Fission yeast Swi2 designates cell-type specific donor and stimulates Rad51-driven strand exchange

A haploid of the fission yeast Schizosaccharomyces pombe expresses either the P or M matingtype, determined by the active, euchromatic, mat1 cassette. Mating-type is switched by Rad51-driven gene conversion of mat1 using a heterochromatic donor cassette, mat2-P or mat3-M. The Swi2-Swi5 complex, a mating-type switching factor, is central to this process by designating a preferred donor in a cell-type-specific manner. Swi2-Swi5 selectively enables one of two cisacting recombination enhancers, SRE2 adjacent to mat2-P or SRE3 adjacent to mat3-M. Here, we identified two functionally important motifs in Swi2, a Swi6 (HP1 homolog)-binding site and two DNA-binding AT-hooks. Genetic analysis demonstrated that the AT-hooks were required for Swi2 localization at SRE3 to select the mat3-M donor in P cells, while the Swi6-binding site was required for Swi2 localization at SRE2 to select mat2-P in M cells. In addition, the Swi2-Swi5 complex promoted Rad51-driven strand exchange in vitro. Taken together, our results show how the Swi2-Swi5 complex would localize to recombination enhancers through a cell-type specific binding mechanism and stimulate Rad51-driven gene conversion at the localization site.