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Two type I DNA topoisomerases have been purified to homogeneity from the nuclei of HeLa cells. One topoisomerase has a peptide molecular weight of 100,000 and the other, a molecular weight of 67,000. Several lines of evidence indicate that these two topoisomerases are closely related, (a) Both exhibit similar enzymatic activities on DNA. (b) The chromatographic properties of the two topoisomerases during purification are similar. (c) Mild proteolysis of the purified molecular weight 100,000 topoisomerase in vitro generates a group of protein bands of molecular weight approximately 67,000, and these bands retain topoisomerase activity. (d) The peptides formed by partial proteolysis of the 67,000 topoisomerase in the presence of NaDodSO4 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 Champoux, J. J. & Dulbecco, R. (1972) Proc. Natl. Acad. Sci. USA 69, 143-146, is probably formed by proteolysis of the 100,000 enzyme.
Liu et al. (Mon,) studied this question.