The LET Factor in Mutagenesis by Ionizing Radiations

Summary

Mutations to wild type of an amber mutant E51 of phage T4 were induced by ionizing radiation after the phage DNA had been injected into cells of a permissive host, Escherichia coli K12 CA 266. Phage bacterial complexes were deposited as monolayers on Millipore filters and exposed to alpha particles or protons of well-defined energies covering a range of LET values from 10 to 230 keV/µm, or to ⁶⁰Co gamma-rays. Complexes were then plated on lawns of CA 266 bacteria for estimation of surviving E51 phage and on lawns of CA 244 bacteria, which were permissive only for wild-type phage, for the scoring of back mutations. The frequency of radiation-induced mutants was proportional to dose with all types of radiation. The observed mutagenic effectiveness of the heavy charged particles relative to gamma-rays fell steadily with increasing LET to a value of 0·43 at 230 keV/µm. This LET dependence was compared with theoretical curves calculated on different assumptions with respect to the type of initial DNA lesion from which a mutation arises. It was concluded that a mutagenic lesion is induced by one ionizing particle which damages one DNA strand as it passes through a phage genome at a point which is probably within or close to its amber triplet.