Mammalian mitochondrial mutants selected for resistance to the cytochromeb inhibitors HQNO or myxothiazol

Mouse LA9 cell lines were selected for increased resistance to either HQNO or myxothiazol, inhibitors of electron transport which bind to the mitochondrial cytochrome b protein. Two phenotypically distinguishable HQNO-resistant mutants were recovered while the myxothiazol-resistant isolates hada common phenotype. All three mutant phenotypes were transmitted cytoplasmically in cybrid crosses. Biochemical studies further established that for all three mutant types, resistance at the cellular level was paralleled by an increase in inhibitor resistance of mitochondrial succinate-cytochromec oxidoreductase, the respiratory complex containing cytochromeb. As with the previously described mitochondrial antimycinresistant mutant, the initial biochemical and genetic studies indicated that these mutations occur within the mitochondrial cytochromeb gene. This conclusion was strongly supported by the results of mtDNA restriction fragment analyses in which it was found that one HQNO-resistant mutant had undergone a small insertion or duplication in the apocytochromeb gene. Finally, all four mitochondrial cytochromeb mutants have been analyzed in both cell plating studies and succinate-cytochromec oxidoreductase assays to determine the pattern of cross-resistance to inhibitors of cytochromeb other than the one used for selection.