Single-strand breaks in DNA and transformation inactivation induced by tritiated water

The induction of single-strand breaks in DNA by tritiated water in aqueous solution was studied by agarose gel electrophoresis. Also the transformation inactivation of the DNA was studied with ampicillin resistance as a marker when the DNA transfer E.coli cells (HB101). Plasmid DNA, pBR322, in SSC buffered solution was kept with tritiated water of different concentrations ranging 5.88 x 10-2 Gy/h - 1.18 x 10-4 Gy/h at 4degC, that is, the DNA was irradiated at several different dose rates. Accumulated hydrogen peroxide in tritiated water was removed by catalase treatment just before adding tritiated water to DNA solution. After the irradiation with tritiated water DNA was rinsed by isotope-free SSC buffer to eliminate the radioactivity using ultra filtration filter, than the DNA was used to analyse its single-strand breaks and transformation inactivation. As the result it was observed that the efficiencies of single-strand breaks and transformation inactivation increased as decreasing dose rate. The values of relative biological effectiveness (RBE) were obtained to be about 0.3 - 0.4 for single-strand breaks and about 1 for transformation inactivation using the result from the irradiation with Co-60 gamma-rays at the similar low dose rate. (author)