Eukaryotic DNA topoisomerases: two forms of type I DNA topoisomerases from HeLa cell nuclei.

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Abstract
Two type I DNA topoisomerases were purified to homogeneity from the nuclei of HeLa [human cervical carcinoma] cells. One topoisomerase has a peptide MW of 100,000 and the other, a MW of 67,000. Several lines of evidence indicate that these topoisomerases are closely related. Both exhibit similar enzymatic activities on DNA. The chromatographic properties of the 2 topoisomerases during purification are similar. Mild proteolysis of the purified MW 100,000 topoisomerase in vitro generates a group of protein bands of MW .apprxeq.67,000, and these bands retain topoisomerase activity. The peptides formed by partial proteolysis of the 67,000 topoisomerase in the presence of NaDodSO4 [sodium dodecyl sulfate] form a subset of those produced from the 100,000 enzyme. The 100,000 topoisomerase is the major type I enzyme in the cell. The 67,000 topoisomerase, which may be identical to the previously identified nicking-closing enzyme is probably formed by proteolysis of the 100,000 enzyme.