The Rad17-Mec3-Ddc1 complex is essential for the cellular response to genotoxic agents and is thought to be important for sensing DNA lesions. Deletion of any of the RAD17, MEC3 or DDC1 genes abolishes the G 1 and G2 and impairs the intra-S DNA-damage checkpoints. We characterize a dominant-negative mec3-dn mutation that has an unexpected phenotype. It inactivates the G1 checkpoint while it leaves the G2 response functional, thus revealing a difference in the requirements of the DNA-damage response in different phases of the cell cycle. In an attempt to identify the molecular defect imparted by the mutation, we dissected step-bystep the signaling cascade, which is triggered by DNA lesions and requires the activity of Mec1 and Rad53 kinases. The analysis of the phosphorylation state of checkpoint factors and critical protein interactions showed that, in mec3-dn cells, the signal transduction cascade is triggered normally, and the central kinase Mec1 can be activated. In G 1 cells expressing the mutation, the signaling cannot proceed any further along the pathway, indicating that the Rad17 complex acts after the activation of Mec1, possibly recruiting targets for the kinase. We also show that the function of the G 2 checkpoint in mutant cells is maintained by an uncharacterized activity of Tel1, the yeast homologue of ATM. This work thus reports a previously undiscovered role for Tel1 in checkpoint control.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.