The Transcription Factor Swi5 Regulates Expression of the Cyclin Kinase Inhibitor p40^SIC1

Abstract

DNA replication in budding yeast cells depends on the activation of the Cdc28 kinase (Cdkl of Saccharomyces cerevisiae) associated with B-type cyclins Clb1 to Clb6. Activation of the kinase depends on proteolysis of the Cdk inhibitor p40^SIC1 in late G₁, which is mediated by the ubiquitin-conjugating enzyme Cdc34 and two other proteins, Cdc4 and Cdc53. Inactivation of any one of these three proteins prevents p40^SIC1 degradation and causes cells to arrest in G₁, with active Cln kinases but no Clb-associated Cdc28 kinase activity. Deletion of SIC1 allows these mutants to replicate. p40^SIC1 disappears at the G₁/S transition and reappears only after nuclear division. Cell cycle-regulated proteolysis seems largely responsible for this pattern, but transcriptional control could also contribute; SIC1 RNA accumulates to high levels as cells exit M phase. To identify additional factors necessary for the inhibition of the Cdk1/Cdc28 kinase in G₁, we isolated mutants that can replicate DNA in the absence of Cdc4 function. Mutations in three loci (SIC1, SWI5, and RIC3) were identified. We have shown that high SIC1 transcript levels at late M phase depend on Swi5. Swi5 accumulates in the cytoplasm during S, G₂, and M phases of the cell cycle but enters the nuclei at late anaphase. Our data suggest that cell cycle-regulated nuclear accumulation of Swi5 is responsible for the burst of SIC1 transcription at the end of anaphase. This transcriptional control may be important for inactivation of the Clb/Cdkl kinase in G₂/M transition and during the subsequent G₁ period.