MCM1 REGULATES DONOR PREFERENCE CONTROLLED BY THE RECOMBINATION ENHANCER IN SACCHAROMYCES MATING-TYPE SWITCHING

Switching of Saccharomyces mating type by replacement of sequences at the MAT locus involves a choice between two donors, HML and HMR. MAT a lpha cells inhibit recombination along the entire left arm of chromoso me III, including HML, whereas MATa cells activate this same region. M ATa-dependent activation of HML depends on a small, cis-acting DNA seq uence designated the recombination enhancer (RE), located 17 kb centro mere-proximal to HML. A comparison of RE sequences interchangeable bet ween Saccharomyces cerevisiae and Saccharomyces carlsbergensis defines a minimum RE of 244 bp. RE activity is repressed in MAT alpha cells b y binding of the Mat alpha 2-Mcm1 corepressor to a site within the RE. Mutation of the two Mat alpha 2 binding sites removes most, but not a ll, of this repression, and RE chromatin structure in MAT alpha cells becomes indistinguishable from that seen in MATa. Surprisingly, a 2-bp mutation in the Mcm1 binding site completely abolishes RE activity in MATa cells; moreover, RE chromatin structure in the MATa mutant becom es very similar to that seen in MAT alpha cells with a normal RE, disp laying highly ordered nucleosomes despite the absence of Mat alpha 2. Further, a mutation that alters the ability of Mcm1 to act with Mat al pha 2 in repressing a-specific genes also alters donor preference in e ither mating type. Thus, Mcm1 is critically responsible for the activa tion as well as the Mat alpha 2-Mcm1-mediated repression of RE activit y.