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 CYCD4-1 and CKS1. Expressed in root tips, shoot apical meristem, leaf primordia vascular tissues and tapetum of anthers. Expressed from early G2 phase and increases to reach a peak at mitosis. Protein abundance may be regulated through proteasome-mediated protein degradation. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.