Mre11 protein complex prevents double-strand break accumulation during chromosomal DNA replication

Mre11 complex promotes repair of DNA double-strand breaks (DSBs). Xenopus Mre11 (X-Mre11) has been cloned, and its role in DNA replication and DNA damage checkpoint studied in cell-free extracts. DSBs stimulate the phosphorylation and 3'-5' exonuclease activity of X-Mre11 complex. This induced phosphorylation is ATM independent. Phosphorylated X-Mre11 is found associated with replicating nuclei. X-Mre11 complex is required to yield normal DNA replication products. Genomic DNA replicated in extracts immunodepleted of X-Mre11 complex accumulates DSBs as demonstrated by TUNEL assay and reactivity to phosphorylated histone H2AX antibodies. In contrast, the ATM-dependent DNA damage checkpoint that blocks DNA replication initiation is X-Mre11 independent. These results strongly suggest that the function of X-Mre11 complex is to repair DSBs that arise during normal DNA replication, thus unraveling a critical link between recombination-dependent repair and DNA replication.