Different cyclin types collaborate to reverse the S-phase checkpoint and permit prompt mitosis.

Precise timing coordinates cell proliferation with embryonic morphogenesis. As Drosophila melanogaster embryos approach cell cycle 14 and the midblastula transition, rapid embryonic cell cycles slow because S phase lengthens, which delays mitosis via the S-phase checkpoint. We probed the contributions of each of the three mitotic cyclins to this timing of interphase duration. Each pairwise RNA interference knockdown of two cyclins lengthened interphase 13 by introducing a G2 phase of a distinct duration. In contrast, pairwise cyclin knockdowns failed to introduce a G2 in embryos that lacked an S-phase checkpoint. Thus, the single remaining cyclin is sufficient to induce early mitotic entry, but reversal of the S-phase checkpoint is compromised by pairwise cyclin knockdown. Manipulating cyclin levels revealed that the diversity of cyclin types rather than cyclin level influenced checkpoint reversal. We conclude that different cyclin types have distinct abilities to reverse the checkpoint but that they collaborate to do so rapidly.