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] Expressed in the dividing region of the root apex and in differentiated cells such as those in the sclerenchyma, pericycle and parenchyma of the central cylinder. Expressed throughout the cell cycle. By submergence. Down-regulated by cytokinin. Phosphorylated at Thr-161 by CDKD-1. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.