Nbs1 is
required for the S-phase DNA damage checkpoint in fission yeast.
Sasirekha Sivakumar, Nick Rhind
Biochemitry, UMass Med School, 364 Plantation, Worcester, MA 01605, USA
(nick.rhind@umassmed.edu)
In response to DNA damage during S-phase, eukaryotic cells reduce their rate of replication. Failure of this response in humans correlates with chromosome instability and predisposition to a wide variety of early onset cancers. This checkpoint requires the Mre11-Rad50-Nbs1/Xrs2 (MRN) heterotrimer, a complex involved in recombinational repair of DNA damage. These results suggest that the checkpoint may act to coordinate recombinational repair with replication during S-phase. Another possibility is that MRN is involved in recognizing the damage and signaling to the checkpoint pathway. We have identified the fission yeast homolog of Nbs1 and shown that the fission yeast homologs of Mre11 (called Rad32), Rad50 and Nbs1 are required for the S-phase DNA damage checkpoint. However, they are not required for activation of the signaling pathway that recognizes the damage. Thus, in response to DNA damage during S-phase, MRN is not required to recognize damage, but rather it acts downstream and is involved in the mechanism that slows replication. We are currently testing the hypothesis that the slowing of replication involves checkpoint induction of replication-coupled recombination. As opposed to current models, in which the primary purpose of the checkpoint is to slow replication to allow time for repair, this model suggests that the checkpoint induces recombination, which slows replication as a secondary consequence.