A comparison of the radiosensitivity of relaxed and supercoiled plasmid DNA

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.

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 400 keV 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.

The yields of dsb were found to be enhanced in the pre-irradiated DNA when Tris was present during irradiation at concentrations varying between 10 and 100 mmol dm(-3) with a maximal enhancement ratio of 1.6 at 80 mmol x dm(-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.

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 (< or =1500 Gy) and short irradiation times (approximately 5 ns) would allow some 'spurs' to overlap and it is proposed that the enhanced yields of dsb are related to this.