A Mutation That Permits the Expression of Normally Silent Copies of Mating-Type Information in SACCHAROMYCES CEREVISIAE

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Studies of heterothallic and homothallic strains of Saccharomyces cerevisiae have led to the suggestion that mating-type information is located at three distinct sites on chromosome 3, although only information at the mating-type (MAT) locus is expressed (Hicks, Strathern and Herskowitz, 1977). We have found that the recessive mutation cmt permits expression of the normally silent copies of mating-type information at the HMa and HMα loci. In haploid strains carrying HMa and HMα, the cmt mutation allows the simultaneous expression of both a and α information, leading to a nonmating ("MATa/MATα") phenotype. The effects of cmt can be masked by changing the mating-type information at HMa or HMα. For example, a cell of genotype MATa hma HMα cmt has an a mating type, while a MATα hma HMα cmt strain is nonmating. Expression of mating-type information at the HM loci can correct the mating and sporulation defects of the mata* and matα10 alleles. Meiotic segregants recovered from cmt/cmt diploids carrying the mat mutations demonstrate that these mutants are not "healed" to normal MAT alleles, as is the case in parallel studies using the homothallism gene HO.—All of the results are consistent with the notion that the HMa and hmα alleles both code for α information, while HMα and hma both code for a information. The cmt mutation demonstrates that these normally silent copies of mating-type and sporulation information can be expressed and that the information at these loci is functionally equivalent to that found at MAT. The cmt mutation does not cause interconversions of mating-type alleles at MAT, and it is not genetically linked to MAT, HMa, HMα or HO. In cmt heterozygotes, cmt becomes homozygous at a frequency greater than 1% when the genotype at the MAT locus in mata*/MATα or matα10/MATa.

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