CDK-activating kinase that modulates CDKD-2 and CDKD-3 activities by phosphorylation of the T-loop. Activates CDKD-2 C-terminal domain (CTD) kinase activity. Activates CDKA-1 probably by phosphorylation. Posseses a CDK kinase activity independently of association with cyclin CYCH1-1. Phosphorylates the 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] Highly expressed in suspension cell culture. Expressed at low levels in all plant organs. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.