CLB5 and CLB6, a new pair of B cyclins involved in DNA replication in Saccharomyces cerevisiae.

The functions of the Cdc28 protein kinase in DNA replication and mitosis in Saccharomyces cerevisiae are thought to be determined by the type of cyclin subunit with which it is associated. G1-specific cyclins encoded by CLN1, CLN2, and CLN3 are required for entry into the cell cycle (Start) and thereby for S phase, whereas G2-specific B-type cyclins encoded by CLB1, CLB2, CLB3, and CLB4 are required for mitosis. We describe a new family of B-type cyclin genes, CLB5 and CLB6, whose transcripts appear in late G1 along with those of CLN1, CLN2, and many genes required for DNA replication. Deletion of CLB6 has little or no effect, but deletion of CLB5 greatly extends S phase, and deleting both genes prevents the timely initiation of DNA replication. Transcription of CLB5 and CLB6 is normally dependent on Cln activity, but ectopic CLB5 expression allows cells to proliferate in the absence of Cln cyclins. Thus, the kinase activity associated with Clb5/6 and not with Cln cyclins may be responsible for S-phase entry. Clb5 also has a function, along with Clb3 and Clb4, in the formation of mitotic spindles. Our observation that CLB5 is involved in the initiation of both S phase and mitosis suggests that a single primordial B-type cyclin might have been sufficient for regulating the cell cycle of the common ancestor of many, if not all, eukaryotes.