[2] S-phase cyclin binding to Cdk1 directly stimulates DNA replication as well as progression to the next phase of the cell cycle.
[3] Clb5 and Clb6 are two of the six B-type cyclins in budding yeast, which contain a short, hydrophobic amino acid sequence that allows targeted degradation and phosphorylation of some proteins that regulate DNA replication.
This degradation occurs in late mitosis and is regulated by the anaphase promoting complex (APC).
[1] During S-phase, Clb5 and Clb6 are simultaneously expressed with other genes encoding proteins required for individual DNA strand replication and separation.
Upon commitment to cell division, G1/S cyclin levels rise, bind Cdk1, and immediately form active complexes.
[1] Thus, Clb5 and Clb6 are engaged in a positive feedback loop to promote their own activation during this period of the cell cycle.
[4] In double mutants for Clb5 and Clb6, the onset of S-phase (rather than the length) is significantly delayed, but will eventually occur as a side result of the buildup of other mitotic cyclins (Clb1-4).
In this hypothesis, cyclin concentrations must rise and accumulate to proceed to the next stage of the cell cycle.
[3] There is also evidence that Clb5 mutant cells are less likely to have DNA recombinations from double strand breaks (DSB), which may be a side effect of this Clb5 regulation.