Serine/threonine-protein kinase (By similarity). Involved in the control of the cell cycle. Required for entry into S-phase and mitosis (By similarity). Probable component of the kinase complex that phosphorylates the repetitive C-terminus of RNA polymerase II (By similarity). 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-17 or Tyr-18 inactivates the enzyme, while phosphorylation at Ser-230 activates it. May form a complex composed of at least the catalytic subunit CRK2 and a cyclin. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.