A new membrane-bound cytochrome c works as an electron donor to the photosynthetic reaction center complex in the purple bacterium, Rhodovulum sulfidophilum

Abstract

A new type of membrane-bound cytochrome c was found in a marine purple photosynthetic bacterium, Rhodovulum sulfidophilum. This cytochrome c was significantly accumulated in cells growing under anaerobic photosynthetic conditions and showed an apparent molecular mass of ∼ 100 kDa when purified and analyzed by SDS-PAGE. The midpoint potential of this cytochrome c was 369 mV. Flash-induced kinetic measurements showed that this new cytochrome c can work as an electron donor to the photosynthetic reaction center. The gene coding for this cytochrome c was cloned and analyzed. The deduced molecular mass was nearly equal to 50 kDa. Its C-terminal heme-containing region showed the highest sequence identity to the water-soluble cytochrome c2, although its predicted secondary structure resembles that of cytochrome cy. Phylogenetic analyses suggested that this new cytochrome c has evolved from cytochrome c2. We, thus, propose its designation as cytochrome c 2m. Mutants lacking this cytochrome or cytochrome c2 showed the same growth rate as the wild type. However, a double mutant lacking both cytochrome c2 and c2m showed no growth under photosynthetic conditions. It was concluded that either the membrane-bound cytochrome c2m or the water-soluble cytochrome c2 work as a physiological electron carrier in the photosynthetic electron transfer pathway of Rvu. sulfidophilum. © 2007 by The American Society for Biochemistry and Molecular Biology, Inc.