Phylogeny of Rieske/cytb complexes with a special focus on the haloarchaeal enzymes

Abstract

Rieske/cytochrome b(Rieske/cytb) complexes are proton pumping quinol oxidases that are present in most bacteria andArchaea. The phylogenyof their subunits followsclosely the16S-rRNAphylogeny, indicating that chemiosmoticcouplingwas alreadypresent in the last universal common ancestor of Archaea and bacteria. Haloarchaea are the only organisms found so far that acquired Rieske/cytb complexes via interdomain lateral gene transfer. They encode two Rieske/cytb complexes in their genomes; one of them is found in genetic context with nitrate reductase genes and has its closest relativesamong Actinobacteria and the Thermus/Deinococcus group. It is likely to function in nitrate respiration. The second Rieske/cytb complex of Haloarchaea features a split cytochrome b sequence as do Cyanobacteria, chloroplasts, Heliobacteria, and Bacilli. It seems that Haloarchaea acquired this complex from an ancestor of the above-mentioned phyla. Its involvement in the bioenergetic reaction chains of Haloarchaea is unknown. We present arguments in favor of the hypothesis that the ancestor of Haloarchaea, which relied on a highly specialized bioenergetic metabolism, that is, methanogenesis, and was devoid of quinones and most enzymes of anaerobic or aerobic bioenergetic reaction chains, integrated laterally transferred genes into its genome to respond to a change in environmental conditions that made methanogenesis unfavorable. © 2012 The Author(s).