Phosphorylation of the budding yeast 9-1-1 complex is required for Dpb11 function in the full activation of the UV-induced DNA damage checkpoint

Following genotoxic insults, eukaryotic cells trigger a phosphorylation based signal transduction cascade known as DNA damage checkpoint response. Full checkpoint activation involves loading onto damaged DNA of an apical kinase and several downstream factors. Chromatin modifications contribute to the recruitment of checkpoint proteins. In budding yeast, methylated H3-K79 provides a binding site for the checkpoint factor Rad9, allowing signal transduction to the Rad53 effector kinase. Loss of Dot1, the specific histone methyltransferase, prevents H3-K79 methylation leading to a checkpoint defect in the G1 phase of the cell cycle and to a partial reduction of checkpoint activation in mitosis. This suggests that another pathway can contribute to Rad9 recruitment to damaged DNA in M phase. In order to identify this second pathway, we sought for mutants that abolished the checkpoint response to UV irradiation in mitosis, when combined with a dot1∆ mutation. We show that the replication factor Dpb11 is the keystone of this second pathway. A dot1∆ dpb11-1 double mutant cannot activate Rad53 when M cells are irradiated and is very sensitive to UV treatment. We suggest that Dpb11 is held in proximity of damaged DNA through an interaction with Mec1-phosphorylated 9-1-1 complex.