Dependence on mating type for the overproduction of iso-2-cytochrome c in the yeast mutant CYC7-H2.

The CYC7-H2 mutation causes an approximately 20-fold overproduction of iso-2-cytochrome c in a and alpha haploid strains of the yeast Saccharomyces cerevisiae due to an alteration in the nontranslated regulatory region that is presumably contiguous with the structural region. In this investigation, we demonstrated that heterozygosity at the mating type locus, a/alpha or a/a/alpha/alpha, prevents expression of the overproduction, while homozygosity, a/a and alpha/alpha, and hemizygosity, a/0 and alpha/0, allow full expression of the CYC7-H2 mutation, equivalent to the expression observed in a and alpha haploid strains. There is no decrease in the overproduction of iso-2-cytochrome c in a/alpha diploid strains containing either of the other two similar mutations, CYC7-H1 and CYC7-H3. It appears as if active expression of one or another of the mating-type alleles is required for the overproduction of iso-2-cytochrome c in CYC7-H2 mutants.