LETHAL AND MUTAGENIC LESIONS INDUCED BY IONIZING RADIATIONS IN E COLI AND DNA STRAND BREAKS

The inactivation doses for E. coli exposed to alpha particles and protons of different LETs and to gamma rays have been measured. Strains derived from E. coli B/r showed a maximum sensitivity at LETs of ∼ 30 keV/μ whilst Bs-1 and other strains known to be deficient in repair capacity had sensitivities which decreased monotonically with increasing LET. These results can be interpreted in terms of two types of lethal damage to the bacterial genome. Damage of type 1 affects only one strand of the DNA macromolecule and is partially reparable in vivo whilst damage of type 2, inflicted by one track intersection of the DNA, is irreparable. The identity of both types of damage is uncertain but type 1 probably gives rise to a lesion recognizable in vitro as a single strand break. Type 2 damage probably corresponds to double strand scission of DNA as observed in vitro. Mutations to prototrophy of three auxotrophic strains of E. coli are induced with an effectiveness which decreases steadily with increasing LET. This form of LET dependence implies that these mutations involve damage to one target only, probably one strand of the DNA duplex. © 1969 Springer-Verlag.