Negative supercoiling increases the sensitivity of plasmid DNA to single-strand break induction by X-rays

Negatively supercoiled topoisomers of the plasmid pIBI 30 were irradiated with 250 kV X-rays and assayed for strand scission by agarose gel electrophoresis. The survival of supercoiled molecules (Form I) decreased exponentially with increasing X-ray exposure and the dose required to reduce the fraction of DNA in Form I to 37% of its value in unirradiated controls (D37) decreased with increasing negative superhelicity. This enhanced radiation sensitivity of underwound DNA is tentatively attributed to the transient denaturation of the double helix that increases the susceptibility of individual strands to free radical attack.