Cyanobacterial Genes for the Cytochrome B6-F Complex: Sequence Homology with Plastid and Bacterial Genes but Divergence of Operon Structure

Abstract

The proton-translocating cytochrome b6-f complex of chloroplast and cyanobacterial thylakoid membranes catalyzes electron transfer from plastoquinone to plastocyanin and is thus required for noncyclic electron flow between the two photosystems (PS) as well as for cyclic flow around PS I. The complex consists of four essential polypeptides: cytochromes f and b6, the Rieske FeS protein, and a subunit IV of unknown function. In the cyanobacterium Anabaena these have apparent molecular weights of 31, 22.5, 22, and 16kd, respectively (1). The b6-f polypeptides occur in equal stoichiometric amounts and share considerable structural homology with components of the mitochondrial-type cytochrome b-cl complex (2). In photosynthetic eukaryotes the genes encoding three of these polypeptides reside in the chloroplast whereas the one for the Rieske FeS protein is located in the nucleus (3). In a purple photosynthetic bacterium the three genes for the essential polypeptides of the b-cl complex are clustered and cotranscribed (4).