Large-Scale Stable Opening of Supercoiled DNA in Response to Temperature and Supercoiling in (A + T)-Rich Regions that Promote Low-Salt Cruciform Extrusion

Bowater, Richard, Aboul-ela, Fareed and Lilley, David M.J. (1991) Large-Scale Stable Opening of Supercoiled DNA in Response to Temperature and Supercoiling in (A + T)-Rich Regions that Promote Low-Salt Cruciform Extrusion. Biochemistry, 30 (49). pp. 11495-11506. ISSN 0006-2960

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Abstract

We have studied the properties of (A + T)-rich sequences derived from ColE1 that promote cruciform extrusion at low ionic strength in supercoiled plasmids. We compared the chemical reactivity of the sequences in negatively supercoiled DNA (using osmium tetroxide and bromoacetaldehyde) with the results of two-dimensional gel electrophoresis performed under the same conditions. Taken together, the results indicate the occurrence of cooperative helix-coil transitions in the (A + T)-rich DNA at low ionic strength, to form stable, denatured regions. The extent of the open region is a function of temperature and superhelix density, with an additional local destabilization brought about by the presence of cruciform structures. We present a simple statistical mechanical model of the helix-coil transition in the (A + T)-rich DNA, from which we have obtained estimates of the free energy for average base-pair opening of 0.31 kcal mol-1 and that for the formation of a helix-coil junction of 4.9 kcal mol-1, in 45 mM Tris-borate, pH 8.3, 0.5 mM EDTA. The results offer a model for the C-type mechanism of cruciform extrusion. Inverted repeats that are incorporated into the melted region undergo hairpin loop formation below 50 °C, and upon closure of the melted region, by reduction of temperature or increased ionic strength, they remain as a fully extruded cruciform structure.

Item Type:	Article
Uncontrolled Keywords:	biochemistry ,/dk/atira/pure/subjectarea/asjc/1300/1303
Related URLs:	https://www.scopus.com/pages/publication...
Depositing User:	LivePure Connector
Date Deposited:	26 May 2026 13:24
Last Modified:	31 May 2026 05:28
URI:	https://ueaeprints.uea.ac.uk/id/eprint/103152
DOI:	10.1021/bi00113a003

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