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Protein kinases play a central role in controlling the cellular metabolism of living organisms. A protein kinase was purified from etiolated oat seedlings by several steps of ion-exchange and affinity chroma-tographies. The kinase was a 150-kDa tetrameric pro-tein and composed of three subunits of 34, 37, and 40 kDa proteins. The 34 and 40 kDa proteins had ATP binding sites, suggesting that they are catalytic sub-units and that the 37-kDa protein is a regulatory sub-unit. In the in vitro phosphorylation of a crude oat cell extract, it intensively phosphorylated a serine residue of a 110-kDa protein. The 110-kDa protein was tenta-tively identified as a DNA topoisomerase I, based on an amino acid sequence homology. Phosphorylation of the 110-kDa protein by the kinase required ATP or GTP as a phosphoryl group donor. The kinase activity was inhibited by 50% at a concentration of 0.05 mg/ml heparin. These results, therefore, indicate that the puri-fied kinase is a CK II protein kinase and may be invol-ved in the regulation of DNA topoisomerase I activity.