  Ubiquinone is an important component of the electron transfer chains in
proteobacteria and eukaryotes. The biosynthesis of ubiquinone requires multiple
steps, most of which are common to bacteria and eukaryotes. Whereas the enzymes
of the mitochondrial pathway that produces ubiquinone are highly similar across
eukaryotes, recent results point to a rather high diversity of pathways in
bacteria. This review focuses on ubiquinone in bacteria, highlighting newly
discovered functions and detailing the proteins that are known to participate
to its biosynthetic pathways. Novel results showing that ubiquinone can be
produced by a pathway independent of dioxygen suggest that ubiquinone may
participate to anaerobiosis, in addition to its well established role for
aerobiosis. We also discuss the supramolecular organization of ubiquinone
biosynthesis proteins and we summarize the current understanding of the
evolution of the ubiquinone pathways relative to those of other isoprenoid
quinones like menaquinone and plastoquinone.
