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 express ed in G2 and M phases. We find that the CLBs play a central role in th e transition from CLNs to CLBs: the CLBs stimulate their own expressio n while repressing that of CLNs. This negative regulation of CLNs may occur via the transcription factor SW14, because CLBs are necessary fo r G2 repression of SCB-regulated genes like CLN1 and CLN2 but not for repression of MCB-regulated genes like DNA polymerase and CLBS. Furthe rmore, SW14 associates with CLB2 protein and is a substrate for the CL B2-associated CDC28 kinase in vitro.