The Saccharomyces cerevisiae recombination enhancer biases recombination during interchromosomal mating-type switching but not in interchromosomal homologous recombination

Haploid Saccharomyces can change mating type through HO-endonuclease cleavage of an expressor locus, MAT, followed by gene conversion using one of two repository loci, HML or HMI as donor. The mating type of a cell dictates which repository locus is used as donor, with alpha cells using HML and a cells using HMR, This preference is established in part by RE, a locus on the left arm of chromosome III that activates the surrounding region, including HML, for recombination in a cells, an activity suppressed by alpha2 protein in a cells. We have examined the ability of RE to stimulate different forms of interchromosomal recombination. . We found that RE exerted an effect on interchromosomal mating-type switching and on intrachromosomal homologous recombination but not on interchromosomal homologous recombination. Also, even in the absence of RE, MATalpha still influenced donor preference in interchromosomal mating-type switching, supporting a role of alpha2 in donor preference independent of RE These results suggest a model in which RE affects competition between productive and nonproductive recombination outcomes. In interchromosome gene conversion, RE enhances both productive and nonproductive pathways, whereas in intrachromosomal gene conversion and mating-type switching, RE enhances only the productive pathway.