The mating-type-related bias of gene conversion in Schizosaccharomyces pombe.

The mating-type bias (mat-bias) of gene conversion was previously described as a phenomenon in which the number of prototrophic recombinants in an ura4A heteroallelic two-factor cross relates to the mating types of the parents. We show now that the mat-bias is restricted neither to ura4A nor to recombination hotspots, but occurs at other genomic loci, too. It is specific for gene conversion and absent in azygotic meiosis. Thus, the mat-bias must originate from mating-type-specific "imprinting" events before karyogamy takes place. Structural variations of the mating-type locus, such as h(+N), h(+S), h(-S), h(+smtDelta), or h(-smtDelta), showed mat-bias manifestation. Mutations in genes coding for histone acetylase (gcn5, ada2) and histone deacetylase (hos2, clr6) activities smooth or abolish the mat-bias. In addition, the mat-bias depends on the presence of Swi5. We propose a new role for Swi5 and the histone acetylation status in mat-bias establishment through directionality of repair from the intact chromatid to the broken chromatid.