Plays a key role in the control of the eukaryotic cell cycle by modulating the centrosome cycle as well as mitotic onset; promotes G2-M transition, and regulates G1 progress and G1-S transition via association with multiple interphase cyclins. Required in higher cells for entry into S-phase and mitosis. May play a role in regulating the amplitude of the cyclic expression of circadian clock genes. 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] Thr-14 and Tyr-15 are phosphorylated maximally during G2 phase, but dephosphorylated abruptly at the G2/M transition. Phosphorylation at Thr-14 and Tyr-15 inactivates the enzyme. During M phase it is also phosphorylated on Thr-161. Finally during G1 phase it is phosphorylated on Ser-277. Forms a stable but non-covalent complex with a regulatory subunit and with a cyclin. Interacts with catalytically active CCNB1 and RALBP1 during mitosis to form an endocytotic complex during interphase. Phosphorylation at Tyr-15 by WEE1 and WEE2 inhibits the protein kinase activity and acts negative regulator of entry into mitosis (G2 to M transition). Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.