Abstract #44

Transcriptional regulation at Start by the G1 cyclin Cln3. Hong-yin Wang, Ying Cai, Bruce Futcher. Microbiology, Stony Brook University, Stony Brook, NY.
   In S. cerevisiae, commitment to the cell cycle occurs at an event called “Start” in G1 phase. At Start, 100 to 200 genes (CLN1, CLN2, CLB5, RNR1, etc.) become activated at the transcriptional level. The transcription factors for these genes are a pair of closely-related factors called SBF (composed of Swi4 and Swi6) and MBF (composed of Mbp1 and Swi6). These two transcription factors are activated at Start in a way that depends on the Cln3-Cdc28 protein kinase complex, and yet there is no clear evidence that direct phosphorylation of either transcription factor is involved. Instead, Cln3-Cdc28 phosphorylates a repressor called Whi5; this leads to the loss of Whi5 from the promoter, and de-repression of SBF- and MBF-regulated genes (Costanzo et al. 2004; de Bruin et al. 2004). Here, we have enquired further into this transcriptional control. We find that even in whi5 null mutants, Start is sensitive to the expression of CLN3, showing that Whi5 is not the only route by which Cln3-Cdc28 affects Start. Using chromatin immunoprecipitation (ChIP), we have monitored transcription-related events at the CLN2 promoter. In the absence of CLN3 expression, we find that SBF, Whi5, and Sin3 (the targeting component of the Rpd3 histone deacetylase complex) are all anchored over the SBF binding sites of the CLN2 promoter. When CLN3 becomes expressed, the Cln3 protein itself becomes associated with the CLN2 promoter. Shortly thereafter, both Sin3 and Whi5 disappear from the promoter. Whether the loss of Sin3 depends on the loss of Whi5, or represents an independent regulatory pathway, is under investigation. After the loss of Sin3 and Whi5, RNA polymerase becomes associated with the promoter, and transcription begins.

Return to YGM 2006 Home at SGD