Radiosensitivity of DNA minicircles.

DNA minicircles of 207 bp were constructed by the ligation of linear restriction fragments in the presence of various concentrations of ethidium bromide. Three topoisomers characterized by linking numbers (Lk) of 20, 19 and 18, and with helical repeats of 10.35, 10.9 and 11.5 bp/turn respectively, were obtained. They are called, respectively, relaxed minicircle or topoisomer 0, topoisomer -1 and topoisomer -2. Owing to the limited flexibility of such small circles, the stress created by the lack of 1 or 2 turns cannot be eliminated by a spatial circle-axis writhing (supercoiling) of the circular molecules. These two undertwisted, stressed topoisomers have to adopt a flat, non-crossed shape, similar to that of the relaxed minicircle. The three minicircles were irradiated with gamma-rays or fast neutrons. The same yields of single-strand breaks, double-strand breaks and alkali-induced single-strand breaks were observed for the three topoisomers showing that their base and sugar moieties are attacked equally by gamma photon- or fast neutron-induced radicals. We conclude that untwisting of a B helix does not modify the radiosensitivity of DNA.