Expression of Chromosomal and Plasmid-mediated Resistance to Ultraviolet Light in Escherichia coli under Aerobic and Anaerobic Conditions

Abstract

Previous work has shown that plasmid-mediated DNA repair, which is active against ultraviolet light-(UV-) induced damage, does not confer resistance to the DNA-damaging drug metronidazole. However, metronidazole-induced bacterial killing is only expressed under anaerobic conditions, and it may be that plasmid-encoded DNA repair does not function under these conditions. We therefore report experiments designed to determine if plasmid-mediated DNA repair is expressed anaerobically. A series of DNA repair-deficient mutants of Escherichia coli strain AB1157 was tested for sensitivity to UV after aerobic or anaerobic incubation. The repair-proficient strain and strains deficient in excision repair, recombination repair or the expression of SOS repair, were all more resistant to UV when incubated aerobically after exposure to UV. Strains deficient in the induction of SOS repair were no more sensitive whether incubated aerobically or anaerobically post-UV. Plasmid R46 mediated a high level of UV resistance in E. coli strain TK501 under anaerobic conditions, although this was lower than the level expressed when cells were incubated aerobically post-UV. The UV-sensitizing effect of plasmid R391 was expressed to a similar extent in both aerobically- and anaerobically-incubated cultures of strain TK501. These results suggest that the previously-reported lack of plasmid-mediated DNA repair activity in metronidazole-damaged TK501 cells is due to the inability of plasmid-encoded repair products to deal with metronidazole-induced damage, rather than to the inability of the repair process to function under anaerobic conditions.