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Published November 1, 1975 | public
Journal Article Open

Action of Nicking-Closing Enzyme on Supercoiled and Nonsupercoiled Closed Circular DNA: Formation of a Boltzmann Distribution of Topological Isomers


Highly purified nicking-closing enzyme from mouse cells in 20-fold enzyme/substrate excess converts closed circular native PM2, ColE1, and Minicol DNA into limit product sets of DNAs. Each set has a mean degree of supercoiling of approximately zero. The individual species in the sets differ by Delta tau = ± 1, ± 2, etc., and the relative masses fit a Boltzmann distribution. It was also demonstrated that ``nonsupercoiled'' closed circular duplex molecules serve as substrates for the nicking-closing enzyme, and that a distribution of topological isomers is generated. Polynucleotide ligase, acting on nicked circular DNA, forms under the same conditions, the same set of closed DNAs. The latter enzyme freezes the population into sets of molecules otherwise in configurational equilibrium in solution.

Additional Information

Copyright © 1975 by the National Academy of Sciences Contributed by Jerome Vinograd, August 25, 1975 We wish to thank H. W. Boyer, University of California, San Francisco, for generously providing the polynucleotide-ligase and the bacterial strains harboring plasmids and R. B. Watson for advice and assistance with the gel electrophoresis experiments. This work was supported in part by USPHS National Institutes of Health grants GM15327 and CA08014. D.E.P. is a recipient of a Medical Research Council of Canada fellowship. This is Contribution no. 5156 from the Division of Chemistry and Chemical Engineering.


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