These tools will no longer be maintained as of December 31, 2024. Archived website can be found here. PubMed4Hh GitHub repository can be found here. Contact NLM Customer Service if you have questions.


Pubmed for Handhelds

PUBMED FOR HANDHELDS

Search MEDLINE/PubMed

  • Title: Evidence for an intermediate with a single-strand break in the reaction catalyzed by the DNA untwisting enzyme.
    Author: Champoux JJ.
    Journal: Proc Natl Acad Sci U S A; 1976 Oct; 73(10):3488-91. PubMed ID: 1068461.
    Abstract:
    The DNA untwisting enzyme relaxes covalently closed circylar DNAs by the sequential breaking (nicking) and closure of one strand of the duplex. The use of highly purified enzyme from rat liver nuclei at very high protein concentrations has permitted the detection of the nicked intermediate in the reaction. The nicking of closed circular simian virus 40 DNA was measured by alkaline sucrose gradient sedimentation or by equilibrium centrifugation in CsCl gradients containing propidium diiodide. The following observations support the hypothesis that the nicked DNA represents an intermediate in the untwisting reaction. The extent of nicking does not increase with time. Nicking is observed in the range of salt concentrations where the enzyme is active (0.01-0.25 M KCl), but is not observed at 0.50 Mkdl, where enzyme activity is undetectable. The nicked DNA that is generated during the reaction carried out in low salt rapidly disappears if the KCl concentration is raised to 0.50 M. At constant enzyme concentration, the number of nicks in the reaction mixture is independent of DNA concentration in the range from 3 to 14 mug/ml. The addition of an excess of unlabeled DNA to a reaction initially containing labeled nicked DNA partially chases the label from the nicked intermediate into covalently closed circular DNA.
    [Abstract] [Full Text] [Related] [New Search]