R.B.E. of 50 kVp X-rays and 660 keV γ-rays (¹³⁷Cs) with Respect to the Production of DNA Damage, Repair and Cell-killing in Escherichia Coli K-12

Summary

We have compared the efficiency of cell-killing, DNA single-strand breakage and double-strand breakage in an Escherichia coli K-12 wild-type strain after irradiation with soft X-rays (50 kVp) and hard γ-rays (660 keV) under aerobic conditions. Irradiation with 50 kVp X-rays resulted in 1·47 times more cell-killing than was observed with ¹³⁷Cs γ-rays based on a comparison of D₀ values evaluated from the survival curves. DNA sedimentation studies showed that, although 50 kVp X-rays were 1·93 times more effective than ¹³⁷Cs γ-rays in producing DNA double-strand breaks, there was no significant difference between the two qualities of radiation with respect to the initial number of single-strand breaks produced. When the cells were irradiated and allowed to repair maximally in minimal medium, 1·57 times more unrepaired DNA single-strand breaks remained per krad after irradiation with 50 kVp X-rays than with ¹³⁷Cs γ-rays. The increased yield of DNA double-strand breaks resulting from 50 kVp X-irradiation may account for most of these additional unrepaired single-strand breaks, since single- and double-strand breaks are indistinguishable on alkaline sucrose gradients. These results suggest that the greater r.b.e. of 50 kVp X-rays may be related to an increased effectiveness for producing DNA double-strand breaks compared with the higher energy ¹³⁷Cs γ-rays.