THE SCHIZOSACCHAROMYCES-POMBE S-PHASE CHECKPOINT DIFFERENTIATES BETWEEN DIFFERENT TYPES OF DNA-DAMAGE

We have identified an S-phase DNA damage checkpoint in Schizosaccharom yces pombe. This checkpoint is dependent on Rad3, the S. pombe homolog of the mammalian ATM/ATR checkpoint proteins, and Cds1. Cds1 had prev iously been believed to be involved only in the replication checkpoint . The requirement of Cds1 in the DNA damage checkpoint suggests that C ds1 may be a general target of S-phase checkpoints. Unlike other check points, the S. pombe S-phase DNA damage checkpoint discriminates betwe en different types of damage. W-irradiation, which causes base modific ation that can be repaired during G1 and S-phase, invokes the checkpoi nt, while gamma-irradiation, which causes double-stranded breaks that cannot be repaired by a haploid cell if induced before replication, do es not invoke the checkpoint. Because the same genes are required to r espond to UV- and gamma-irradiation during G2, this discrimination may represent an active suppression of the gamma response during S-phase.