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Mol. Cell. Biol., 02 1996, 657-668, Vol 16, No. 2
X Wu, JK Moore and JE Haber
During homothallic switching of the mating-type (MAT) gene in Saccharomyces
cerevisiae, a- or alpha-specific sequences are replaced by opposite
mating-type sequences copied from one of two silent donor loci, HML alpha
or HMRa. The two donors lie at opposite ends of chromosome III,
approximately 190 and 90 kb, respectively, from MAT. MAT alpha cells
preferentially recombine with HMR, while MATa cells select HML. The
mechanisms of donor selection are different for the two mating types. MATa
cells, deleted for the preferred HML gene, efficiently use HMR as a donor.
However, in MAT alpha cells, HML is not an efficient donor when HMR is
deleted; consequently, approximately one- third of HO HML alpha MAT alpha
hmr delta cells die because they fail to repair the HO endonuclease-induced
double-strand break at MAT. MAT alpha donor preference depends not on the
sequence differences between HML and HMR or their surrounding regions but
on their chromosomal locations. Cloned HMR donors placed at three other
locations to the left of MAT, on either side of the centromere, all fail to
act as efficient donors. When the donor is placed 37 kb to the left of MAT,
its proximity overcomes normal donor preference, but this position is again
inefficiently used when additional DNA is inserted in between the donor and
MAT to increase the distance to 62 kb. Donors placed to the right of MAT
are efficiently recruited, and in fact a donor situated 16 kb proximal to
HMR is used in preference to HMR. The cis-acting chromosomal determinants
of MAT alpha preference are not influenced by the chromosomal orientation
of MAT or by sequences as far as 6 kb from HMR. These data argue that there
is an alpha-specific mechanism to inhibit the use of donors to the left of
MAT alpha, causing the cell to recombine most often with donors to the
right of MAT alpha.
Copyright © 1996, American Society for Microbiology
Mechanism of MAT alpha donor preference during mating-type switching of Saccharomyces cerevisiae
Rosenstiel Basic Medical Sciences Research Center, Brandeis University, Waltham, Massachusetts 02254-9110, USA.
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