Single- and double-strand breaks induced in plasmid DNA irradiated by ultra-soft X-rays

In order to investigate the molecular consequences of a carbon K photo-ionization located on DNA, dry pBS plasmid samples were irradiated with ultra-soft X-rays at energies below and above the carbon K-threshold ( eV). Single- and double-strand breaks (ssb and dsb) were quantified after resolution of the three plasmid forms (supercoiled, relaxed circular, linear) by gel electrophoresis. A factor of 1.2 was found between the doses required at 250 eV and 380 eV to induce the same number of dsb per plasmid.