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 G 1 is promoted by the G 1 cyclin Cln3 and its associated cyclin-dependent kinase, Cdc28. We show here that all known aspects of the function of Cln3 in G 1 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 G 1 -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.

A Systematic Screen for Transcriptional Regulators of the Yeast Cell Cycle

“…Pramila et al (2006) showed that a Dhcm1 mutant spends longer in G 2 /M, consistent with our result (Table 1). Deletion of the G 1 transcription factors Swi4 (Gray et al 1997) and Swi6 (Wijnen et al 2002) have each been reported to result in an accumulation of cells in G 2 /M, which we also observed. In the case of the Dswi4 mutant, the G 2 /M phenotype is due to the fact the Dswi4 cells are defective in budding but not in DNA replication (Gray et al 1997).…”

Bck2 is a phase-independent activator of cell cycle-regulated genes in yeast

“…Most likely, this is due to overexpressed Cln3 being able to target the Clb/Cdc28 inhibitor Sic1 for degradation. 41 In contrast to GAL-CLN3, induction of GAL-BCK2 gave rise to cell cycle entry both in the SWI6 and in the swi6Δ background. The swi6Δ strain behaved similarly to the SWI6 counterpart, as judged by flow cytometry and budding index measurements.…”

“…To control for non-specific expression changes we used a double deletion cln3Δ bck2Δ strain, again kept alive by MET3pCLN2. Cln3 function requires Swi6, 41 whereas Bck2's role in gene expression is at least partially independent of SBF and MBF. 42,43 To investigate further the Swi6 requirement for Bck2-and Cln3-mediated gene expression, we performed similar analyses in strains deleted for SWI6.…”

Potential and flux landscapes quantify the stability and robustness of budding yeast cell cycle network

“…Checking the network, we found that at bistability k6 and k51 repress and activate Cdc20, respectively. From formal discussions and previous studies (26)(27)(28)(29), the presence of Cdc20 makes Cdh1 (Enemies) more active and represses CycB/CDK and cell exits miosis (G1 state again). This means that the system is inclined to be in the G1 state (monostable state) when Cdc20 increases, making the bistable state less stable.…”