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). 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-169 is required for enzymatic activity. Probably associates with cyclin-H (ccnh) and mat1 to form a multimeric active enzyme. Phosphorylation of Ser-163 during mitosis inactivates the enzyme. Phosphorylation of Thr-169 is required for activity. Phosphorylated at Ser-163 and Thr-169 by CDK2 (By similarity). Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.