SUMMARY: The effect of X-rays on the synthesis of induced β-D-galactosidase was examined with strain B and three of its mutants, i.e. the sensitive strain BS and two resistant strains B/r and B/H. With the parent strain B the extent of radiation-induced lethal damage depended on conditions of culture after irradiation, whereas radiation-induced inhibition of β-D-galactosidase was unaffected by the composition of the culture medium. The sensitivity of radiation damage to induction of β-D-galactosidase was about the same for strains B, B/r and B/H, the dose required to give 37% survival being 42 ± 6 krad; in BS the sensitivity to radiation was three times as great. Enzyme induction was one-half to one-third as radiosensitive as viability for the strains B/r, B/H and BS but with strain B it was one-tenth as sensitive when Oxoid Nutrient Medium was used. Oxygen enhancement ratios for lethal effects were always smaller than those for inhibition of enzyme synthesis.


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