Mating-type Mutations in Schzzosaccharomyces Pombe:

Mutants defective in various steps of the sexual cycle have been isolated from homothallic strains of Schizosaccharomyces pombe by BRESCH, MULLER and EGEL (1968). These mutants include heterothallic h+ and hstrains. We have isolated additional h+ and hmutants from homothallic strains. Those mutants which are due to mutations in the mating-type region were analyzed in detail. Our results show that the mating-type gene mat2 not only has a function in copulation and meiosis, but that it also regulates the formation of the mapi gene product (mapi is a mating-type auxiliary gene). Some of the hmutants have lost only one of the three functions while others are defective in at least two, and perhaps all three, €unctions. Further, we show that the matiallele of h90 strains can mutate to m a t i f but that mutations in mat2 appear to affect the mutational behavior of mati. Finally, we describe a new inactive mating-type allele, mat2*, which is different from mat20 in that it can mutate to mat2+. I N this paper we report experiments on the mating-type genes of Schizosaccharomyces pombe. S. pombe is a haploid fission yeast; its life cycle and genetics have recently been reviewed by GUTZ et al. (1974). Homothallic and heterothallic mating types in S. pombe (LEUPOLD 1950) are determined by a short region on chromosome I1 (LEUPOLD 1958; GUTZ et al. 1974). LEUPOLD (1958) distinguished two closely linked subunits in the matingtype region. Whether these subunits are part of one gene or whether they represent two distinct genes is not known. Further, the use of the term “gene” in connection with the subunits may be an oversimplification; both subunits may be part of one gene cluster or may represent two different gene clusters rather than single cistrons. LEUPOLD (1958) has proposed a two-gene model for this region; the postulated mating-type genes are now called mat1 and mat2 ( GUTZ and DOE 1973). The various mating types known in S. pombe and their interpretation in LEUPOLD’S two gene scheme are summarized in Table 1. The mat1 gene can This work represents a partial fulfillment of requirements of the University of Texas at Dallas for the degree of Doctor of Philosophy awarded to J.H.M. in 1975. Institute of General Medical Sciences, DHEW, and by the University of Texas at Dallas Research Fund. * This work was supported by Natlonal Science Foundation Grant GB-15148, by Grant GM-19849 awarded by National Present Address. Department of Physics, Kansas State University, Manhattan, Kansas 6650G. Genetics 83: 259-273 June. 1976. 260 J. H. MEADE A N D H. GUTZ