Release of Ku and MRN from DNA Ends by Mre11 Nuclease Activity and Ctp1 Is Required for Homologous Recombination Repair of Double-Strand Breaks
Figure 8
Model for resection in Schizo. pombe.
Wild type: Ku and MRN recognize the DSB, and MRN recruits Ctp1. Ctp1 initiates resection. The action of the Mre11 nuclease and Ctp1 releases Ku from the DNA end, after which MRN is released from the DNA. Resection can be extended by Exo1. Localization of RPA to ssDNA recruits Rad3-Rad26 and activates the checkpoint pathway by phosphorylating Chk1. The break is repaired. Ctp1Δ: MRN recognizes the DSB, but absence of Ctp1 inhibits release of Ku and resection initiation. Inefficient Ku release by Mre11 nuclease allows Exo1 to substitute for Ctp1 in the initiation of resection at some DNA ends. Exo1 also extends resection. This step is inefficient and results in strongly decreased RPA binding to the ssDNA and prevents HR repair. Mre11-ND: M*RN recognizes the DSB, and recruits Ctp1. The lack of the nuclease function of Mre11 inhibits Ku release. Ctp1 can initiate resection for several hundred base pairs, allowing Exo1 to extent resection despite the presence of Ku on the DNA end. The inability to release Ku and M*RN from the DNA end interferes with RPA localization at the ssDNA and prevents repair. Ctp1 is however able to release Ku from some DNA ends. Resection and efficient RPA binding to the ssDNA overhangs allows repair of the DSB.