Cryptic c 6-Like and c M Cytochromes of Cyanobacteria

Abstract

Cytochromes c6 are widely-distributed monoheme proteins that transfer electrons from the cytochrome b6f complex to photosystem I in cyanobacteria and certain eukaryotic algae. Cytochrome c6 has been replaced by plastocyanin in all known plant chloroplasts and in many cyanobacteria and algae, and was thought to be absent from plants until the discovery of chloroplast cytochrome c6-like proteins, now named cytochromes c6A. These have midpoint potentials substantially lower than those of typical cytochrome c6 or plastocyanin indicating that they could not effectively transfer electrons from the cytochrome b6f complex to photosystem I. Additional c6-like cytochromes have been discovered in cyanobacteria. The first of these was cytochrome cM, discovered in Synechocystis sp. PCC 6803 followed by discovery of a c6-like cytochrome encoded by the petJ2 gene in Synechococcus sp. PCC 7002. Subsequent phylogenetic analysis revealed that most, but not all, cyanobacteria carry several genes for putative c6-like cytochromes. Two deeply-branching clusters, designated cytochromes c6B and c6C, are distinct from either the classical c6 cytochromes, chloroplast cytochromes c6A, cyanobacterial cytochromes cM or c550, or bacterial cytochromes c555. Like cytochrome c6A, representative cytochromes c6C, c6B, and cM have been analyzed and found to have low midpoint potentials unfavorable for electron transfer from the cytochrome b6f complex. High-resolution structures have been solved for c6C and c6B cytochromes and key residues identified that govern their low midpoint potentials. However, despite these advances and the widespread distribution of these cryptic cytochromes among cyanobacteria, their functions have remained largely mysterious. Here we review the discovery of cyanobacterial c6-like cytochromes and current understanding of their properties and possible functions.