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

Cell. 1980 Nov;22(1 Pt 1):277-89. doi: 10.1016/0092-8674(80)90175-0.


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.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Alleles
  • Chromosomes
  • Conjugation, Genetic*
  • DNA Transposable Elements*
  • Genetic Markers
  • Recombination, Genetic*
  • Saccharomyces cerevisiae / genetics*
  • Saccharomyces cerevisiae / physiology
  • Threonine / pharmacology


  • DNA Transposable Elements
  • Genetic Markers
  • Threonine