The Spike of S Phase Cyclin Cig2 Expression at the G1–S Border in Fission Yeast Requires Both APC and SCF Ubiquitin Ligases

We describe a novel set of oscillation mechanisms for the fission yeast S phase cyclin Cig2, which contains an authentic destruction box and is destroyed at anaphase via the APC/cyclosome (APC/C). Unlike the mitotic cyclin Cdc13, however, Cig2 mRNA and protein peak at the G1/S boundary and decline to low levels in G2 and M phases. We show here that SCF Pop1, Pop2 plays a role in transcriptional periodicity, as pop mutations result in constitutive cig2 + transcripts. The instability of Cig2 during G2 and M is independent of either the APC/C or Pop1/Pop2, but requires Skp1, a core component of SCF. These data indicate that the APC/C and SCF control Cig2 levels differentially at different stages of the cell cycle.