Molecular rearrangement of mating-type genes in fission yeast.

In the wild-type homothallic strain (h90) of the fission yeast Schizosaccharomyces pombe, the mating type of a cell changes at almost every cell division1. Genetic analysis has suggested that mating-type switching results from copy transposition of genes from one of two storage cassettes into an active site2. This model is functionally similar to that already established for Saccharomyces cerevisiae3 but is different in that the three mating-type genes involved are tightly linked in S. pombe. Heterothallic strains in which mating-type switching is much reduced or absent arise from the homothallic wild type either through the irreversible loss of one of the silent storage cassettes (generating heterothallic stable minus strain h−S) or through a genetic change which has been formally interpreted as a change in the switching signal at the mating-type locus (generating heterothallic normal plus (h+N) and unstable minus (h−U) strains. We have now isolated one of the mating-type genes mat-P, and have used it as a hybridization probe for sequence changes at the mating-type locus. We confirm that the genetic loss suggested for h−S is due to deletion and demonstrate that the impairment of switching in h+N and h−U is correlated with a duplication.