-specific metabolic pathways is still poorly ex- (oxidases), ERG26 (NADP-dependent dehydrogenases), plored. Here we have reported a phylogenomics analysis and DHCR24 (bacterial FAD/FMN-containing dehydro- of one such eukaryotic-specific metabolic pathway. The genases). These have been likely recruited from preexisting presence of sterols is an essential characteristic of all eu- enzymes in parallel to the emergence of the sterol pathway karyotic membranes and thus presumably accompanied in the lineage leading to the LECA. By contrast, four en- the very emergence of this domain of life. In particular, zymes (ERG28 and EBP, ERG24, and ERG6) do not have the appearance of sterols may have allowed an increase flu- any bacterial homologue and have thus presumably arisen idity of eukaryotic membranes and thus possibly repre- in the eukaryotic lineage. The same is likely true for five sented an important step toward increasing cell additional enzymes: Three (ERG4, DHCR7, and ERG2) size. Additionally, the emergence of sterols may have pro- have very few bacterial homologs other than P. pacifica vided protection against oxidative stress (Galea and Brown and these clearly derive from HGT from eukaryotes 2009). (ERG4, Coxiella) and DHCR7 (Coxiella and Protochlamy- We have shown that eight enzymes of the sterol path- dia), ERG2 (Mycobacteria), and two (CPI1 and FK) have way belong to large gene families that include distant bac- only P. pacifica as bacterial homolog (Supplementary Ma- terial homologs (if the few sterol-making bacteria are not terial 1, Supplementary Material online). considered). These are ERG1 (monoxygenases), ERG7 We have highlighted that P. pacifica harbors the largest (SHCs), ERG5-ERG11 (cytochrome P450), ERG3-ERG25 reported set of homologs of eukaryotic sterol-synthesizing Phylogenomics of Sterol Synthesis 379 enzymes. It may be put forward that this myxobacterium is at ERG1, we think that the emergence of ERG7 in an ancestor the origin of these eukaryotic enzymes, which may lend sup- of eukaryotes was likely a key event in the assembly of the port to the hypothesis that Myxobacteria might have been the eukaryotic sterol pathway. Moreover, we tentatively infer potential partners of a syntrophic association at the origin of that the LECA harbored a cycloartenol route rather than eukaryotes (Lopez-Garcia and Moreira 1999). Indeed, P. pa- a lanosterol route, consistent with previous hypotheses cifica sequences appear to branch basally to eukaryotes, (Bloch 1991; Ourisson and Nakatani 1994). Importantly, which may be consistent with this bacterium being the source because the action of ERG7 and the subsequent steps in of eukaryotic homologs. However, for all enzymes previ- the sterol pathway are highly linked to oxygen and because ously mentioned that belong to large gene families, P. pacif- we inferred the presence of a complex sterol pathway in the ica sequences do not branch with their typical bacterial LECA, this suggests that this ancestor lived in a fully ox- homologs, indicating that they may not have arisen from ygenated environment. gene duplication of native genes followed by functional Our study allowed reconstructing the potential capaci- switch. Nevertheless, this may also be due to an acceleration ties for making specific sterols in the eukaryotic organisms of evolutionary rates that prevent assessing the real evolu- where the pathways have not yet been characterized. The tionary relationships of these P. pacifica enzymes. However, emerging enzymatic landscape highlighted in this study in- we have shown that it is possible that P. pacifica has acquired dicates that the differences in the characterized pathways its pathway for sterol synthesis via HGT from eukaryotes from fungi, land plants, and vertebrates, as well as those (either ancient or not). By the same reasoning, as proposed inferred in the noncharacterized organisms, arose by differ- previously (Pearson et al. 2003) it is possible that ERG7 has ent organization of a common set of conserved enzymes, been transferred to one of the few sterol-producing bacteria loss of specific enzymes, and the addition of a few novel and then among them. However, we put forward the possi- enzymes. In particular, the different order of steps does bility that ERG1 in these bacteria does not derive from HGT not seem to be linked to a cycloartenol or lanosterol route from eukaryotes. Consequently, it is possible that the ERG1 because, for example, kinetoplastids have a set of enzymes function was recruited into the sterol pathway of these bac- that is very similar to that of fungi, whereas they use a lan- teria from a native enzyme only after HGT from eukaryotes osterol route with an order of steps very similar to that of bringing along the ERG7 copy. If this were so, ERG1 homo- land plants (e.g., C-4 demethylation occurring through two logs would have been recruited independently in these bac- nonconsecutive steps; Lepesheva et al. 2004). Thus, the re- teria and eukaryotes to function with ERG7. Interestingly, organization of a similar order of steps observed in fungi phylogenetic analysis of the eukaryotic enzymes involved and animals may be specific to these two lineages and pos- in the pathway leading from IPP to squalene do not show sibly linked to the recruitment of their unique ERG27, any particular evolutionary relationships with the sterol- which replaced an ancestral analogous enzyme. Interest- producing bacteria (data not shown), supporting the hypoth- ingly, this would have occurred twice independently in esis that the capacity of making sterols in these bacteria arose fungi and animals because choanoflagellates appear to have via HGT from eukaryotes and that the eukaryotic sterol retained the ancestral C-3 ketoreductase and possibly an or- pathway originated in the LECA from the branching off der of steps similar to that of land plants. on a preexisting native pathway leading to squalene. In this Importantly, we showed that phylogenomics ap- regard, the evolutionary proximity of P. pacifica and eukary- proaches are a powerful tool to identify still-missing en- otic sterol enzymes should not be taken as support for an or- zymes, such is the case of the elusive ERG27 equivalent igin of eukaryotes from a symbiosis between myxobacteria in land plants and probably in other eukaryotic phyla. In this and Archaea because in this case the previous steps should respect, more experimental data on a wider sampling of eu- also show a similar pattern. Similarly, the proposal that karyotic diversity are surely necessary to test our predictions. actinobacteria are at the origin of the eukaryotic pathway Finally, because sterols are key markers for indicating (Cavalier-Smith 2006) is clearly weakened by the fact that the presence of specific eukaryotic lineages in the fossil re- the few homologs present in mycobacteria very likely derive cord, our study may be a useful reference for palaeogeo- from HGT from eukaryotes. More importantly, no actino- chemistry studies. bacteria have so far been shown to synthesize sterols (Rez ˇen et al. 2004) and these enzymes in mycobacteria are likely Supplementary Material used to process sterols from their eukaryotic hosts. Our analysis also indicates that the LECA had the po- Supplementary Material is available at Genome Biol- tential to make a wide array of different sterols and that sub- ogy and Evolution online (http://www.oxfordjurnals.org/ sequent evolution occurred through tinkering via our_journals/gbe/). differential enzyme losses and specializations in the various eukaryotic lineages in parallel with their divergence from Acknowledgments the LECA. This is consistent with the idea that the ability of synthesizing elaborated sterols would have paved the The authors declare that they have no competing in- way toward specific eukaryotic characters such as cell sig- terest. S.G. conceived the study. E.D. performed all anal- naling and multicellularity (Bloch 1991). Our analysis does yses. E.D. and S.G. wrote the manuscript, which was not allow us to predict the order of steps leading to the as- read, edited, and approved by both authors. 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Genome Biology and EvolutionOxford University Press

Published: Sep 10, 2009

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