MATING-TYPE DIFFERENTIATION BY TRANSPOSITION OF CONTROLLING ELEMENTS IN SACCHAROMYCES CEREVISIAE

Takehiro Oshima ¹ and Isamu Takano ¹

¹ The Central Research Institute, Suntory Ltd., Wakayamadai, Shimamoto-cho, Mishima-gun, Osaka 618, Japan

The nonfunctional mutation of the homothallic gene HML, designated hml, produced two mutant alleles, hml-1 and hml-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 hml-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 hml-1 mutant. In tetrad dissection of prototrophic diploids that were obtained by rare-mating of hml-1 mutants with a heterothallic strain having the MATa ho HMRa HMLa genotype, many mating-deficient haploid segregants were found, while mating-type segregants were observed in a similar diploid using an hml-2 mutant. The mating-type-deficient haploid segregants were supposed to have the sterile mating-type allele because the nonmating genetic trait always segregated with the mating-type locus. Sporogenous diploid cells obtained in the hml-2 mutant clone had the MATa/MAT HO/HO HMRa/HMRa hml-2/hml-2 genotype. These observations suggested that the hml-1 allele produces a transposable element that gives rise to the sterile mating type by transposition into the mating-type locus, and that the hml-2 allele produces an element that provides mating-type information, but is defective in the structure for transposition.