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The nature of the 3H activity released from M. radiodurans after γ-ray doses of 500 krads which cause no decrease in cellular viability has been investigated by gel filtration and alkaline sucrose-gradient centrifugation. Immediately after irradiation, oligonucleotides ∼7·0 × 105 daltons are released which are subsequently rapidly degraded in broth to thymine by exonucleolytic enzymes. Such enzymes are also capable of degrading mammalian DNA. Treatment with iodoacetamide 2 × 10-5 M results in a sensitization expressed as a loss of the shoulder of the dose-response curve. The study of DNA degradation products in M. radiodurans treated with iodoacetamide suggests that sensitization is due to inhibition of an enzyme which excises DNA from a DNA-membrane complex. Iodoacetamide appears not to inhibit the removal of thymine from irradiated DNA. The results suggest that at least two enzymes are involved in the repair of γ-ray damage in M. radiourans and that repair of the DNA-membrane complex is mainly responsible for radioresistance in these bacteria.

The effect of post-irradiation treatments with exogenous DNA, deoxyribonucleotides and deoxyribonucleosides on the rate of DNA synthesis and on the survival of x-irradiated L-V166 cells has been studied. The addition of either DNA or deoxyribonucleosides to the cells immediately after irradiation increased the rate of DNA synthesis. The magnitude of this effect is not dose-dependent and can be related to the increase in the cell survival. The experiments with synchronized cultures have shown that the increase in survival is a function of cell position in the intermitotic cycle: the highest increase in survival was observed in S-phase cells. Further experiments were concerned with the fate of DNA, when added to the serum-containing growth medium. The results show that most of DNA was broken down to the level of deoxyribonucleosides. When DNA breakdown was prevented, the addition of DNA had no effect on either survival or on the rate of DNA synthesis. All these results indicate that small molecules, probably deoxyribonucleosides, are utilized by irradiated cells and are involved in the repair of the induced damage.