Assembly of the mitochondrial membrane system XIX

Summary

Nineteen mutants of S. cerevisiae exhibiting a double deficiency in cytochrome oxidase and coenzyme QH₂-cytochrome c reductase (also cytochrome b deficient) have been studied. The mutants have been crossed to a set of ρ ⁻ tester strains with different segments of mitochondrial DNA. The mutants have also been crossed to mit⁻ testers with defined genetic lesions. In addition, crosses were performed with a respiratory competent strain to ascertain whether mitotic and meiotic segregants could be isolated with only one of the two enzymatic deficiencies.

The ρ ₋ testers allowed the doubly deficient mutants to be separated into two classes. Mutants in class 1 were not restored by any of the ρ ₋ testers and appeared to have separate mutations, one in cytochrome oxidase and the other in cytochrome b. Mutants in class 2 were restored by a set of ρ ₋ clones whose retained segments of mitochondrial DNA contained the cytochrome b but not the cytochrome oxidase loci. These appeared to behave as single hit mutations. Further studies, however, indicated that both class 1 and class 2 mutants carried separate mutations in two different loci. Mitotic and meiotic segregants with a single enzymatic deficiency could be isolated. In a number of strains, the mutations were mapped in known cytochrome oxidase and cytochrome b loci. The apparent discrepancy of the ρ ₋ tests for the class 2 mutants was shown to be probably due to a high unstability in one of the mutations.

It has been concluded that all the doubly deficient strains carry two mutations in previously described cytochrome oxidase and cytochrome b loci. This conclusion argues against the existence of a single gene on mitochondrial DNA that controls the biosynthesis of the two respiratory enzymes.