We have purified and characterized topoisomerase I from Drosophila melanogaster. The molecular weight of the enzyme is 135,000; 100,000, 90,000, and 65,000 molecular weight products result from degradation of the enzyme. The enzyme relaxes both positive and negative supercoiled DNA. Mg++ is not absolutely required, but stimulates the enzymatic activity considerably.
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Javaherian et al. (1982) studied this question.