Institut Pasteur, Unité de Biologie Moléculaire du gène chez les Extrêmophiles, Paris, France.
Genome Biol Evol. 2009 Sep 10;1:364-81. doi: 10.1093/gbe/evp036.
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.
从广泛的真核生物多样性样本中获得完整基因组，使得人们能够应用系统发生基因组学方法来研究独特的真核细胞结构的起源和进化，但这些方法在研究独特的真核代谢途径方面的应用仍然很少。甾醇就是一个很好的例子，因为它们是真核生物膜的一个重要特征。甾醇途径在脊椎动物、真菌和陆生植物中已经得到了很好的剖析。然而，尽管在其他真核生物谱系中已经鉴定出不同类型的甾醇，但它们的途径尚未完全阐明。我们对甾醇途径的酶在大量真核生物谱系中的分类分布和系统发育进行了广泛分析。这使我们能够初步确定在尚未对甾醇途径进行特征描述的生物体中的甾醇途径特征，并指出了一些有待发现酶的步骤。我们还推断，最后一个真核生物共同祖先已经拥有了大量合成甾醇的酶，随后在真核生物树中的进化是通过修补来实现的，主要是通过基因丢失。我们强调了粘细菌 Plesiocystis pacifica 具有很高的甾醇合成能力，并支持这样的假说，即少数拥有甾醇途径同源物的细菌可能是通过从真核生物横向基因转移获得这些途径的。最后，我们提出了一个可能的候选酶，用于在陆地植物中进行难以捉摸的 C-3 酮还原（ERG27 等效物），可能在其他真核生物门中也是如此。
