The effects of a chemical radioprotectant, beta-mercaptoethylamine (MEA), on gamma-induced single-strand breaks in DNA from three strains (B/r, <tex-math>${\rm B}_{{\rm s}-1}$</tex-math>, and 15TAU-bar) of Escherichia coli have been studied. The amount of DNA single-strand breakage in irradiated cultures was similar in all three strains. Although MEA enhanced radiation survival in both logarithmic- and stationary-phase cultures, it protected against DNA strand breakage only in the stationary-phase cultures. Single-strand breakage of DNA could not be correlated with radiation killing, even in the very radiosensitive strain <tex-math>${\rm B}_{{\rm s}-1}$</tex-math>. The effects of MEA on DNA strand breakage and killing in cultures of strain 15TAU-bar starved of amino acids with and without methionine provided further evidence that single-strand breaks in DNA are not significant lesions in gamma-ray killing of bacteria.

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