Natural resistance to inhibitors of the ubiquinol cytochrome c oxidoreductase of Rubrivivax gelatinosus: sequence and functional analysis of the cytochrome bc(1) complex.

Biochemical analyses of Rubrivivax gelatinosus membranes have revealed that the cytochrome bc(1) complex is highly resistant to classical inhibitors including myxothiazol, stigmatellin, and antimycin. This is the first report of a strain exhibiting resistance to inhibitors of both catalytic Q(0) and Q(i) sites. Because the resistance to cytochrome bc(1) inhibitors is primarily related to the cytochrome b primary structure, the petABC operon encoding the subunits of the cytochrome bc(1) complex of Rubrivivax gelatinosus was sequenced. In addition to homologies to the corresponding proteins from other organisms, the deduced amino acid sequence of the cytochrome b polypeptide shows (i) an E303V substitution in the highly conserved PEWY loop involved in quinol/stigmatellin binding, (ii) other substitutions that could be involved in resistance to cytochrome bc(1) inhibitors, and (iii) 14 residues instead of 13 between the histidines in helix IV that likely serve as the second axial ligand to the b(H) and b(L) hemes, respectively. These characteristics imply different functional properties of the cytochrome bc(1) complex of this bacterium. The consequences of these structural features for the resistance to inhibitors and for the properties of R. gelatinosus cytochrome bc(1) are discussed with reference to the structure and function of the cytochrome bc(1) complexes from other organisms.