A CIS-Acting Mutation within the MATa Locus of SACCHAROMYCES CEREVISIAE That Prevents Efficient Homothallic Mating-Type Switching

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Homothallic strains of Saccharomyces cerevisiae are able to switch from one mating-type to the other as frequently as every cell division. We have identified a cis-dominant mutation of the MATa locus, designated MATa-inc, that can be converted to MATα at only about 5% of the normal efficiency. In homothallic MATa-inc/mata* diploids, the MATa-inc locus switched to MATα in only one of 30 cases, while the mata* locus switched to MATα in all 30 cases. The MATa-inc mutation can be "healed" by a series of switches, first to MATα and then to a normal allele of MATa. These data are consistent with the "cassette" model of Hicks, Strathern and Herskowitz (1977), in which mating conversions involve the transposition of wild-type copies of a or α information from silent genes elsewhere in the genome. The MATa-inc mutation appears to alter a DNA sequence necessary for the replacement of MATa by MATα. The MATa-inc mutation has no other effect on MATa functions. In beterothallic backgrounds, the mutation has no effect on the sensitivity to α-factor, synthesis of a-factor, expression of barrier phenotype or ability to mate or sporulate.——The MATa-inc allele does, however, exhibit one pleiotropic effect. About 1% of homothallic MATa-inc cells become completely unable to switch mating type because of mutations at HMa, the locus proposed to carry the silent copy of α information.——In addition, we have isolated a less efficient allele of the HO gene.

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