The G₁ Cyclin Cln3 Promotes Cell Cycle Entry via the Transcription Factor Swi6

Abstract

In Saccharomyces cerevisiae (budding yeast), commitment to cell division in late G₁ is promoted by the G₁ cyclin Cln3 and its associated cyclin-dependent kinase, Cdc28. We show here that all known aspects of the function of Cln3 in G₁ phase, including control of cell size, pheromone sensitivity, cell cycle progress, and transcription, require the protein Swi6. Swi6 is a component of two related transcription factors, SBF and MBF, which are known to regulate many genes at the G₁-S transition. The Cln3-Cdc28 complex somehow activates SBF and MBF, but there was no evidence for direct phosphorylation of SBF/MBF by Cln3-Cdc28 or for a stable complex between SBF/MBF and Cln3-Cdc28. The activation also does not depend on the ability of Cln3 to activate transcription when artificially recruited directly to a promoter. The amino terminus and the leucine zipper of Swi6 are important for the ability of Swi6 to respond to Cln3 but are not essential for the basal transcriptional activity of Swi6. Cln3-Cdc28 may activate SBF and MBF indirectly, perhaps by phosphorylating some intermediary protein.

We thank Martine Lessard for excellent technical assistance; Kim Nasmyth for communicating unpublished results; and Kim Arndt, Brenda Andrews, Kim Nasmyth, and members of the Futcher laboratory for reagents.

This work was supported by grant RO1 GM39978 from the National Institutes of Health.