CLB5-DEPENDENT ACTIVATION OF LATE REPLICATION ORIGINS IN SACCHAROMYCES-CEREVISIAE

Replication origins in chromosomes are activated at specific times dur ing the S phase. We show that the B-type cyclins are required for prop er execution of this temporal program. clb5 cells activate early origi ns but not late origins, explaining the previously described long clb5 S phase. Origin firing appears normal in clb6 mutants. In clb5 clb6 d ouble mutant cells, the late origin firing defect is suppressed, accou nting for the normal duration of the phase despite its delayed onset. Therefore, Clb5p promotes the timely activation of early and late orig ins, but Clb6p can activate only early origins. In clb5 clb6 mutants, the other B-type cyclins (Clb1-4p) promote an S phase during which bot h early and late replication origins fire.