Cytochrome oxidase and the cta operon of cyanobacteria

The discovery of aa(3)-type cytochrome-e oxidase in cyanobacteria is briefly reviewed, starting with earlier experiments on whole cells, intact spheroplasts and crude membrane preparations, eventually culminating in the isolation and purification of the enzyme from Anacystis and Synechocystis. The most prominent feature of cyanobacterial cytochrome oxidase as identified in 25 different strains and species, is its striking similarity to the mitochondrial enzyme. The purified preparation, however, contains four major and a minor subunit only. In Synechocystis these proteins are encoded by a cta operon comprising mitochondria-like ctaC-D-E genes (encoding subunits II, I and III), an orf4 whose putative gene product (44 amino acids) is not related to other small bacterial subunits IV, and a ctaF gene which may code for subunit IV (187 amino acids, one or two transmembrane helices). This subunit is also detected in the isolated enzyme by specific immunological cross-reaction towards nuclear-encoded subunits IV (> 50% amino acid similarity with yeast). Subunit IV of the cyanobacterial cytochrome-e oxidase exhibits amino acid domains very similar to adenylate-binding proteins and may thus explain the regulation of the enzyme by ATP and ADP. The liposomal enzyme also pumps protons with a H+/e(-) ratio close to 1. The aa(3)-type cytochrome oxidase of cyanobacteria can reside in both plasma and thylakoid membranes relative shares critically depending on the species and on growth conditions. In micro-aerophilic growth conditions heme A can be partly replaced by heme O without any change in the function of the enzyme which is always a cytochrome-c, and not a quinol, oxidase.