Combining Inhibitor Resistance-conferring Mutations in Cytochrome b Creates Conditional Synthetic Lethality in Saccharomyces cerevisiae.

The mitochondrial cytochrome bc1 complex is an essential respiratory enzyme in oxygen-utilizing eukaryotic cells. Its core subunit, cytochrome b, contains two sites, center P and center N, that participate in the electron transfer activity of the bc1 complex and that can be blocked by specific inhibitors. In yeast, there are various point mutations that confer inhibitor resistance at center P or center N. However, there are no yeast strains in which the bc1 complex is resistant to both center P and center N inhibitors. We attempted to create such strains by crossing yeast strains with inhibitor-resistant mutations at center P with yeast strains with inhibitor-resistant mutations at center N. Characterization of yeast colonies emerging from the cross revealed that there were multiple colonies resistant against either inhibitor alone but that the mutational changes were ineffective when combined and when the yeast were grown in the presence of both inhibitors. Inhibitor titrations of bc1 complex activities in mitochondrial membranes from the various yeast mutants showed that a mutation that confers resistance to an inhibitor at center P, when combined with a mutation that confers resistance to an inhibitor at center N, eliminates or markedly decreases the resistance conferred by the center N mutation. These results indicate that there is a pathway for structural communication between the two active sites of cytochrome b and open new possibilities for the utilization of center N as a potential drug target.