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

Repair of DNA double-strand breaks (DSBs) by homologous recombination (HR) requires resection of 5′-termini to generate 3′-single-strand DNA tails(1). Key components of this reaction are Exonuclease 1 and the bifunctional endo/exonuclease, Mre11(2-4). Mre11 endonuclease activity is critical when DSB termini are blocked by bound protein—such as by the DNA end-joining complex(5), topoisomerases(6), or the meiotic nuclease, Spo11(7-13)—but a specific function for the Mre11 3′-5′ exonuclease activity has remained elusive. Here, we 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 resection to traverse unidirectionally(1). Using a combination of physical assays for 5′-end-processing, our results suggest an alternative mechanism involving bidirectional resection. First, Mre11 nicks the strand to be resected up to 300 nucleotides from the 5′-terminus of the DSB—much further away than previously assumed. Second, this nick enables resection in a bidirectional manner, using Exo1 in the 5′-3′ direction away from the DSB, and Mre11 in the 3′-5′ direction towards the DSB end. Finally, Mre11 exonuclease activity confers resistance to DNA damage in cycling cells, suggesting that Mre11-catalysed resection may be a general feature of various DNA repair pathways.