RAD9 and DNA polymerase epsilon form parallel sensory branches for transducing the DNA damage checkpoint signal in Saccharomyces cerevisiae.

In response to DNA damage and replication blocks, yeast cells arrest at distinct points in the cell cycle and induce the transcription of genes whose products facilitate DNA repair. Examination of the inducibility of RNR3 in response to UV damage has revealed that the various checkpoint genes can be arranged in a pathway consistent with their requirement to arrest cells at different stages of the cell cycle. While RAD9, RAD24, and MEC3 are required to activate the DNA damage checkpoint when cells are in G1 or G2, POL2 is required to sense UV damage and replication blocks when cells are in S phase. The phosphorylation of the essential central transducer, Rad53p, is dependent on POL2 and RAD9 in response to UV damage, indicating that RAD53 functions downstream of both these genes. Mutants defective for both pathways are severely deficient in Rad53p phosphorylation and RNR3 induction and are significantly more sensitive to DNA damage and replication blocks than single mutants alone. These results show that POL2 and RAD9 function in parallel branches for sensing and transducing the UV DNA damage signal. Each of these pathways subsequently activates the central transducers Mec1p/Esr1p/Sad3p and Rad53p/Mec2p/Sad1p, which are required for both cell-cycle arrest and transcriptional responses.