Transcriptional regulation by targeted recruitment of cyclin-dependent CDK9 kinase in vivo

The CDK9 kinase in association with Cyclin T is a component of the transcri ption positive-acting complex pTEFb which facilitates the transition from a bortive to productive transcription elongation by phosphorylating the carbo xyl-terminal domain of RNA polymerase II. The Cyclin T1/CDK9 complex is imp licated in Tat transactivation, and it has been suggested that Tat function s by recruiting this complex to RNAPII through cooperative binding to RNA. Here, we demonstrate that targeted recruitment of Cyclin T1/CDK9 kinase com plex to specific promoters, through fusion to a DNA-binding domain of eithe r Cyclin T1 or CDK9 kinase, stimulates transcription in vivo. Transcription al enhancement was dependent on active CDK9, as a catalytically inactive fo rm had no transcriptional effect. We determined that, unlike conventional a ctivators, DNA-bound CDK9 does not activate enhancerless TATA-promoters unl ess TBP is overexpressed, suggesting that CDK9 acts in vivo at a step subse quent to TFIID recruitment DNA-bound, Finally, we determined that CDK9-medi ated transcriptional activation is mediated by preferentially stimulating p roductive transcription elongation.