Schizosaccharomyces pombe cells switch mating type by replacing genetic information at the expressed mat1 locus with sequences copied from mat2-P or mat3-M silent donor loci. The choice of donor locus is dictated by cell type, such that mat2 is the preferred donor in M cells and mat3 is the preferred donor in P cells. Donor choice involves a recombination-promoting complex (RPC) containing Swi2 and Swi5. In P cells, the RPC localizes to a specific DNA element located adjacent to mat3, but in M cells it spreads across the silent mating-type region, including mat2-P. This differential distribution of the RPC regulates nonrandom choice of donors. However, celltype-specific differences in RPC localization are not understood. Here we show that the mat1-M-encoded factor Mc, which shares structural and functional similarities with the male sex-determining factor SRY, is highly enriched at the swi2 and swi5 loci and promotes elevated levels of RPC components. Loss of Mc reduces Swi2 and Swi5 to levels comparable to those in P cells and disrupts RPC spreading across the mat2/3 region. Mc also localizes to loci expressed preferentially in M cells and to retrotransposon LTRs. We demonstrate that Mc localization at LTRs and at swi2 requires Abp1, a homolog of transposon-derived CENP-B protein and that loss of Abp1 impairs Swi2 protein expression and the donor choice mechanism. These results suggest that Mc modulates levels of recombination factors, which is important for mating-type donor selection and for the biased gene conversion observed during meiosis, where M cells serve as preferential donors of genetic information. (1) and mating-type switching in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe (2-5). In these distantly related yeast species, cells alternate between two distinct mating-type alleles by copying genetic information from one of the two silent donor loci to the active mating-type locus via highly orchestrated recombination processes (6-8).The mating-type region of S. pombe contains three linked locimat1, mat2, and mat3-located on chromosome 2. In wild-type homothallic strains, designated h 90 , mat2 is located ∼17 kb centromere-distal to mat1, whereas mat3 is separated from mat2 by an 11-kb interval ( Fig. 1) (8, 9). The mating type of a haploid cell is determined by the presence of P or M information at mat1 (8). Each allele consists of two divergently transcribed genes (10). mat1-P contains the Pc and Pi genes, and mat1-M contains the Mc and Mi genes. mat2 and mat3 contain the same genetic information as mat1-P and mat1-M, respectively, but are maintained in a silent state. These silent mating-type cassettes are embedded in a 20-kb heterochromatin domain surrounded by inverted repeat (IR-R and IR-