Serine/threonine kinase involved in cell cycle control and in RNA polymerase II-mediated RNA transcription. Cyclin-dependent kinases (CDKs) are activated by the binding to a cyclin and mediate the progression through the cell cycle. Each different complex controls a specific transition between 2 subsequent phases in the cell cycle. Required for both activation and complex formation of cdk1/cyclin-B during G2-M transition, and for activation of cdk2/cyclins during G1-S transition (but not complex formation). cdk7 is the catalytic subunit of the CDK-activating kinase (CAK) complex. CAK activates the cyclin-associated kinases cdk1, cdk2, cdk4 and cdk6 by threonine phosphorylation, thus regulating cell cycle progression. CAK complexed to the core-TFIIH basal transcription factor activates RNA polymerase II by serine phosphorylation of the repetitive C-terminal domain (CTD) of its large subunit (polr2a), allowing its escape from the promoter and elongation of the transcripts (By similarity). Involved in negative regulation of the meiotic maturation of Xenopus oocytes. 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-176 is required for enzymatic activity. Probably associates with cyclin-H (ccnh) and mat1 to form a multimeric active enzyme. In the prominent nucleus of oocytes. Expressed only in non-mature oocytes. Phosphorylation of Ser-170 during mitosis inactivates the enzyme. Phosphorylation of Thr-176 is required for activity. Phosphorylated at Ser-170 and Thr-176 by CDK2 (By similarity). Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.