Homothallic conversions of yeast mating-type genes occur by intrachromosomal recombination.

The switching of yeast mating-type alleles involves a transposition of a copy of a sequence from HML or HMR to replace the sequences at MAT. Using diploid strains of yeast we have discovered that about 1% of the homothalic conversions of MAT alleles are accompanied by large intrachromosomal rearrangements. These rearrangements are highly specific fusions of part of MAT either with HMR (to produce a deficiency ring chromosome). We conclude that the mechanism of MAT conversions involves a highly specific pairing between the homologous sequences at MAT and the donor genes HML or HMR followed by a specialized gene conversion event, in which the original allele is replaced by a sequence copied from HMR or HML. At about a 1% frequency conversion of the MAT locus is accompanied by a reciprocal recombination event that results in an intrachromosomal deletion. This same preferential pairing is reflected in a high frequency (> 10(-3)) of site-specific mitotic recombination between MAT alleles on differenat chromosomes. A gene conversion model also allows us to explain the "illegal" transpositions of MAT alleles to HMR or HML that occur when normal excision of MAT is prevented.

Duke Scholars

Author John Henry McCusker Molecular Genetics and Microbiology