Mechanism of MATa Donor Preference during Mating-Type Switching of Saccharomyces cerevisiae

During homothallic switching of the mating-type (MAT) gene in Saccharomyces cerevisiae, a- or a-specific sequences are replaced by opposite mating-type sequences copied from one of two silent donor loci, HMLa or HMRa. The two donors lie at opposite ends of chromosome III, approximately 190 and 90 kb, respectively, from MAT. MATa cells preferentially recombine with HMR, whileMATa 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 MATa cells, HML is not an efficient donor when HMR is deleted; consequently, approximately one-third of HO HMLa MATa hmrD cells die because they fail to repair the HO endonuclease-induced double-strand break atMAT. MATa 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 andMAT 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 MATa preference are not influenced by the chromosomal orientation ofMAT or by sequences as far as 6 kb fromHMR