Influence of Ultrafast Repair Processes (Independent of DNA Polymerase I) on the Yield of DNA Single-Strand Breaks in Escherichia coli K-12 X-Irradiated in the Presence or Absence of Oxygen

Abstract

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) ...