Assembly of the mitochondrial membrane system XVIII

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Abstract
Fourteen cytoplasmic mutants of Saccharomyces cerevisiae with a specific deficiency of cytochrome b have been studied. The mutations have been shown to occur in two separate genetic loci, COB 1 and COB 2. These loci can be distinguished by mit−xmit− crosses. Pairwise crosses of cytochrome b mutants belonging to different loci yield 4–6% wild type recombinants corresponding to recombinational frequencies of 8–12%. In intra-locus crosses, the recombinational frequencies range from 1% to less than 0.01%. The two loci can also be distinguished by mit− × ρ − crosses. Twenty ρ − testers have been isolated of which ten preferentially restore mutations in COB 1 and ten others in COB 2. The COB 1 and COB 2 loci have been localized on mitochondrial DNA between the two antibiotic resistance loci OLI 1 and OLI 2 in the order OLI 2-COB 2-COB 1-OLI 1. The results of mit−xmit− and mit− × ρ − crosses have also been used to map the cytochrome b mutations relative to each other. The maps obtained by the two independent methods are in good agreement. Mutations in COB 1 have been found to be linked to the OLI1 locus in some but not in other strains of S. cerevisiae. This evidence suggests that there may be a spacer region between the two loci whose length varies from strain to strain. Two mutations in COB 2 have been found to cause a loss of a mitochondrial translation product corresponding to the cytochrome b apoprotein. Instead of the wild type protein the mutants have a new low-molecular weight product which is probably a fragment of cytochrome b. The fact that the mutations revert suggests that they are nonsense mutations in the structural gene of cytochrome b.