Molecular basis for resistance to antimycin and diuron, Q-cycle inhibitors acting at the Qi site in the mitochondrial ubiquinol-cytochrome c reductase in Saccharomyces cerevisiae.

The bc1 complex of the mitochondrial respiratory chain transfers electrons from ubiquinol to cytochrome c oxidase. Cytochrome b, a transmembranous protein, is thought to form a transmembrane electron circuit, transferring electrons between two ubiquinone redox sites, (Qi) and (Qo), respectively, near the inner and outer sides of the inner mitochondrial membrane. Antimycin and diuron appear to block cytochrome b oxidation-reduction at one ubiquinone site, presumably Qi. The cytochrome b gene is carried by the organelle DNA. Yeast mutants resistant to antimycin and diuron have been previously isolated and mapped to specific loci of the cytochrome b gene. In the present work the mutated amino acid residues from nine antimycin- and three diuron-resistant mutants have been identified by sequencing the relevant segments of the resistant cytochrome b gene. The sequencings were performed by primer extension in the presence of dideoxynucleotides on total mitochondrial RNA preparations using reverse transcriptase. Regions of the cytochrome b protein affecting the inhibitor and putative quinone-binding sites have been defined.