A RECOMBINATIONALLY REPRESSED REGION BETWEEN MAT2 AND MAT3 LOCI SHARES HOMOLOGY TO CENTROMERIC REPEATS AND REGULATES DIRECTIONALITY OF MATING-TYPE SWITCHING IN FISSION YEAST

Cells of the fission yeast Schizosaccharomyces pombe switch mating typ e by replacing genetic information at the transcriptionally active mat 1 locus with sequences copied from one of two closely Linked silent lo ci, mat2-P or mat3-M. By a process referred to as directionality of sw itching, cells predominantly sw itch to the opposite mat1 allele; the mat1-P allele preferentially recombines with mat3, while mat1-M select s the mat2. In contrast to efficient recombination at mat1, recombinat ion within the adjoining mat2-mat3 interval is undetectable. We define d the role of sequences between mat2 and mat3, designated the K-region , in directionality as well as recombinational suppression. Cloning an d sequencing analysis revealed that a part of the K-region is homologo us to repeat sequences present at centromeres, which also display tran scriptional and recombinational suppression. Replacement of 7.5 kb of the K-region with the ura4(+) gene affected directionality in a varieg ated manner. Analysis of the swi6-mod locus, which was previously show n to affect directionality, in K Delta::ura4(+) strains suggested the existence of at least two overlapping directionality mechanisms. Our w ork furthers the model that directionality is regulated by cell-type s pecific organization of the heterochromatin-like structure in the mati ng-type region and provides evidence that the K-region contributes to silencing of the mat2-mat3 interval.