Interaction between P-TEFb and the C-terminal domain of RNA polymerase II activates transcriptional elongation from sites upstream or downstream of target genes.

Transcriptional elongation by RNA polymerase II (RNAPII) is regulated by the positive transcription elongation factor b (P-TEFb). P-TEFb is composed of Cdk9 and C-type cyclin T1 (CycT1), CycT2a, CycT2b, or CycK. The role of the C-terminal region of CycT1 and CycT2 remains unknown. In this report, we demonstrate that these sequences are essential for the activation of transcription by P-TEFb via DNA, i.e., when CycT1 is tethered upstream or downstream of promoters and coding sequences. A histidine-rich stretch, which is conserved between CycT1 and CycT2 in this region, bound the C-terminal domain of RNAPII. This binding was required for the subsequent expression of full-length transcripts from target genes. Thus, P-TEFb could mediate effects of enhancers on the elongation of transcription.