Cryptic C(6)- Like And C(M) Cytochromes Of Cyanobacteria

Cytochromes c(6) are widely-distributed monoheme proteins that transfer electrons from the cytochrome b(6)f complex to photosystem I in cyanobacteria and certain eukaryotic algae. Cytochrome c(6) has been replaced by plastocyanin in all known plant chloroplasts and in many cyanobacteria and algae, and was thought to be absent from plants until the discovery of chloroplast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 c(6)-like proteins, now named cytochromes c(6A). These have midpoint potentials substantially lower than those of typical cytochrome c(6) or plastocyanin indicating that they could not effectively transfer electrons from the cytochrome b(6)f complex to photosystem I. Additional c(6)-like cytochromes have been discovered in cyanobacteria. The first of these was cytochrome c(M), discovered in Synechocystis sp. PCC 6803 followed by discovery of a c(6)-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 c(6)-like cytochromes. Two deeply-branching clusters, designated cytochromes c(6B) and c(6C), are distinct from either the classical c(6) cytochromes, chloroplast cytochromes c(6A), cyanobacterial cytochromes c(M) or c(550), or bacterial cytochromes c(555). Like cytochrome c(6A), representative cytochromes c(6C), c(6B), and cM have been analyzed and found to have low midpoint potentials unfavorable for electron transfer from the cytochrome b(6)f complex. High-resolution structures have been solved for c(6C) and c(6B) 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 c(6)-like cytochromes and current understanding of their properties and possible functions.