Mating-Type Differentiation by Transposition of Controlling Elements in SACCHAROMYCES CEREVISIAE

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The nonfunctional mutation of the homothallic gene HMLα, designated hmlα, produced two mutant alleles, hmlα-1 and hmlα-2. Both mutant clones were mixed cultures consisting of a mating-type cells and nonmating haploid cells. The frequencies of the two cell types were different, and a few diploid cells able to sporulate were found in the hmlα-2 mutant. Conversions of an a mating-type cell to nonmater, and vice versa, were observed in both mutants. The conversion of an a mating phenotype to nonmating is postulated to occur by alteration of the a mating type to the sterile mating-type allele in the hmlα-1 mutant. In tetrad dissection of prototrophic diploids that were obtained by rare-mating of hmlα-1 mutants with a heterothallic strain having the MATa ho HMRa HMLa genotype, many mating-deficient haploid segregants were found, while α mating-type segregants were observed in a similar diploid using an hmlα-2 mutant. The mating-type-deficient haploid segregants were supposed to have the sterile α mating-type allele because the nonmating genetic trait always segregated with the mating-type locus. Sporogenous diploid cells obtained in the hmlα-2 mutant clone had the MATa/MATα HO/HO HMRa/HMRa hmlα-2/hmlα-2 genotype. These observations suggested that the hmlα-1 allele produces a transposable element that gives rise to the sterile α mating type by transposition into the mating-type locus, and that the hmlα-2 allele produces an element that provides α mating-type information, but is defective in the structure for transposition.

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