The Evolution of the Cytochrome c(6) Family of Photosynthetic Electron Transfer Proteins

During photosynthesis, electrons are transferred between the cytochrome b(6)f complex and photosystem I. This is carried out by the protein plastocyanin in plant chloroplasts, or by either plastocyanin or cytochrome c(6) in many cyanobacteria and eukaryotic algal species. There are three further cytochrome c(6) homologs: cytochrome c(6A) in plants and green algae, and cytochromes c(6B) and c(6C) in cyanobacteria. The function of these proteins is unknown. Here, we present a comprehensive analysis of the evolutionary relationship between the members of the cytochrome c(6) family in photosynthetic organisms. Our phylogenetic analyses show that cytochromes c(6B) and c(6C) are likely to be orthologs that arose from a duplication of cytochrome c(6), but that there is no evidence for separate origins for cytochromes c(6B) and c(6C). We therefore propose renaming cytochrome c(6C) as cytochrome c(6B). We show that cytochrome c(6A) is likely to have arisen from cytochrome c(6B) rather than by an independent duplication of cytochrome c(6), and present evidence for an independent origin of a protein with some of the features of cytochrome c(6A) in peridinin dinoflagellates. We conclude with a new comprehensive model of the evolution of the cytochrome c(6) family which is an integral part of understanding the function of the enigmatic cytochrome c(6) homologs.