The G1-S checkpoint in fission yeast is not a general DNA damage checkpoint

Inhibitory mechanisms called checkpoints regulate progression of the cell cycle in the presence of DNA damage or when a previous cell-cycle event is not finished. In fission yeast exposed to ultraviolet light the G1-S transition is regulated by a novel checkpoint that depends on the Gcn2 kinase. The molecular mechanisms involved in checkpoint induction and maintenance are not known. Here we characterise the checkpoint further by exposing the cells to a variety of DNA-damaging agents. Exposure to methyl methane sulphonate and hydrogen peroxide induce phosphorylation of eIF2 beta, a known Gcn2 target, and an arrest in G1 phase. By contrast, exposure to psoralen plus long-wavelength ultraviolet light, inducing DNA adducts and crosslinks, or to ionizing radiation induce neither eIF2 alpha phosphorylation nor a cell-cycle delay. We conclude that the G1-S checkpoint is not a general DNA-damage checkpoint, in contrast to the one operating at the G2-M transition. The tight correlation between eIF2 alpha phosphorylation and the presence of a G1-phase delay suggests that eIF2 alpha phosphorylation is required for checkpoint induction. The implications for checkpoint signalling are discussed.