R.b.e. of 50 k Vp x-rays and 660 keV gamma-rays (137 Cs) with respect to the production of DNA damage, repair and cell-killing in Escherichia coli K-12.

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 (50kVp) and hard gamma-rays (600 keV) under aerobic conditions. Irradiation with 50 kVP x-rays resulted in 1.47 times more cell killing than was observed with 137Cs gamma-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 gamma-rays in producing DNA double-strand breaks, there was no sigificant 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 gamma-rays. The increased yield of DNA double-strand breaks resulting from 50 kVp X-radiation 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 gamma-rays.