Search

DNA-membrane complexes were isolated from lysed E. coli B/r and Bs-1, either by low g forces from a low salt solution, or by high g forces through a discontinuous sucrose gradient. The latter method was more gentle. Irradiation of the intact bacteria had no effect on the membrane macromolecules or on RNA components of these complexes. DNA loss was not significant after irradiation under anoxic conditions but complexes isolated from from Bs-1 irradiated in air showed an appreciable decrease in DNA content. In the presence of the appropriate nucleotide mixture, both 'free' DNA, found in the supernatant fractions, and rapidly sedimented membrane-associated DNA were able to synthesize DNA in the absence of added polymerase. DNA synthesis associated with 'free' DNA was more sensitive to radiation than that associated with DNA bound to the membrane, which appeared to moderate the effects of radiation on new DNA synthesis. It is concluded that the depression of DNA synthesis is primarily a result of irradiation-induced changes on genome-DNA. The interpretation of earlier work from our laboratories that DNA-membrane complexes contained the macromolecular structure which responded to radiation with a high o.e.r. is not supported by the evidence in this work.

SummaryDNA-membrane complexes from three strains of E. coli were irradiated and changes in the rates of DNA synthesis were observed. Doses from 1–10 krad to complexes from W3110 and pol A1 strains gave up to a 100 per cent increase in DNA synthesis; under the same conditions, no change was observed in Bs − 1. The degree of stimulation did not depend on the presence of oxygen during irradiation, and a post-irradiation incubation was necessary to achieve activation. The properties of all three complexes were similar when unirradiated. Irradiation of intact organisms under conditions which produced marked, oxygen-dependent inhibition of the Bs − 1 complex had no significant effect on those from W3110 and pol A1.Enhanced DNA synthesis is concluded to be due wholly to repair of preexisting DNA. It is further postulated that DNA synthesis in untreated complexes (E. coli B's, W3110 and pol A1) is mainly of the repair-type and does not necessarily take place at the site of DNA-membrane attachment.

SummaryIrradiation of E. coli B/r and Bs−1 depressed the DNA synthetic activity of the DNA-membrane complexes from these bacteria. The depression of this activity in both strains in the presence of oxygen was seven to eight times greater than under anoxic conditions when irradiation was with fast electrons. The oxygen-enhancement ratios for neutrons were 4·0 and 5·2 for E. coli B/r and E. coli Bs−1, respectively.The DNA-synthesizing abilities of complexes from the radiation-resistant E. coli B/r and the radisation-sensitive E. coli Bs−1 were equally inhibited by electron irradiation, but with fast neutrons the synthetic activity of the DNA-membrane complex from E. coli B/r was more inhibited than that from E. coli Bs−1.

THE model proposed by Alper1 for lethal radiation damage to cells is based on inferential evidence that there are two important sites of damage by ionizing radiation. At one site, damage referred to as type “N” is associated with a low oxygen enhancement ratio (OER) and is probably to nucleic acid, while at the other site, type “O” damage is associated with a considerably higher oxygen enhancement value and is to a non-nucleic acid target. The model demands that the two values of OER are respectively less and greater than that observed for the overall lethal effect. More recently2 Alper reviewed further inferential evidence3 that cell membranes are the site of type O damage, though there may be subsequent interaction with the lesions following energy deposition in DNA4.