Kinetics of cruciform formation and stability of cruciform structure in superhelical DNA.

This is a study of the kinetics of formation of a cruciform structure from the longest palindromic sequence in plasmid pAO3 DNA. DNA was prepared so as to be free of cruciforms even in topoisomers whose negative superhelicity was great enough to induce cruciform formation. Samples of such DNA were incubated at various temperatures, the incubation time varying over a wide range. Then the state was frozen by chilling. Two-dimensional electrophoretic analysis made it possible to estimate the fraction of molecules that got the cruciform structure during incubation. Precautions were taken for electrophoresis conditions to rule out any spontaneous conformational changes within the palindromic region. The relaxation time at the midpoint of the transition ranged from 30 min at 30 C to 50 hrs at 20 C, both in 0.1SSC. An increase in the negative superhelical density by 0.01 led to a 500-fold reduction of the relaxation time at 30 C but had little effect at 20 C. The probability of cruciform formation has been examined as a function of temperature. It has been shown that the cruciform state is no longer the predominant one at elevated temperatures: the cruciformation probability drops to an insignificant value for all of the topoisomers involved. Data have been obtained suggesting that the cruciform formation at the major palindromic site is not the only structural transition possible in pAO3 DNA.