Cyclin-dependent kinase inhibits reinitiation of a normal S-phase program during G2 in fission yeast

To achieve faithful replication of the genome once in each cell cycle, reinitiation of S phase is prevented in G(2) and origins are restricted from refiring within S phase. We have investigated the block to rereplication during G(2) in fission yeast. The DNA synthesis that occurs when G(2)/M cyclin-dependent kinase (CDK) activity is depleted has been assumed to be repeated rounds of S phase without mitosis, but this has not been demonstrated to be the case. We show here that on G(2)/M CDK depletion in G(2), repeated S phases are induced, which are correlated with normal G(1)/S transcription and attainment of doublings in cell size. Mostly normal mitotic S-phase origins are utilized, although at different efficiencies, and replication is essentially equal across the genome. We conclude that CDK inhibits reinitiation of S phase during G(2), and if G(2)/M CDK is depleted, replication results from induction of a largely normal S-phase program with only small differences in origin usage and efficiency.