CDK-activating kinase that may control G1/S phase progression. May control the rate of cell differentiation to accomplish proper development of organs, or in response to a changing environment. Forms a complex with cyclin CYCH1-1 that phosphorylates CDKA-1 and the C-terminal domain (CTD) of the large subunit of RNA polymerase II. 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] Interacts with CYCH1-1. Expressed in actively dividing cells of roots, leaves and shoots. Expressed in the intercalary meristem and the elongation zone of internodes. Expression reaches a peak in the G1/S phases and then decreases in the G2/M phases. By gibberellic acid (GA3) and submergence in the meristematic zone of internodes. Down-regulated by auxin. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.