Microbial ubiquinones: multiple roles in respiration, gene regulation and oxidative stress management

Spectacular advances have been made in recent pears in our understanding of the structure and function of the membrane-bound protein complexes in respiratory and photosynthetic electron-transport chains. Perhaps as a result of this focus of attention, there is a temptation to regard quinones as a passive pool of redox carriers whose only function is to shuttle reducing equivalents between more 'interesting' protein complexes. However, it is now clear that quinones play additional vital roles in management of oxidative stress and, apparently, gene regulation. The purpose of this review is to give an overview of the different roles quinones fulfil in the prokaryotic cell with emphasis on Escherichia coli, although recent advances in yeast are also covered. The ubiquinone biosynthetic pathway has now been elucidated in both prokaryotes and lower eukaryotes, and a comparison of the numerous enzymes involved in these pathways is included. The literature cited is necessarily selective, with an emphasis on recent work; reviews have been used to cover early work.