Alkaline sucrose-density gradient results indicate that aerobic irradiation of log phase E. coli B/r CSH is 4.5 times more effective than anoxic irradiation in producing single-strand breaks in DNA. If 0.02 M EDTA, presumed to inhibit the rejoining enzyme, is present during irradiation, the yield of breaks is increased 6-fold in air and 10-fold in nitrogen, thus aerobic conditions are still more effective than anoxic in producing breaks. Holding bacteria at 0°C in buffer after aerobic or anoxic radiation resulted in 80-90% of the observed breaks being rejoined. Since rejoining takes place during irradiation, the yield of breaks is dose-rate dependent. The oxygen effect on single-strand breaks in log phase E. coli B/r may be related to radiation survival: however, survival is not improved by lowering the dose rate nor by postirradiation holding at 0°C conditions which result in the rejoining of breaks. Holding irradiated bacteria in buffer at 37°C results in only a small amount of rejoining, while considerable loss of label takes place due to DNA degradation. Holding at 37°C in growth medium, however, results in DNA degradation accompanied by a marked increase in molecular weight of TCA-insoluble DNA.

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