Mating Type Switch
Donor Preference

Yeast haploid cells can exist in one of two mating types, either a or alpha. The two mating types can mate with each other to form an a/alpha diploid that can undergo meiosis (and DNA recombination) resulting in four meiotic products or haploid spores. These spores are either a or alpha and can undergo further rounds of mating and sporulation under the appropriate conditions. Wild type yeast cells can undergo mating type switching as often as once per generation. To accomplish this, the MAT locus, which controls the cell’s mating type, is changed from an a-specific sequence to an alpha-specific sequence or vice versa by gene conversion induced by the HO endonuclease (see Introduction).

The donor sequences for gene conversion are located at HMLalpha and HMRa on the left and right arms of chromosome III respectively. These loci are kept in a silent state where they cannot be expressed and they cannot be cleaved by the HO endonuclease. The phenomenon of "donor preference" refers to the ability of MATa cells to choose the HMLalpha locus as a donor about 90% of the time. Likewise MATalpha cells choose HMRa about 90% of the time.

Our lab has shown that this process is controlled by a small locus located 17 kb away from HML called the recombination enhancer (RE). In MATa cells, the RE appears to activate the entire left arm of chromosome III encouraging cells to switch cell type using the silent cassette at HMLalpha. Interestingly, the RE can also enhance spontaneous ectopic recombination between leu2 alleles. On the other hand, in MATalpha cells, the left arm becomes recombinationally cold preventing use of HML as a donor to heal a chromosomal break at MAT. HML cannot be used as a donor even if HMR has been deleted.
Sequence analysis of the RE has been carried out by comparing the RE region from the related yeast S. carlbergensis and by deletion and point mutation analysis. The RE is a 270 bp sequence composed of four cis-acting regions called A, B, C and D. More recently we have shown that an additional sequence called E is also important for donor preference. Various combinations and orientations of these elements can restore donor preference to varying degrees.
The C element is of particular interest because it contains a binding site for a complex of the Mcm1 and the alpha2 proteins. The alpha2 protein is expressed from the MAT Y alpha sequence and controls expression of alpha-specific genes in combination with the Mcm1 protein. The Mcm1 protein is a transcriptional activator of both a and alpha-specific genes. A 2-bp mutation at the Mcm1p binding site is sufficient to reduce HML usage from 90% to 20% in MATa cells.
It is most intriguing that the chromatin structure of the RE region is quite different between a and alpha cells. In alpha cells the RE and its surrounding region is covered by highly organized nucleosomes; however, this feature is absent in MATa cells but re-appears when the Mcm1p binding site has been mutated as described above. This suggests Mcm1p is responsible for removing nucleosomes from the RE region and that the binding of the alpha2 protein aids in positioning the phased nucleosomes.