Organization and structure of the genes for the cytochromeb/c 1 complex in purple photosynthetic bacteria. A phylogenetic study describing the homology of theb/c 1 subunits between prokaryotes, mitochondria, and chloroplasts

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Abstract
The cytochromeb/c 1 complex is an ubiquitous energy transducing enzyme, part of the electron transport chain of prokaryotes, mitochondria, and chloroplasts (b 6/f). In the ancient purple photosynthetic bacteria, theb/c 1 complex occupies a central metabolic role, being part of their photosynthetic and respiratory electron transport chain. InRhodobacter the three subunits of theb/c 1 complex are FeS protein, cytochromeb, and cytochromec 1, and they are encoded by a constitutively expressed operon namedfbc. The organization of the genes for the cytochromeb/c 1 complex, the modality of transcription, and the biogenesis of the encoded polypeptides will be described. TheRhodobacter species used to isolate thefbc genes, previously reported asR. sphaeroides was identified asR. capsulatus. Further biochemical characterization of the prokaryoticb/c 1 complex indicated that the three polypeptides encoded by thefbc operon comprise the entire catalytic structure: ubiquinol-cytochrome-c reductase. The amino acid sequences of the threeb/c 1 subunits from the photosynthetic bacteriumRhodobacter capsulatus were compared with the corresponding sequences from yeast mitochondria and spinach chloroplasts. The high homology found between the sequences of all three redox polypeptides fromR. capsulatus and yeast mitochondria (cytochromeb 41%, FeS protein 46%, cytochromec 1 31%) provided further evidence that mitochondria arose from the phylogenetic line of purple bacteria. The structure of cytochromeb also exhibited considerable homology to chloroplast cytochromeb 6 plus subunit IV (26%). The amino acid sequence of the Rieske FeS protein fromR. capsulatus and chloroplasts were found to be conserved only in the C-terminal part (14% total identity), whereas the homology between cytochromec 1 and cytochromef is very weak (12%), despite similar topology of the two polypeptides. Analysis of the homology suggested that the catalytic sites quinol oxidase (Qo) and quinone reductase (Qi) arose monophonetically, whereas cytochromec and plastocyanin reductase sites are not homologous and could derive from diverse ancestral genes by convergent evolution.