MARl-A REGULATOR OF THE HMa AND HMa LOCI IN SACCHAROMYCES CEREVISIAE

A mutation in the MARI (mating-type regulator) locus causing sterility in Saccharomyces cerevisiae is reported. The mutation maps on the left arm of linkage group IV between trpl and cdc2 at a distance of about 27 CM from trpl and about 31 cM from cdc2. Haploid strains with genotype MAT& HMa HMa marl-I and MATa HMa HMa marl-I are sterile. However, MATa hma HMa marl-I and MATa HMa hma marl-I strains exhibit a and a mating type, respectively. The sterile strains can be “rare mated” with standard strains as a consequence of mutational changes at HMa and HMa. It is proposed that the MARI locus blocks the expression of MATa and MATa information thought to exist at HMa and HMa loci, respectively (HICKS, STRATHERN and HERSKOWITZ, 1977). In a marl-1 mutant, the expression of both HMa and HMa information leads to a nonmating phenotype similar to that of MATa/MATa diploids. The genetic evidence reported here is consistent with a central feature of the “cassette model”, namely that HMa and hma carry MATa information and HMa and hma carry MATa information. ETEROTHALLIC (ho) strains of bakers yeast, Saccharomyces cerevisiae, Hdisplay a or a mating type. The mating type is controlled by two stable allelic forms of the mating-type locus, MATa and MATa, although rare switches between the two allelic states occur at low frequencies of about IO-* in standard laboratory strains (HAWTHORNE 1963a; &BIN 1970). In contrast, homothallic ( H O ) strains can change their mating types as often as every generation (WINGE and ROBERTS 1949; HAWTHORNE 1963b; OSHIMA and TAKANO 1971; HICKS and HERSKOWITZ 1976; STRATHERN 1977). These switches comprise stable heritable changes at the mating-type locus (MAT). The continued presence of the HO gene is not required for the maintenance of the altered allele. Mitotic products of a single haploid HO cell may express opposite mating types due to switching and therefore fuse to produce MATa/MATa diploids that are not subject to further switching. Present address: Cold Spring Harbor Laboratory, P.O. Box 100, Cold Spring Harbor, New York 11724. Genetics 93: 37-50 September, 1979. 38 A. S . S. KLAR, S. FOGEL A N D K. MACLEOD Two other genes, HMa (alternate allele hma) and H M a (alternate allele hma) control the direction of switching. HO and H M a or hma are required for M A T a to MATa, and HO and HMa or hma are required for M A T a to M A T a interconversions (TAKANO and OSHIMA 1970; NAUMOV and TOLSTORUKOV 1973; HARASHIMA, NOGI and OSHIMA 1974; KLAR and FOGEL 1977; HICKS, STRATHERN and HERSKOWITZ 1977). The H M a and HMa loci are located on opposite arms of chromosome 3; and to date HO has not been mapped. The H M a locus displays loose linkage to M A T (HARASHIMA and OSHIMA 1976; KLAR and FOGEL 1977), which is situated about 20 map units from the centromere on the right arm of chromosome 3 (MORTIMER and HAWTHORNE 1969). Several molecular models have been proposed to explain interconversions at M A T . Variously they posit DNA modification of the regulatory site at M A T (HAWTHORNE, quoted in HOLLIDAY and PUGH 1975), inversion of such a regulatory site by recombination events (BROWN 1976; HICKS and HERSKOWITZ 1977), insertion of a “controlling element” into M A T (OSHIMA and TAKANO 1971 ) , or gene replacement (HICKS, STRATHERN and HERSKOWITZ 1977). The models are summarized in HICKS and HERSKOWITZ (1977) and KLAR, FOGEL and RADIN (1979). OSHIMA and TAKANO (1971) proposed that HO controls insertion or removal of a regulatory element into M A T . The regulatory elements are proposed to function in a manner analogous to that of the controlling elements described for maize (MCCLINTOCK 1956). The HMa and H M a loci are hypothesized to produce the mating-type-specific controlling elements. The attachment of an H M a element differentiates the M A T locus to an a allele and the attachment of an HMa element differentiates the M A T locus into an a allele. HICKS, STRATHERN and HERSKOWITZ (1977) proposed a similar but more specific scheme, the “cassette model.” Here, the HMa and hma loci are presumed to be blocks of unexpressed M A T a information and the H M a and hma loci are blocks of unexpressed M A T a information. It is suggested that the switch is brought about by insertion of this silent information or its copy (i.e., “cassette”) into M A T by the action of the HO gene. It is proposed that the M A T information located at the “silent genes,” HMa and H M a (and alternate hma and hma), are silent perhaps due to the lack of an essential regulatory site. We isolated a spontaneous mutation at a locus designated MAR2 (matingtype regulator). Overall, the genetic evidence provided by an analysis of the marl-l mutant and the alterations of HMa and H M a is consistent with the suggestion that the loci H M a and hma carry MATa information and that HMa and hma carry M A T a information. MATERIALS AND METHODS Strains: All strains used are described in Table 1. Techniques: All media for growth and sporulation and techniques for micromanipulation and tetrad analysis have been described (MORTIMER and HAWTHORNE 1969). For rare-mating experiments, freshly grown cells of the parental strains were mixed at 108 cells per ml (1:l) in sterile water, and 0.2 ml of the suspension was spread on each of five plates containing a complex, rich, nonselective medium. After 48 hr of growth, the confluent lawn was transCONTROL OF HM,a AND HMa LOCI