Plays a key role in the control of the eukaryotic cell cycle. Component of the kinase complex that phosphorylates the repetitive C-terminus of RNA polymerase II. 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] Phosphorylation at Thr-11 or Tyr-12 inactivates the enzyme, while phosphorylation at Thr-158 activates it. Found in most organs including root, young leaf, stem, vegetative meristem and flower bud. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.