Origins and early evolution of the mevalonate pathway of isoprenoid biosynthesis in the three domains of life.

Isoprenoids are a very diverse family of organic compounds widespread in the three domains of life. Although they are produced from the condensation of the same precursors in all organisms (isopentenyl pyrophosphate and dimethylallyl diphosphate), the evolutionary origin of their biosynthesis remains controversial. Two independent nonhomologous metabolic pathways are known: the mevalonate (MVA) pathway in eukaryotes and archaea and the methylerythritol phosphate (MEP) pathway in bacteria and several photosynthetic eukaryotes. The MVA pathway is also found in a few bacteria, what has been explained in previous works by recent acquisition by horizontal gene transfer (HGT) from eukaryotic or archaeal donors. To reconsider the question of the evolutionary origin of the MVA pathway, we have studied the origin and the evolution of the enzymes of this pathway using phylogenomic analyses upon a taxon-rich sequence database. On the one hand, our results confirm the conservation in archaea of an MVA pathway partially different from eukaryotes. This implies that each domain of life possesses a characteristic major isoprenoid biosynthesis pathway: the classical MVA pathway in eukaryotes, a modified MVA pathway in archaea, and the MEP pathway in bacteria. On the other hand, despite the identification of several HGT events, our analyses support that the MVA pathway was ancestral not only in archaea and eukaryotes but also in bacteria, in contradiction with previous claims that the presence of this pathway in bacteria was due to HGT from other domains. Therefore, the MVA pathway is likely an ancestral metabolic route in all the three domains of life, and hence, it was probably present in the last common ancestor of all organisms (the cenancestor). These findings open the possibility that the cenancestor had membranes containing isoprenoids.