Mechanisms that help the yeast cell cycle clock tick: G2 cyclins transcriptionally activate G2 cyclins and repress G1 cyclins.

In budding yeast, G1 cyclins such as CLN1 and CLN2 are expressed in G1 and S phases, while mitotic cyclins such as CLB1 and CLB2 are expressed in G2 and M phases. We find that the CLBs play a central role in the transition from CLNs to CLBs: the CLBs stimulate their own expression while repressing that of CLNs. This negative regulation of CLNs may occur via the transcription factor SWI4, because CLBs are necessary for G2 repression of SCB-regulated genes like CLN1 and CLN2 but not for repression of MCB-regulated genes like DNA polymerase and CLB5. Furthermore, SW14 associates with CLB2 protein and is a substrate for the CLB2-associated CDC28 kinase in vitro.