Ataxia-telangiectasia (A-T) and Nijmegen breakage syndrome (NBS) are recessive genetic disorders with susceptibility to cancer and similar cellular phenotypes. The protein product of the gene responsible for A-T, designated ATM, is a member of a family of kinases characterized by a carboxy-terminal phosphatidylinositol 3-kinase-like domain. The NBS1 protein is specifically mutated in patients with Nijmegen breakage syndrome and forms a complex with the DNA repair proteins Rad50 and Mrel1. Here we show that phosphorylation of NBS1, induced by ionizing radiation, requires catalytically active ATM. Complexes containing ATM and NBS1 exist in vivo in both untreated cells and cells treated with ionizing radiation. We have identified two residues of NBS1, Ser 278 and Ser 343 that are phosphorylated in vitro by ATM and whose modification in vivo is essential for the cellular response to DNA damage. This response includes S-phase checkpoint activation, formation of the NBS1/Mrel1/Rad50 nuclear foci and rescue of hypersensitivity to ionizing radiation. Together, these results demonstrate a biochemical link between cell-cycle checkpoints activated by DNA damage and DNA repair in two genetic diseases with overlapping phenotypes.

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Genetic studies in yeast have indicated a role of the RAD50 and MRE11 genes in homologous recombination, telomere length maintenance, and DNA repair processes. Here, we purify from nuclear extract of Raji cells a complex consisting of human Rad50, Mre11, and another protein factor with a size of about 95 kDa (p95), which is likely to be Nibrin, the protein encoded by the gene mutated in Nijmegen breakage syndrome. We show that the Rad50-Mre11-p95 complex possesses manganesedependent single-stranded DNA endonuclease and 3 to 5 exonuclease activities. These nuclease activities are likely to be important for recombination, repair, and genomic stability.Genetic studies on Saccharomyces cerevisiae mutants sensitive to ionizing radiation and to other agents that cause DNA double-stranded breaks have identified a large number of genetic loci required for the repair of such breaks. Many of these genes, including RAD50, RAD51, RAD52, RAD54, RAD55, RAD57, RAD59, RDH54, MRE11, and XRS2, show epistasis and are collectively known as the RAD52 epistasis group. Mutants of the RAD52 group also have defects of varying degrees in mitotic and meiotic recombination, which are initiated via DNA double-stranded break formation. Because meiotic recombination is essential for the proper segregation of homologous chromosomal pairs during meiosis I, the RAD52 group mutants often exhibit severe meiotic abnormalities, including inviability (see Refs. 1 and 2 for discussions and references).Extensive genetic evidence in yeast indicates that DNA double-stranded breaks are processed exonucleolytically, yielding 3Ј overhanging single-stranded (ss) 1 tails of about 600 bases in length (3, 4). According to the double-stranded break repair model for recombination (5), the 3Ј ssDNA tails formed as a result of break processing are bound by recombination proteins, which then mediate a search for the chromosomal homolog and heteroduplex DNA formation with the homolog (5). The RAD52 group genes may be divided into two categories. The first class consists of the RAD50, MRE11, and XRS2 genes, whose protein products are thought to be involved in the nucleolytic processing of DNA double-stranded breaks (6). Consistent with this classification, the Rad50 and Mre11 proteins have been shown to be homologous to the Escherichia coli SbcC and SbcD proteins, which combine to form a complex with endonuclease and exonuclease activities (7). The second category of the RAD52 group genes includes RAD51, RAD52, RAD54, RAD55, RAD57, and RDH54, whose products nucleate onto the ssDNA tails generated from break processing and then mediate the formation of heteroduplex DNA between the recombining chromosomes (1, 2). Whether the Rad59 protein, which is homologous to Rad52 (8), also has a role in heteroduplex DNA formation remains to be established.Important insights concerning the mechanism by which the RAD52 group proteins form heteroduplex DNA have been garnered through biochemical studies of purified human and yeast proteins (9 -11). However, no information as to the bi...