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Slow Cruciform Transitions in Palindromic DNA
Martin Gellert, Mary H. O'Dea and Kiyoshi Mizuuchi
Proceedings of the National Academy of Sciences of the United States of America
Vol. 80, No. 18, [Part 1: Biological Sciences] (Sep. 15, 1983), pp. 5545-5549
Published by: National Academy of Sciences
Stable URL: http://www.jstor.org/stable/14375
Page Count: 5
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Extrusion of cruciform structures in self-complementary regions of DNA is known to be favored by negative supercoiling of DNA. We show here that, in moderately supercoiled DNA, cruciform extrusion is a very slow process. In plasmid pUC7 DNA, with a 48-base-pair palindrome, the half-time of extrusion at 50 degrees C is typically several hours; rates are even slower at lower temperature. The rates increase significantly with increasing DNA supercoiling but are only slightly faster in DNA species with much longer palindromes. The reabsorption of cruciform arms is also very slow. The equilibrium between cruciform and regular DNA structures is sensitive to changes in the linking number. Measurement of this equilibrium leads to an estimate of 18 kcal/mol (75.3 kJ/mol) for the free energy required to generate a cruciform structure. In bacterial cells, cruciform DNA may be rare, even when it is thermodynamically favored, because of its slow formation.
Proceedings of the National Academy of Sciences of the United States of America © 1983 National Academy of Sciences