Transcriptional activity of P-TEFb kinase in vivo requires the C-terminal domain of RNA polymerase II

Phosphorylation of the carboxyl-terminal domain (CTD) of RNA polymerase II (RNAPII) is an important step in transcription and the positive transcripti on elongation factor b (P-TEFb) has been proposed to facilitate elongation at many genes. The P-TEFb contains a catalytic subunit (Cdk9) that, in asso ciation with a cyclin subunit (cyclinT1): has the ability to phosphorylate the CTD substrate in vitro. Here, we demonstrate that cyclinT1/Cdk9-mediate d transcription requires CTD-containing RNAPII, suggesting that the CTD is the major target of the cyclinT1/Cdk9 complex in vivo. Unlike Cdk7 and Cdk8 , two other cyclin-dependent kinases that are capable of phosphorylating th e CTD in vitro, we found that only the Cdk9 activates gene expression in a catalysis-dependent manner. Finally, unlike cyclinT1 and T2, we found that. the targeted recruitment to promoter DNA of cyclinK (a recently described alternative partner of Cdk9) does not stimulate transcription in vivo. Coll ectively, our data strongly indicate that the P-TEFb kinase subunits cyclin T/Cdk9 are specifically involved in transcription and the CTD domain of RNA PII is the major functional target of this complex in vivo. (C) 2000 Elsevi er Science B.V. All rights reserved.