By phosphorylating negative elongation factors and the C-terminal domain of RNA
polymerase II (RNAPII), positive transcription elongation factor b (P-TEFb), which is
composed of CycT1 or CycT2 and CDK9, activates eukaryotic transcription elongation. In
growing cells, it is found in active and inactive forms. In the former, free P-TEFb is a potent
transcriptional coactivator. In the latter, it is inhibited by HEXIM1 or HEXIM2 in the 7SK small
nuclear ribonucleoprotein (snRNP), which contains, additionally, 7SK snRNA, methyl …