Factors Controlling Promoter-Proximal Pausing By Rna Polymerase Ii
Most gene expression is regulated at the level of transcription, and the transition from initiation to productive elongation is a key point of regulation. This transition is accompanied by pausing of transcriptionally engaged polymerase in the promoter-proximal region of several heat shock genes. Although this mechanism of regulation was long thought to be limited to a few genes, recent evidence has indicated that pausing is wide-spread in higher eukaryotes. Therefore, it is increasingly important to understand the mechanisms controlling the paused polymerase. I have investigated how the site of pausing on Hsp70 is specified using highresolution mapping of polymerase on reporter genes with shifted pausing site sequences. The results indicate that the downstream sequence dictates pause position and the overall level of pausing. I have also used RNAi knock-down in Drosophila cell culture to study the roles of several factors in establishing, maintaining, and releasing the paused polymerase. These experiments have shown GAGA factor is required for pausing on many of its target genes, and the knock-down effects indicate it is involved in establishing the pause. In contrast, Spt5, a protein previously shown to enhance pausing in vitro, reduces pausing genome-wide by increasing levels of elongating polymerase. Two kinases, P-TEFb and CDK12, function in productive elongation. Previously our lab showed that P-TEFb inhibition prevented the transition into elongation, limiting the polymerase to the 5' end of the heat shock-induced Hsp70 gene. I mapped these polymerases in high resolution to show they occupied sites further downstream than the normal pause sites, suggesting P-TEFb activity may not solely release the paused polymerase. I also determined the localization of CDK12 on active genes. Its localization downstream of P-TEFb suggests that these kinases may have distinct functions. Finally, I have examined the role of Fcp1 in Hsp70 transcription. Our lab previously showed the CTD phosphatase Fcp1 was required for optimum expression of Hsp70 mRNA. Fcp1 knock-down reduced the heat shock levels of Pol II and increased phosphorylation of nonchromatin bound Pol II, indicating that Fcp1 recycling of RNA polymerase II to an initiationcompetent form is required for optimal Hsp70 heat shock transcription.
promoter-proximal pausing; transcription regulation; RNA polymerase II
Lis, John T
Roberts, Jeffrey Warren; Pleiss, Jeffrey A.
Molecular and Cell Biology
Ph. D., Molecular and Cell Biology
Doctor of Philosophy
dissertation or thesis