DNA polymerase-alpha is essential for mating-type switching in fission yeast

In the fission yeast Schizosaccharomyces pombe, the double-stranded chromosomal break (DSB) at the mating-type locus (mat1) initiates recombination during mating-type switching. A constant DSB level is maintained throughout the cell-cycle. In the strand-segregation model for mating-type switching, it was postulated that if the DSB is generated during or soon after mat1 replication, one of the chromatids could be repaired and switched during replication in the next cell cycle, while the other chromatid inherits the break. Here we report a molecular characterization of swi7, one of the genes required for DSB formation. Surprisingly, a gene complementing the swi7 mutation maps to chromosome I and encodes S. pombe DNA polymerase-alpha. Disruption of this gene is lethal in both switching and non-switching strains, as expected. S. pombe DNA polymerase-alpha must therefore play a role in generating the DSB at mat1, suggesting that DSB formation is coupled with DNA replication.