Homologous recombination mutant lethality differs between h- and h+ Schizosaccharomyces pombe strains due to mat1 heterochromatin

Homologous recombination (HR) is generally considered dispensable in yeast and vertebrates, yet mounting evidence indicates that its essentiality depends on cellular context. Here, we dissect the basis of this context dependency in Schizosaccharomyces pombe. In the homothallic h90 strain, regarded as wild type, mating-type switching (MTS) occurs every other cell division and requires HR to repair programmed double-strand breaks (DSBs) at the mat1 locus. We show that the widely used heterothallic h-S strain is likewise dependent on HR for viability. HR-deficient h-S mutants (rad51{Delta}, rad52{Delta}, or rad54{Delta}), still frequently employed in the literature, survive only when carrying secondary suppressor mutations that abolish mat1 DSB formation, such as smt-0, swi1{Delta}, or fml1{Delta}. In contrast, HR is dispensable in the h+N strain, where duplication of the mat2/3 region into mat1 introduces the cenH and REIII elements. These elements nucleate H3K9 methylation and heterochromatin spreading across the imprint site, blocking imprintosome recruitment and thereby preventing both imprinting and DSB formation. Disruption of this heterochromatin, via deletion of cenH or key chromatin modifiers, restores DSB formation in h+N cells and reinstates HR essentiality in the absence of the Clr4 methyltransferase. Collectively, our findings demonstrate that HR is indispensable for S. pombe survival due to its critical role in repairing mat1 DSBs, except under genetic or epigenetic conditions that suppress their formation.