Environmental influences on DNA superhelicity. The effect of ionic strength on superhelix conformation in solution

George W. Brady, Michael Satkowski, David Foos, Craig J. Benham

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21 Scopus citations

Abstract

The techniques of small-angle X-ray scattering and analysis that have been developed by the authors are used to investigate the influence of ionic strength on the superhelical conformation of native COP608 plasmid DNA in solution. For salt concentrations below 0.1 m, the superhelicity is partitioned between twisting (Tw) and writhing (Wr) in the ratio Δ Tw Wr = 2. Near the physiological salt concentration, [Na+] = 0.2 m, a co-operative transition is observed in which the pitch angle of the toroidal superhelix is drastically decreased. This results in an almost complete relaxation of writhe. At salt concentrations in excess of the threshold for this transition, the superhelical partitioning occurs in the ratio Δ Tw Wr > 25. Energetic considerations support the suggestion that this transition results from co-operative, superhelical B to Z transconformation reactions at susceptible sites. A method is discussed that will enable the direct measurement of this secondary structural transition by means of X-ray scattering.

Original languageEnglish
Pages (from-to)185-191
Number of pages7
JournalJournal of Molecular Biology
Volume195
Issue number1
DOIs
StatePublished - 5 May 1987
Externally publishedYes

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