Abstract
Switching of
Saccharomyces
mating type by replacement of sequences at the
MAT
locus involves a choice between two donors,
HML
and
HMR. MAT
α cells inhibit recombination along the entire left arm of chromosome III, including
HML,
whereas
MAT
a cells activate this same region.
MAT
a-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
MAT
α 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
MAT
α cells becomes indistinguishable from that seen in
MAT
a. Surprisingly, a 2-bp mutation in the Mcm1 binding site completely abolishes RE activity in
MAT
a cells; moreover, RE chromatin structure in the
MAT
a mutant becomes very similar to that seen in
MAT
α 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.