Chromosome instability and defective recombinational repair in knockout mutants of the five Rad51 paralogs

The Rad51 protein, a eukaryotic homologue of Escherichia coli RecA, plays a central role in both mitotic and meiotic homologous DNA recombination (HR) in Saccharomyces cerevisiae and is essential for the proliferation of vert ebrate cells. Five vertebrate genes, RAD51B, -C, and -D and XRCC2 and -3, a re implicated in HR on the basis of their sequence similarity to Rad51 (Rad 51 paralogs). We generated mutants deficient in each of these proteins in t he chicken B-lymphocyte DT40 cell line and report here the comparison of fo ur new mutants and their complemented derivatives with our previously repor ted rad51b mutant. The Rad51 paralog mutations all impair HR, as measured b y targeted integration and sister chromatid exchange. Remarkably, the mutan t cell lines all exhibit very similar phenotypes: spontaneous chromosomal a berrations, high sensitivity to killing by cross-linking agents (mitomycin C and cisplatin), mild sensitivity to gamma rays, and significantly attenua ted Rad51 focus formation during recombinational repair after exposure to g amma rays. Moreover, all mutants show partial correction of resistance to D NA damage by overexpression of human Rad51. We conclude that the Rad51 para logs participate in repair as a functional unit that facilitates the action of Rad51 in HR.