THE UBIQUINOL - CYTOCHROME C2 C OXIDOREDUCTASE OF CHROMATIUM-VINOSUM/

Chromatophore membranes isolated from the photosynthetic purple sulfur bacterium Chromatium vinosum exhibit a quinol: cytochrome c2(C) oxido reductase activity that is sensitive to two specific inhibitors of cyt ochrome bc1 complexes, antimycin A and myxothiazol. Digests of C vinos um DNA hybridize to probes constructed from portions of the pet(fbc) o perons that code for the cytochrome bc1 complexes of the photosyntheti c purple non-sulfur bacteria Rhodobacter capsulatus, Rhodobacter sphae roides and Rhodospirillum rubrum. Despite the fact that it has not yet proven possible to isolate a detergent-solubilized, purified cytochro me bc1 complex from C vinosum, these new results, when combined with s pectroscopic and kinetic data available from the literature, indicate that this purple sulfur bacterium contains a cytochrome bc1 complex si milar in structure to the well-characterized complexes found in purple non-sulfur bacteria. Equine cytochrome c and cytochromes c2 isolated from the purple non-sulfur bacteria Rhodospirillum rubrum and Rhodopse udomonas viridis all function as effective electron acceptors from the C vinosum cytochrome bc1 complex. In contrast, HiPIP (high-potential iron protein) isolated from two purple sulfur bacteria, C. vinosum and Chromatium tepidum, are reduced only at low rates by quinol in the pr esence of C vinosum membranes and their reduction is not inhibited by either antimycin A or myxothiazol. These observations support the hypo thesis that a protein structurally related to cytochrome c2, rather th an HiPIP, is the physiological electron acceptor for the C vinosum cyt ochrome bc1 complex. Although it has not yet proven possible to purify the putative C vinosum cytochrome c2 to homogeneity, we report here t hat digests of C vinosum DNA hybridize with probes constructed from po rtions of the cycA genes coding for cytochromes c2 from Rb. capsulatus , Rps. viridis and Rb. sphaeroides. These DNA hybridization results pr ovide further support for the hypothesis that a cytochrome c2-like pro tein is present in C vinosum.