Serine/threonine kinase involved in cell cycle control and in RNA polymerase II-mediated RNA transcription. Required for maintaining chromosome ploidy. May phosphorylate the large subunit of RNA polymerase II, ama-1. 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] Catalytic component which, in association with cyclin H (cyh-1) and mat1, is likely to form the CAK complex. RNAi-mediated knockdown results in an arrest at approximately 50-cell embryonic stage. The phenotype is more severe when done in a cdk7(ax224) mutant background where the embryonic arrest occurs at the 1-cell stage due to an arrest in early meiosis. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.