The rate of production of DNA single-strand breaks has been studied in several polA1 derivatives of Escherichia coli K-12 x-irradiated in the presence or absence of oxygen. For log phase cells irradiated in phosphate-buffered saline, pH 7.3, the rate of production of breaks per single-strand genome per krad was 2.13 in the presence of air, and 0.66 in its absence [an oxygen-nitrogen breaks ratio (ONBR) of 3.2]. To determine if this oxygen effect was due to a difference in the absolute yield of breaks or to a differential ability for repair, the yield of single-strand breaks was studied in cells which had been inactivated either by heat treatment (52°C) or cold shock (0°C). In both cases there was a large increase in the yield of radiation-induced anoxic breaks (2.8-fold) and a small increase (1.25-fold) in aerobic breaks. Attempts were also made to inhibit repair using chemicals. Sodium cyanide (1 mM) used at 0°C had no effect on the anoxic yield of breaks. However, in the presence of quinacrine (0.2 mM) the level of anoxic breaks increased above that seen in untreated <tex-math>$pol^{-}$</tex-math> cells, while the aerobic level was the same. N-ethylmaleimide, NEM (0.5 mM), had a similar effect, but another SH-enzyme inhibitor, iodoacetic acid (1 mM), had no effect. Hydroxyurea (10 mM) had an effect qualitatively similar to NEM. These results suggest that E. coli possess an ultrafast repair system, which operates mainly on anoxic breaks and can be inhibited by physical or chemical pretreatment of the cells. This inhibition permits the demonstration that the initial yield of x-ray-induced DNA single-strand breaks in vivo is largely independent of the presence of oxygen, but that the yield of breaks in anoxic cells is very rapidly modified by repair.

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