A high-potential soluble cytochrome c-551 from the purple phototrophic bacterium Chromatium vinosum is homologous to cytochrome c8 from denitrifying pseudomonads

A minor cytochrome c-551 component of Chromatium vinosum was previously found to efficiently couple electron transfer between the cytochrome bc(1) complex and the photosynthetic reaction center. We have now determined the amino acid sequence of this cytochrome c-551 and find that it is homologous to cytochrome c(8) (formerly called Pseudomonas cytochrome c-551). It is most similar to Methylophilus methylotrophus, Rhodocyclus tenuis, and Azotobacter vinelandii cytochromes c(8) (respectively, 57%, 52% and 51%). The C. vinosum cytochrome c(8) has a single residue insertion relative to Pseudomonas and Azotobacter cytochromes c(8). It has fewer charged residues than its homologs and is essentially neutral, which may explain why it is less soluble than the others. The cytochromes c(8) are only very distantly related to the cytochromes c(2) found in other species of purple bacteria which are much larger in size and which usually mediate electron transfer between the cytochrome bc(1) complex and the reaction center. The photosynthetic pathway in Chromatium thus appears to be radically different from that in purple non-sulfur bacteria