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 HMR, as donor. The mating type of a cell dictates which repository locus is used as donor, with a cells using HML and alpha 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 alpha 2 protein in alpha 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, MAT alpha still influenced donor preference in interchromosomal mating-type switching, supporting a role of alpha 2 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.