Cruciform transitions in DNA.

The rates of transition between the cruciform and linear conformations of a perfectly inverted repeated lac operator DNA sequence have been measured using a trimethylpsoralen intrastrand cross-linking assay. The rate and extent of the linear to cruciform transition were dependent on temperature and on the superhelical density of the DNA. Apparent half-lives for this transition were between 4-9 min at 37 degrees C for supercoiled DNAs as isolated from cells. The half-life for the cruciform to linear transition in relaxed DNA was about 30 s at 37 degrees C. Mg2+ stabilized both conformations but stabilized the linear form to a greater degree than the cruciform. The rates of transition were temperature dependent suggesting enthalpies of activation of 26.3 kcal mol-1 for the cruciform to linear transition and 33.4 kcal mol-1 for the linear to cruciform transition. The rate of the linear to cruciform transition was slower at 50 than 37 degrees C. Heating above 70 degrees C resulted in the loss of the cruciform structure.