Checkpoint activation in response to double-strand breaks requires the Mre11/Rad50/Xrs2 complex

Studies of human Nijmegen breakage syndrome (NBS) cells have led to the pro posal that the Mre11/Rad50/NBS1 complex, which is involved in the repair of DNA double-strand breaks (DSBs), might also function in activating the DNA damage checkpoint pathways after DSBs occur(1,2). We have studied the role of the homologous budding yeast complex, Mre11/Rad50/Xrs2, in checkpoint a ctivation in response to DSB-inducing agents. Here we show that this comple x is required for phosphorylation and activation of the Rad53 and Chk1 chec kpoint kinases specifically in response to DSBs. Consistent with defective Rad53 activation, we observed defective cell-cycle delays after induction o f DSBs in the absence of Mrell. Furthermore, after gamma -irradiation phosp horylation of Rad9, which is an early event in checkpoint activation, is al so dependent on Mre11. All three components of the Mre11/Rad50/Xrs2 complex are required for activation of Rad53, however, the Ku80, Rad51 or Rad52 pr oteins, which are also involved in DSB repair, are not. Thus, the integrity of the Mre11/Rad50/Xrs2 complex is specifically required for checkpoint ac tivation after the formation of DSBs.