The evolution of the cytochrome c6 family of photosynthetic electron transfer proteins

Abstract

During photosynthesis, electrons are transferred between the cytochrome b6f complex and photosystem I. This is carried out by the protein plastocyanin in plant chloroplasts. In contrast, electron transfer can be carried out by either plastocyanin or cytochrome c6 in many cyanobacteria and eukaryotic algal species. There are three further cytochrome c6 homologues: cytochrome c6A in plants and green algae, and cytochromes c6B and c6C 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 c6 family in photosynthetic organisms. Our phylogenetic analyses show that cytochrome c6B and cytochrome c6C are likely to be orthologues that arose from a duplication of cytochrome c6, but that there is no evidence for separate origins for cytochrome c6B and c6C. We therefore propose re-naming cytochrome c6C as cytochrome c6B. We show that cytochrome c6A is likely to have arisen from cytochrome c6B rather than by an independent duplication of cytochrome c6, and present evidence for an independent origin of a protein with some of the features of cytochrome c6A in peridinin dinoflagellates. We conclude with a new comprehensive model of the evolution of the cytochrome c6 family which is an integral part of understanding the function of the enigmatic cytochrome c6 homologues.