Abstract
Abstract Mating type (MAT) switching in Saccharomyces cerevisiae results from a double-strand break (DSB) at the MAT locus induced by HO endonuclease and uses one of two donors, HML and HMR to repair the break by gene conversion. MATα cells recombine with HML 90% of the time and arc strongly activated by a recombination enhancer (RE) located 17kb proximal to HML. MATα cells prefers to recombine with HMR and is not activated by RE. When RE is deleted in MATα cells, they behave like MATa cells, reversing the donor preference. We studied the effect of the RE location in MATα cells where the HML. donor is deleted in a competition assay where the DSB at MAT can be repaired by inter-chromosomal MAT switching or by single-strand annealing (SSA) between two URA3 genes flanking MAT. When RE is inserted near the "wrong" donor HMR, gene conversion or MAT switching events increased from 7% to 26% in competition with SSA. The increased usage of the "wrong" donor HMR in MATa hmlΔ cells is comparable when RE is located on both arms on chromosome III and also when RE is only located near HMR. In another system, we explored the influence of the length of shared homology between the "wrong" donor HML and MAT on cell viability when the preferred donor HMR is deleted in MATa reΔ cells. We found that increasing the amount of homology between HML and MAT increases cell viability from about 50% to about 80% of all switches.