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The mating-type information residing at the HML and HMR loci in Saccharomyces cerevisiae is kept unexpressed by the action of at least four MAR (or SIR) loci. To determine possible interactions between the MAR/SIR gene products and to find new regulatory loci, we sought extragenic suppressors of the mar1-1 mutation. A strain with the genotype HML  a  MATα HMR  a  mar1-1 is unable to mate because of the simultaneous expression of a and α information. A mutant of this strain was isolated that exhibits an α phenotype and, therefore, presumably fails to express the HML and HMR loci. We designate the new locus SUM1(supressor of mar). The mutation is recessive, centromere unlinked and does not correspond to the MAT, HML, HMR, SIR1, MAR1, MAR2(SIR3) or SIR4 loci. The sum1 mutation affects expression of both a and α information at the HM loci. Suppression by sum1-1 is neither allele specific nor locus specific as it suppresses a deletion mutation of the MAR1 locus and mutations in SIR3 and SIR4. The sum1-1 mutation has no discernible phenotype in a Mar+ strain. We propose that the MAR/SIR gene produts negatively regulate the SUM1 locus, the gene product of which is necessary for expression of the HM loci.

SUM1, AN APPARENT POSITIVE REGULATOR OF THE CRYPTIC MATING-TYPE LOCI IN SACCHAROMYCES CEREVISIAE