Abstract
Saccharomyces mating-type (
MAT
) switching occurs by gene conversion using one of two donors,
HML
α and
HMR
a
, located near the ends of the same chromosome.
MAT
a
cells preferentially choose
HML
α, a decision that depends on the recombination enhancer (RE) that controls recombination along the left arm of chromosome III (III-L). When RE is inactive, the two chromosome arms constitute separate domains inaccessible to each other; thus
HMR
a
, located on the same arm as
MAT
, becomes the default donor. Activation of RE increases
HML
α usage, even when RE is moved 50 kb closer to the centromere. If
MAT
is inserted into the same domain as
HML
, RE plays little or no role in activating
HML
, thus ruling out any role for RE in remodeling the silent chromatin of
HML
in regulating donor preference. When the donors
MAT
and RE are moved to chromosome V, RE increases
HML
usage, but the inaccessibility of
HML
without RE apparently depends on other chromosome III-specific sequences. Similar conclusions were reached when RE was placed adjacent to
leu2
or
arg4
sequences engaged in spontaneous recombination. We propose that RE's targets are anchor sites that tether chromosome III-L in
MAT
α cells thus reducing its mobility in the nucleus.