MECHANISM OF MAT-ALPHA DONOR PREFERENCE DURING MATING-TYPE SWITCHING OF SACCHAROMYCES-CEREVISIAE

During homothallic switching of the mating-type (MAT) gene in Saccharo myces cerevisiae, a- or alpha-specific sequences are replaced by oppos ite mating-type sequences copied from one of two silent donor loci, HM L 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 m echanisms of donor selection are different for the two mating types, M ATa tells, deleted for the preferred HML gene, efficiently use HMR as a donor, However, in MAT alpha cells, HML is not an efficient donor wh en HMR is deleted; consequently, approximately one-third of HO HML alp ha MAT alpha hmr Delta cells die because they fail to repair the HO en donuclease-induced double-strand break at MAT, MAT alpha donor prefere nce depends not on the sequence differences between HML and HMR or the ir surrounding regions but on their chromosomal locations. Cloned HMR donors placed at three other locations to the left of MAT, on either s ide of the centromere, all fail to act as efficient donors, When the d onor is placed 37 kb to the left of MAT, its proximity overcomes norma l donor preference, but this position is again inefficiently used when additional DNA is inserted in between the donor and MAT to increase t he distance to 62 kb, Donors placed to the right of MAT are efficientl y recruited, and in fact a donor situated 16 kb proximal to HMR is use d 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 th ere 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 dono rs to the right of MAT alpha.