Analysis of microsatellite mutations in the mitochondrial DNA of Saccharomyces cerevisiae

In the nuclear genome of Saccharomyces cerevisiae, simple, repetitive DNA s equences (microsatellites) mutate at rates much higher than nonrepetitive s equences. Most of these mutations are deletions or additions of repeat unit s. The yeast mitochondrial genome also contains many microsatellites. To ex amine the stability of these sequences, we constructed a reporter gene (arg 8(m)) containing out-of-frame insertions of either poly(AT) or poly(CT) tra cts within the coding sequence. Yeast strains with this reporter gene inser ted within the mitochondrial genome were constructed. Using these strains, we showed that poly(CT) tracts were considerably less stable than poly(Ai) tracts and that alterations usually involved deletions rather than addition s of repeat units. In contrast, in the nuclear genome, poly(CT) and poly(AT ) tracts had similar stabilities, and alterations usually involved addition s rather than deletions. Poly(CT) tracts were more stable in the mitochondr ia of diploid cells than in haploids, In addition, an msh1 mutation destabi lized poly(GT) tracts in the mitochondrial genome.