Regulation of Transcription at the Saccharomyces cerevisiae Start Transition by Stb1, a Swi6-Binding Protein

InSaccharomyces cerevisiae , gene expression in the late G1 phase is activated by two transcription factors, SBF and MBF. SBF contains the Swi4 and Swi6 proteins and activates the transcription of G1 cyclin genes, cell wall biosynthesis genes, and theHO 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. TheSTB1 transcript was cell cycle periodic and peaked in late G1 phase. In vivo accumulation of Stb1 phosphoforms was dependent onCLN1 ,CLN2 , andCLN3 , which encode G1 -specific cyclins for the cyclin-dependent kinase Cdc28, and Stb1 was phosphorylated by Cln-Cdc28 kinases in vitro. Deletion ofSTB1 caused an exacerbated delay in G1 progression and the onset of Start transcription in acln3 Δ strain. Our results suggest a role forSTB1 in controlling the timing of Start transcription that is revealed in the absence of the G1 regulatorCLN3 , and they implicate Stb1 as an in vivo target of G1 -specific cyclin-dependent kinases.