Localization of melted regions in supercoiled DNA

Diethyl pyrocarbonate (DEPC) was used as a probe of local denatured regions in ccDNA pAO3 plasmid. It was found that in native ccDNA molecules only adenosine residues in the loop of the cruciform structure react with DEPC. Denaturation of ccDNA is accompanied by the appearance of two short regions (20 bp long) at both borders of the cruciform structure. Further increase in the denaturation process is associated with considerable expansion of the region located to the left of the cruciform, while the cruciform structure itself and the denatured region located to the right of it disappear.