NUCLEASE ACTIVITIES IN A COMPLEX OF HUMAN RECOMBINATION AND DNA-REPAIR FACTORS RAD50, MRE11, AND P95

Genetic studies in yeast have indicated a role of the RAD50 and MRE11 genes in homologous recombination, telomere length maintenance, and DN A repair processes. Here, we purify from nuclear extract of Raji cells a complex consisting of human Rad50, Mre11, and another protein facto r 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 manganese-dependent s ingle-stranded DNA endonuclease and 3' to 5' exonuclease activities. T hese nuclease activities are likely to be important for recombination, repair, and genomic stability.