A comparison of the radiosensitivity of relaxed and supercoiled plasmid DNA

Abstract

Purpose: The aim of the work was to compare critically the radiosensitivity of the supercoiled and relaxed forms of a plasmid DNA system commonly used in DNA damage assays. Materials and methods: The yields of single- (ssb) and double-strand breaks (dsb) in pBR322 DNA over a range of scavenging capacities were measured in the presence of Tris after irradiation with a single pulse of 400keV electrons. The response was compared with DNA that had been given a preliminary dose of gamma -rays such that an average of one ssb per molecule was present. Results: The yields of dsb were found to be enhanced in the preirradiated DNA when Tris was present during irradiation at concentrations varying between 10 and 100mmoldm-3 with a maximal enhancement ratio of 1.6 at 80mmoldm-3 Tris. This increased yield was not observed when both the pre-irradiation and the experimental doses were given by gamma -irradiation. The increased response was not found in DNA that had been enzymatically relaxed by the introduction of a nick in each molecule using the enzyme gpII. Conclusions: The observations suggest that the conformation per se does not influence the radiosensitivity. The enhanced yields observed in the gamma -ray pre-irradiated DNA with pulsed irradiation appear to be a consequence of the high dose rate used. A proposed explanation is that the combination of high dose levels (1500Gy) and short irradiation times (5ns) would allow some 'spurs' to overlap and it is proposed that the enhanced yields of dsb are related to this.