Homologous Recombination, but Not DNA Repair, Is Reduced in Vertebrate Cells Deficient in RAD52

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Abstract
Rad52 plays a pivotal role in double-strand break (DSB) repair and genetic recombination in Saccharomyces cerevisiae, where mutation of this gene leads to extreme X-ray sensitivity and defective recombination. Yeast Rad51 and Rad52 interact, as do their human homologues, which stimulates Rad51-mediated DNA strand exchange in vitro, suggesting that Rad51 and Rad52 act cooperatively. To define the role of Rad52 in vertebrates, we generatedRAD52−/− mutants of the chicken B-cell line DT40. Surprisingly, RAD52−/− cells were not hypersensitive to DNA damages induced by γ-irradiation, methyl methanesulfonate, or cis-platinum(II)diammine dichloride (cisplatin). Intrachromosomal recombination, measured by immunoglobulin gene conversion, and radiation-induced Rad51 nuclear focus formation, which is a putative intermediate step during recombinational repair, occurred as frequently inRAD52−/− cells as in wild-type cells. Targeted integration frequencies, however, were consistently reduced inRAD52−/− cells, showing a clear role for Rad52 in genetic recombination. These findings reveal striking differences between S. cerevisiae and vertebrates in the functions of RAD51 and RAD52.