The ubiquinol:cytochrome [formula omitted] oxidoreductase of Chromatium vinosum

Abstract

Chromatophore membranes isolated from the photosynthetic purple sulfur bacterium Chromatium vinosum exhibit a quinol:cytochrome c 2( c) oxidoreductase activity that is sensitive to two specific inhibitors of cytochrome bc 1 complexes, antimycin A and myxothiazol. Digests of C. vinosum DNA hybridize to probes constructed from portions of the pet(fbc) operons that code for the cytochrome bc 1 complexes of the photosynthetic purple non-sulfur bacteria Rhodobacter capsulatus, Rhodobacter sphaeroides and Rhodospirillum rubrum. Despite the fact that it has not yet proven possible to isolate a detergent-solubilized, purified cytochrome bc 1 complex from C. vinosum, these new results, when combined with spectroscopic and kinetic data available from the literature, indicate that this purple sulfur bacterium contains a cytochrome bc 1 complex similar in structure to the well-characterized complexes found in purple non-sulfur bacteria. Equine cytochrome c and cytochromes c 2 isolated from the purple non-sulfur bacteria Rhodospirillum rubrum and Rhodopseudomonas viridis all function as effective electron acceptors from the C. vinosum cytochrome bc 1 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 presence of C. vinosum membranes and their reduction is not inhibited by either antimycin A or myxothiazol. These observations support the hypothesis that a protein structurally related to cytochrome c 2, rather than HiPIP, is the physiological electron acceptor for the C. vinosum cytochrome bc 1 complex. Although it has not yet proven possible to purify the putative C. vinosum cytochrome c 2 to homogeneity, we report here that digests of C. vinosum DNA hybridize with probes constructed from portions of the cycA genes coding for cytochromes c 2 from Rb. capsulatus, Rps. viridis and Rb. sphaeroides. These DNA hybridization results provide further support for the hypothesis that a cytochrome c 2-like protein is present in C. vinosum.