Natural Resistance to Inhibitors of the Ubiquinol Cytochrome c Oxidoreductase of Rubrivivax gelatinosus : Sequence and Functional Analysis of the Cytochrome bc 1 Complex

Abstract

ABSTRACT 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.