Induction of a G1-S checkpoint in fission yeast.

Entry into S phase is carefully regulated and, in most organisms, under the control of a G(1)-S checkpoint. We have previously described a G(1)-S checkpoint in fission yeast that delays formation of the prereplicative complex at chromosomal replication origins after exposure to UV light (UVC). This checkpoint absolutely depends on the Gcn2 kinase. Here, we explore the signal for activation of the Gcn2-dependent G(1)-S checkpoint in fission yeast. If some form of DNA damage can activate the checkpoint, deficient DNA repair should affect the length of the checkpoint-induced delay. We find that the cell-cycle delay differs in repair-deficient mutants from that in wild-type cells. However, the duration of the delay depends not only on the repair capacity of the cells, but also on the nature of the repair deficiency. First, the delay is abolished in cells that are deficient in the early steps of repair. Second, the delay is prolonged in repair mutants that fail to complete repair after the incision stage. We conclude that the G(1)-S delay depends on damage to the DNA and that the activating signal derives not from the initial DNA damage, but from a repair intermediate(s). Surprisingly, we find that activation of Gcn2 does not depend on the processing of DNA damage and that activated Gcn2 alone is not sufficient to delay entry into S phase in UVC-irradiated cells. Thus, the G(1)-S delay depends on at least two different inputs.