A transcription unit petCB from Chlorobium limicola is described. The leading gene petC codes for a Rieske FeS-protein of 19.04 kDa with 181 amino acid residues. The following gene petB codes for a cytochrome b of 47.48 kDa with 428 amino acid residues. The transcription unit lacks a third gene pet-A for cytochrome c 1 or-f, which is found in the fbc-operons of gram-negative bacteria. In the derived amino acid sequence for the Rieske FeS-protein the four cysteines and the 2 histidines are conserved in the peptides binding the 2Fe2S-cluster, although the redox potential of the cluster is about 150 mV more negative in Chlorobium. The gene for cytochrome b includes the coding region for an N-terminal, positively charged extension which is typical for Chlorobium. The gene is not split into two parts for cytochrome b 6 and subunit IV. However, a fourteenth amino acid between the two histidines in the fourth, putative transmembrane helix, and the lack of an eighth transmembrane helix at the C-terminus, among other features, clearly resemble the cytochrome b 6 f-complexes. Therefore, the separation into b 6 f- and bc 1-type complexes during evolution must have occurred before the split of the gene.