The Release of a Deoxyribonucleic Acid Fragment after X-irradiation ofMicrococcus Radiodurans

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Abstract
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.