Component of the positive transcription elongation factor b (P-TEFb) which consists of cdk9 and pch1, and which phosphorylates the C-terminal domain (CTD) of RNA polymerase II and spt5. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] [DNA-directed RNA polymerase] + ATP = ADP + H(+) + phospho-[DNA-directed RNA polymerase] May be activated by autophosphorylation or phosphorylation by a separate activating kinase. Interacts with pch1 cyclin via its N-terminal domain. Via its C-terminal domain, interacts with RNA triphosphatase pct1 which is involved in mRNA capping. Interacts also with pcm1. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.