| Abstract: | Switching of Saccharomyces mating type by replacement of sequences at the MAT locus involves a choice between two donors, HML and HMR. MATα cells inhibit recombination along the entire left arm of chromosome III, including HML, whereas MATa cells activate this same region. MATa-dependent activation of HML depends on a small, cis-acting DNA sequence designated the recombination enhancer (RE), located 17 kb centromere-proximal to HML. A comparison of RE sequences interchangeable between Saccharomyces cerevisiae and Saccharomyces carlsbergensis defines a minimum RE of 244 bp. RE activity is repressed in MATα cells by binding of the Matα2–Mcm1 corepressor to a site within the RE. Mutation of the two Matα2 binding sites removes most, but not all, of this repression, and RE chromatin structure in MATα 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 becomes very similar to that seen in MATα cells with a normal RE, displaying highly ordered nucleosomes despite the absence of Matα2. Further, a mutation that alters the ability of Mcm1 to act with Matα2 in repressing a-specific genes also alters donor preference in either mating type. Thus, Mcm1 is critically responsible for the activation as well as the Matα2-Mcm1-mediated repression of RE activity. |
