Diethyl pyrocarbonate: a chemical probe for DNA cruciforms.

Two palindromic DNA sequences were analyzed with respect to their chemical reactivities with diethyl pyrocarbonate. In negatively supercoiled plasmid templates enhanced N7 carbethoxylation was found with individual purines located in presumptive single-stranded loops of DNA cruciform structures. No enhanced reactivity at these positions was observed in linear, relaxed or low superhelical density plasmids. Hyperreactivity was found over a narrow region only, indicating that stable cruciforms contain loops of minimal size. No enhanced chemical reactivity was found with the four-way junction at the base of cruciforms. Diethyl pyrocarbonate has proved a sensitive structural probe for the analysis, with single nucleotide resolution, of DNA cruciform structures.