Nuclear mutations affecting the stability of the mitochondrial genome in S. cerevisiae

We have studied a pleiotropic mutation petD in S. cerevisiae which both confers the inability to grow on glycerol (Gly-) and greatly increases the frequency of cytoplasmic petites (Het). The first phenotype, Gly-, is recessive, whereas the second, Het, is dominant. Genetic and biochemical analysis showed that the majority of the petites in petD strains are not of the rho degree type (completely lacking mit-DNA), but of the rho- type (containing partially deleted mit-DNA). This finding and the fact that the phenotype Het is dominant argue in favour of the involvement of the petD product in the excision process of the mit-DNA. Another nuclear mutation, mod, was shown to exhibit a dominant epistasy with respect to the Het phenotype of the mutation petD. Two types of Gly+ revertants from petD mutants were isolated: rpa revertants, which restore completely the wild-type phenotype, and rpb revertants, which restore only the growth on glycerol, but still allow the production of high frequencies of cytoplasmic petites. Thus the mutations mod and rpb permit the genetic uncoupling of two phenotypes induced by the mutation petD.