THE CDC7 PROTEIN-KINASE IS REQUIRED FOR ORIGIN FIRING DURING S-PHASE

The Cdc7p protein kinase plays an essential, but undefined, role promo ting S phase in the budding yeast, Saccharomyces cerevisiae. Previous experiments have shown that the essential function of Cdc7 is executed near the G(1)-S boundary; after Start but before the elongation phase of DNA replication. Origins of DNA replication fire throughout S phas e in budding yeast. Therefore, the G(1)-S transition is a cell-cycle e vent that precedes, and is distinct from, the activation of individual origins. Consequently, we have asked whether Cdc7 is only required fo r S-phase entry or if it plays a role during S phase in origin firing. In this article, we show that partial loss of Cdc7 function results i n slow progression through S phase rather than slow entry into S phase and that Cdc7 is still required for the timely completion of S phase after a block to elongation with hydroxyurea. This is because Cdc7 is still required for the activation of late-firing origins after the hyd roxyurea block. These experiments show that, rather than acting as a g lobal regulator of the G(1)-S transition, Cdc7 appears to play a more direct role in the firing of replication origins during S phase.