DIRECTIONALITY OF FISSION YEAST MATING-TYPE INTERCONVERSION IS CONTROLLED BY THE LOCATION OF THE DONOR LOCI

Cells of homothallic strains of Schizosaccharomyces pombe efficiently switch between two mating types called P and M. The phenotypic switche s are due to conversion of the expressed mating-type locus (mat1) by t wo closely linked silent loci, mat2-P and mat3-M, that contain unexpre ssed information for the P and M mating types, respectively. In this p rocess, switching-competent cells switch to the opposite mating type i n 72-90% of the cell divisions. Hence, mat2-P is a preferred donor of information to mat1 in M cells, whereas mat3-M is a preferred donor in P cells. We investigated the reason for the donor preference by const ructing a strain in which the genetic contents of the donor loci were swapped. We found that switching to the opposite mating type was very inefficient in that strain. This shows that the location of the silent cassettes in the chromosome, rather than their content, is the decidi ng factor for recognition of the donor for each cell type. We propose a model in which switching is achieved by regulating accessibility of the donor loci, perhaps by changing the chromatin structure in the mat ing-type region, thus promoting an intrachromosomal folding of mat2 or mat3 onto mat1 in a cell type-specific fashion. We also present evide nce for the involvement of the Swi6 and Swi6-mod trans-acting factors in the donor-choice mechanism. We suggest that these factors participa te in forming the proposed folded structure.