DNA double-strand breaks (DSBs) are repaired by non-homologous end joining (NHEJ) or homologous recombination (HR). HR requires 5 0 DSB end degradation that occurs in the presence of cyclin-dependent kinase (CDK) activity. Here, we show that a lack of any of the NHEJ proteins Yku (Yku70-Yku80), Lif1 or DNA ligase IV (Dnl4) increases 5 0 DSB end degradation in G1 phase, with ykuD cells showing the strongest effect. This increase depends on MRX, the recruitment of which at DSBs is enhanced in ykuD G1 cells. DSB processing in G2 is not influenced by the absence of Yku, but it is delayed by Yku overproduction, which also decreases MRX loading on DSBs. Moreover, DSB resection in ykuD cells occurs independently of CDK activity, suggesting that it might be promoted by CDK-dependent inhibition of Yku.

SummaryThe Mre11–Rad50–Nbs1 (MRN) complex tethers, processes and signals DNA double strand breaks, promoting genomic stability. To understand the functional architecture of MRN, we determined the crystal structures of the Schizosaccharomyces pombe Mre11 dimeric catalytic domain alone and in complex with a fragment of Nbs1. Two Nbs1 subunits stretch around the outside of Mre11’s nuclease domains, with one subunit additionally bridging and locking the Mre11 dimer via a highly conserved asymmetrical binding motif. Our results reveal that Mre11 forms a flexible dimer and suggest that Nbs1 is not only a checkpoint adaptor, but also functionally impacts on Mre11-Rad50. Clinical mutations in Mre11 are located along the Nbs1 interaction sites and weaken the Mre11–Nbs1 interaction. However, they differentially affect DNA repair and telomere maintenance in Saccharomyces cerevisiae, potentially providing insight into their different human disease pathologies.

The DNA-damage checkpoint kinase Chk1 is essential in higher eukaryotes due to its role in maintaining genome stability in proliferating cells. CHK1 gene deletion is embryonically lethal, and Chk1 inhibition in replicating cells causes cell-cycle defects that eventually lead to perturbed replication and replication-fork collapse, thus generating endogenous DNA damage. What is the cause of replication-fork collapse when Chk1 is inactivated, however, remains poorly understood. Here, we show that generation of DNA double-strand breaks at replication forks when Chk1 activity is compromised relies on the DNA endonuclease complex Mus81/Eme1. Importantly, we show that Mus81/Eme1-dependent DNA damage—rather than a global increase in replication-fork stalling—is the cause of incomplete replication in Chk1-deficient cells. Consequently, Mus81/Eme1 depletion alleviates the S-phase progression defects associated with Chk1 deficiency, thereby increasing cell survival. Chk1-mediated protection of replication forks from Mus81/Eme1 even under otherwise unchallenged conditions is therefore vital to prevent uncontrolled fork collapse and ensure proper S-phase progression in human cells.