Characterization of RAD52 homologs in the fission yeast Schizosaccharomyces pombe

The RAD52 gene of Saccharomyces cerevisiae is essential for repair of DNA double-strand breaks (DSBs) by homologous recombination. Inactivation of this gene confers hypersensitivity to DSB-inducing agents and defects in most forms of recombination. The rad22⁺ gene in Schizosaccharomyces pombe (here referred to as rad22A⁺) has been characterized as a homolog of RAD52 in fission yeast. Here, we report the identification of a second RAD52 homolog in Schizosaccharomyces pombe, called rad22B⁺. The amino acid sequences of Rad22A and Rad22B show significant conservation (38% identity). Deletion mutants of respectively, rad22A and rad22B, show different phenotypes with respect to sensitivity to X-rays and the ability to perform homologous recombination as measured by the integration of plasmid DNA. Inactivation of rad22A⁺ leads to a severe sensitivity to X-rays and a strong decrease in recombination (13-fold), while the rad22B mutation does not result in a decrease in homologous recombination or a change in radiation sensitivity. In a rad22A–rad22B double mutant the radiation sensitivity is further enhanced in comparison with the rad22A single mutant. Overexpression of the rad22B⁺ gene results in partial suppression of the DNA repair defects of the rad22A mutant strain. Meiotic recombination and spore viability are only slightly affected in either single mutant, but outgrowth of viable spores is almost 31-fold reduced in the rad22A–rad22B double mutant. The results obtained imply a crucial role for rad22A⁺ in repair and recombination in vegetative cells just like RAD52 in S. cerevisiae. The rad22B⁺ gene presumably has an auxiliary role in the repair of DSBs. The drastic reduced spore viability in the double mutant suggests that meiosis in S. pombe is dependent on the presence of either rad22A⁺ or rad22B⁺.