The LET Factor in Mutagenesis by Ionizing Radiations

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Abstract
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.