Protein kinase involved in the regulation of transcription. Member of the cyclin-dependent kinase pair (CDK9/cyclin-T) complex, also called positive transcription elongation factor b (P-TEFb), which facilitates the transition from abortive to productive elongation by phosphorylating the CTD (C-terminal domain) of the large subunit of RNA polymerase II (RNAP II) polr2a, supt5h and rdbp. This complex is inactive when in the 7SK snRNP complex form. Regulates cytokine inducible transcription networks by facilitating promoter recognition of target transcription factors. P-TEFb is also involved in cotranscriptional histone modification, mRNA processing and mRNA export. 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] Component of the super elongation complex (SEC). Associates with ccnt1/cyclin-T1, ccnt2/cyclin-T2 or ccnk/cyclin-K to form active P-TEFb. Expressed from 24 hours post-fertilization (hpf) (PubMed:26542022). Expression increases between 24 and 72 hpf and then decreases towards 120 hpf (PubMed:26542022). In embryonic hearts expression peaks at 48 hpf and again at 96 hpf, decreasing at 120 hpf to levels then detected in adult hearts (PubMed:26542022). Morpholino knockdown of the protein causes in larvae causes high levels of lethality. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.