Mating type switches in the yeast Saccharomyces cerevisiae occur by transposition of a replica of the "source" unexpressed loci HML and HMR to the mating type locus (MAT). The incoming information replaces previously expressed DNA, resulting in an interconversion of MAT alleles. A strain of genotype HML alpha/HML alpha MAT alpha/mata-missense HMR alpha/hmra-nonsense HO/ho generates cells with the genotype HML alpha/HML alpha MAT alpha/MAT a HMR alpha/hmra-nonsense HO/ho; that is, wild-type MATa+ recombinants are produced efficiently by a strain in which the incoming a information and the resident mata allele bear different mutations. Production of the wild-type MATa recombinants requires the homothallism (switching) function, and the incoming a information and the resident mata allele must bear different mutations. This result is consistent with the formation of a heteroduplex between the incoming and the outgoing DNA at MAT. Thus a process of unidirectional gene conversion as a mechanism for mating type gene transposition is favored. A molecular model based on a single-strand transfer is proposed. Results also favor the idea that the direction of switching is controlled by cell's mating phenotype rather than by the genetic content of MAT.

• PMID: 6253082
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Journal Article | Research Support, U.S. Gov't, Non-P.H.S. | Research Support, U.S. Gov't, P.H.S.

Authors

Klar AJ, McIndoo J, Strathern JN, Hicks JB

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