The yeast mitotic cyclin Clb2 cannot substitute for S phase cyclins in replication origin firing

Cyclin-dependent kinases (CDKs) drive the cell cycle, central to which is the accurate control of chromosome replication. In Saccharomyces cerevisiae, six closely related B-type cyclins (Clb1–6) drive the events of S phase and mitosis. Either Clb5 or Clb6 can activate early-firing replication origins, whereas only Clb5 can activate late origins. Clb1–4 are expressed later in the cell cycle. Whether Clb cyclins differ only in timing of expression, or else impart different kinase specificities is under ongoing investigation. This study shows that the expression of Clb2 during S phase in cells lacking Clb5 failed to rescue late origin activation. Early expression of Clb2 in cells lacking both Clb5 and Clb6 did not activate early origins on schedule to restore the correct S phase entry time. Therefore, Clb2 cannot drive timely activation of either early or late replication origins, demonstrating that Clb2-directed CDK has a specificity distinct from that driven by Clb5 and Clb6.