👤 Michael Andrews

In Saccharomyces cerevisiae, gene expression in the late G(1) phase is activated by two transcription factors, SBF and MBF. SBF contains the Swi4 and Swi6 proteins and activates the transcription of G(1) cyclin genes, cell wall biosynthesis genes, and the HO gene. MBF is composed of Mbp1 and Swi6 and activates the transcription of genes required for DNA synthesis. Mbp1 and Swi4 are the DNA binding subunits for MBF and SBF, while the common subunit, Swi6, is presumed to play a regulatory role in both complexes. We show that Stb1, a protein first identified in a two-hybrid screen with the transcriptional repressor Sin3, binds Swi6 in vitro. The STB1 transcript was cell cycle periodic and peaked in late G(1) phase. In vivo accumulation of Stb1 phosphoforms was dependent on CLN1, CLN2, and CLN3, which encode G(1)-specific cyclins for the cyclin-dependent kinase Cdc28, and Stb1 was phosphorylated by Cln-Cdc28 kinases in vitro. Deletion of STB1 caused an exacerbated delay in G(1) progression and the onset of Start transcription in a cln3Delta strain. Our results suggest a role for STB1 in controlling the timing of Start transcription that is revealed in the absence of the G(1) regulator CLN3, and they implicate Stb1 as an in vivo target of G(1)-specific cyclin-dependent kinases. Show less

In budding yeast, entry into the mitotic cell cycle, or Start, requires the Cdc28 cyclin-dependent kinase (Cdk) and one of its three associated G1 cyclins, Cln1, Cln2, or Cln3. In addition, two other G1 cyclins, Pcl1 and Pcl2, associate with a second Cdk, Pho85, to contribute to Start. Although Pho85 is not essential for viability, Pcl1,2-Pho85 kinase complexes become essential for Start in the absence of Cln1,2-Cdc28 kinases. In addition, Pho85 interacts with a third cyclin, Pho80, to regulate acid phosphatase gene expression. Other cellular roles for Pho85 cyclin-Cdk complexes are suggested by the multiple phenotypes associated with deletion of PHO85, in addition to Start defects and deregulated acid phosphatase gene expression. Strains with pho80, pcl1, and pcl2 deletions show only a subset of the pho85 mutant phenotypes, suggesting the existence of additional Pho85 cyclins (Pcls). We used two-hybrid screening and database searching to identify seven additional cyclin-related genes that may interact with Pho85. We found that all of the new genes encode proteins that interacted with Pho85 in an affinity chromatography assay. One of these genes, CLG1, was previously suggested to encode a cyclin, based on the protein's sequence homology to Pcl1 and Pcl2. We have named the other genes PCL5, PCL6, PCL7, PCL8, PCL9, and PCL10. On the basis of sequence similarities, the PCLs can be divided into two subfamilies: the Pcl1,2-like subfamily and the Pho80-like subfamily. We found that deletion of members of the Pcl1,2 class of genes resulted in pronounced morphological abnormalities. In addition, we found that expression of one member of the Pcl1,2 subfamily, PCL9, is cell cycle regulated and is decreased in cells arrested in G1 by pheromone treatment. Our studies suggest that Pho85 associates with multiple cyclins and that subsets of cyclins may direct Pho85 to perform distinct roles in cell growth and division. Show less