Bidirectional resection of DNA double-strand breaks by Mre11 and Exo1

Repair of DNA double-strand breaks (DSBs) by homologous recombination requires resection of 5-termini to generate 3-single-strand DNA tails1. Key components of this reaction are exonuclease 1 and the bifunctional endo/exonuclease, Mre11 (refs 24). Mre11 endonuclease activity is critical when DSB termini are blocked by bound proteinsuch as by the DNA end-joining complex5, topoisomerases6 or the meiotic transesterase Spo11 (refs 713)but a specific function for the Mre11 35 exonuclease activity has remained elusive. Here we use Saccharomyces cerevisiae to reveal a role for the Mre11 exonuclease during the resection of Spo11-linked 5-DNA termini in vivo. We show that the residual resection observed in Exo1-mutant cells is dependent on Mre11, and that both exonuclease activities are required for efficient DSB repair. Previous work has indicated that resection traverses unidirectionally1. Using a combination of physical assays for 5-end processing, our results indicate an alternative mechanism involving bidirectional resection. First, Mre11 nicks the strand to be resected up to 300 nucleotides from the 5-terminus of the DSBmuch further away than previously assumed. Second, this nick enables resection in a bidirectional manner, using Exo1 in the 53 direction away from the DSB, and Mre11 in the 35 direction towards the DSB end. Mre11 exonuclease activity also confers resistance to DNA damage in cycling cells, suggesting that Mre11-catalysed resection may be a general feature of various DNA repair pathways.