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Molecular and Cellular Biology, August 1999, p. 5267-5278, Vol. 19, No. 8
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

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

Yuen Ho,1 Michael Costanzo,1 Lynda Moore,1 Ryuji Kobayashi,2 and Brenda J. Andrews1,*

Department of Molecular and Medical Genetics, University of Toronto, Toronto, Canada M5S 1A8,1 and Cold Spring Harbor Laboratory, Cold Spring Harbor, New York 117242

Received 19 March 1999/Returned for modification 28 April 1999/Accepted 30 April 1999

In Saccharomyces 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 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 G1 phase. In vivo accumulation of Stb1 phosphoforms was dependent on CLN1, CLN2, and CLN3, which encode G1-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 G1 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 G1 regulator CLN3, and they implicate Stb1 as an in vivo target of G1-specific cyclin-dependent kinases.

* Corresponding author. Mailing address: Department of Molecular and Medical Genetics, University of Toronto, Rm. 4285, Medical Sciences Building, 1 Kings College Circle, Toronto, Ontario, Canada M5S 1A8. Phone: (416) 978-8562. Fax: (416) 978-6885. E-mail: brenda.andrews{at}utoronto.ca.

Molecular and Cellular Biology, August 1999, p. 5267-5278, Vol. 19, No. 8
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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