Mitochondrial Assembly in Respiration-deficient Mutants of Saccharomyces cerevisiae

Eighteen respiration-deficient yeast mutants were isolated from wild type cells after mutagenesis with ethylmethane sulfonate. Segregation analysis indicated that each mutant phenotype resulted from the mutation of a single nuclear gene. All mutants except one efficiently retained functional mitochondrial DNA as measured by ability to complement mitochondrial DNA-less tester strains. When the mutants were crossed pairwise and the resulting zygotes checked for functional respiration, the 18 strains could be classified into seven complementation groups. Three of these groups were characterized by a specific loss of cytochrome aa3. Another group included pleiotropic mutants which lacked cytochrome aa3, b, and c1, but not oligomycin-sensitive mitochondrial ATPase. Still another group was represented by a pleiotropic mutant that was not only deficient in cytochromes aa3, b, and c1 but in mitochondrial ATPase as well. The two remaining complementation groups included mutants that were completely deficient in cytochrome aa3 and partially deficient in other mitochondrial constituents. All of the mutants still exhibited mitochondrial protein synthesis. However, when the proteins synthesized by the mutant mitochondria in vivo were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis, they usually lacked several species made by wild type mitochondria. These results show that nuclear mutations can affect either the synthesis of mitochondrial translation products or their integration into the mitochondrial inner membrane. Since mitochondrial protein synthesis can still be detected in single gene mutants with multiple mitochondrial deficiencies, we suggest that some nuclear genes may code for mitochondrial organizer proteins that control the correct assembly of the mitochondrial inner membrane.