CLB5-Dependent Activation of Late Replication Origins in S. cerevisiae

Abstract

Replication origins in chromosomes are activated at specific times during the S phase. We show that the B-type cyclins are required for proper execution of this temporal program. clb5 cells activate early origins but not late origins, explaining the previously described long clb5 S phase. Origin firing appears normal in cIb6 mutants. In clb5 clb6 double mutant cells, the late origin firing defect is suppressed, accounting for the normal duration of the phase despite its delayed onset. Therefore, Clb5p promotes the timely activation of early and late origins, but Clb6p can activate only early origins. In clb5 clb6 mutants, the other B-type cyclins (Clb1-4p) promote an S phase during which both early and late replication origins fire.

Swe1 regulation and transcriptional control restrict the activity of mitotic cyclins toward replication proteins in Saccharomyces cerevisiae

“…Previous studies have shown that late origin firing is defective in cells lacking Clb5 and that early expression of Clb2 does not compensate for lack of Clb5 (12,25). However, we suspected that regulation by Swe1 could explain the failure of Clb2 to activate late origins in the previous study.…”

“…In clb5⌬ clb6⌬ and clb5⌬ clb6⌬ swe1⌬ cells, ARS603 initiated with efficiency similar to wild-type cells; thus, late origin activation can be attributed to one or more of the mitotic cyclins present (Clb1-Clb4) (Fig. 2C) (25). In clb5::CLB2 clb6⌬ swe1⌬ cells, the early S-phase entry wholly depends on Clb2 ( Fig.…”

Chk1–cyclin A/Cdk1 axis regulates origin firing programs in mammals

“…It remains to be determined whether the Chk1-Cdc25 axis regulates a particular set of cyclinCdk complexes or multiple cyclin-Cdk complexes to complete the S phase program. Clb5-dependent Cdk activity was found to be indispensable for activation of late origins in budding yeast, showing that Clb5-deficient cells failed to activate these origins and subsequently showed an extended S phase (Donaldson et al 1998). Interestingly, this late origin firing defect is suppressed although entry into S phase and is significantly delayed in Clb5/Clb6 double-mutant cells, suggesting that other B-type cyclins promote firing of both early and late replication origins.…”

“…In budding yeast, Cdk phosphorylation of an initiation factor, Sld2/Drc1, is also critical for the initiation of DNA replication (Masumoto et al 2002). In contrast to those involved in early origin firing, trans-factors that regulate late origin firing have been less clearly identified, although Clb5-dependent Cdk activity is known to be indispensable for activation of late replication origins in budding yeast (Donaldson et al 1998;Noguchi et al 2002). These results suggest the existence of a specific trans-factor(s) for late origin activation as there is for early origin.…”

Perpetuating the double helix: molecular machines at eukaryotic DNA replication origins

“…Inhibition of pre-IC formation at late origins could be mediated by CDKs and DDK, which are essential for temporally regulated origin firing. (101)(102)(103) In fact, a recent study has shown that etoposideinduced DNA damage triggered a checkpoint response in Xenopus that inactivated DDK, preventing Cdc45 from loading onto chromatin. (104) More studies will be required to unravel the possible checkpoint functions of the proteins that control initiation of DNA replication.…”