Catalytic part of CAK which activates cyclin-associated CDK1/CDK2/CDK4 by threonine phosphorylation, thereby allowing MPF activation. 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] Probably associates with cyclin H and mat1 to form a multimeric active enzyme. Only expressed during vegetative cell growth. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.