Multiple Rieske genes in prokaryotes: exchangeable Rieske subunits in the cytochrome bc1-complex of Rubrivivax gelatinosus

Bacterial cytochrome bc1-complex encoded by the petABC operon consists of three subunits, the Rieske iron-sulphur protein, the b-type cytochrome, and the c1-type cytochrome. Disruption of the petA gene of Rubrivivax gelatinosus is not lethal under photosynthetic growth conditions. However, deletion of both petA and petB results in a photosynthesis-deficient strain, suggesting the presence of a second gene encoding a Rieske protein and rescuing a functional cytochrome bc1-complex in the PETA1 mutant. The corresponding petA2 gene was identified and the PETA2 mutant could also grow under photosynthetic conditions. The double mutant PETA12, however, was unable to grow photosynthetically. The presence of a photo-induced cyclic electron transfer was tested by monitoring the kinetics of cytochrome photo-oxidation on intact cells; the data confirm the capacity of petA2 to replace petA1 in the bc1-complex to support photosynthesis. Soluble forms of both PetA1 and PetA2 Rieske proteins were purified from Escherichia coli and found to contain correctly inserted [2Fe-2S] clusters. Electron paramagnetic resonance (EPR) spectroscopy and midpoint potential measurements showed typical [2Fe-2S] signals and E(m) values of +275 mV for both Rieske proteins. The high amino acid sequence similarity and the obtained midpoint potential values argue for a functional role of these proteins in the cytochrome bc1-complex. The presence of duplicated Rieske genes is not restricted to R. gelatinosus. Phylogenetic trees of Rieske genes from Rubrivivax and other proteobacteria as well as from cyanobacteria were reconstructed. On the basis of the phylogenetic analyses, differing evolutionary origins of duplicated Rieske genes in proteo- and cyanobacteria are proposed.