Modulation of the H2O2-induced SOS response in escherichia coli PQ300 by amino acids,metal chelators,antioxidants, and scavengers of reactive oxygen species

The SOS chromotest is a simple colorimetric genotoxicity assay that monitors DNA repair by measuring the induction of the gene sfiA in Escherichia coli K-12. E. coli PQ300, a diagnostic SOS tester strain for the detection of oxidative genotoxins, carries a mutation in a key gene for antioxidative defense, _oxyR. This mutation renders PQ300 more sensitive to oxidative genotoxins, particularly to H₂O₂. We found that induction of the SOS response by H₂O₂ in E. coli PQ300 is dependent on the composition of the incubation medium; a substantially reduced response was obtained in minimal phosphate buffered saline (PBS) as opposed to complex Luria broth (LB) medium. Supplementation of PBS with histidine or cysteine stimulated H₂O₂-induced SOS induction to levels exceeding those found in LB medium. Low concentrations of glutathione (20–70 μM) also enhanced the H₂O_2-induced SOS response in E. coli PQ300, whereas higher concentrations (>150 μM) were protective. Preincubation of tester cells with the chelators o-phenanthroline, 2,2-dipyridyl, and ethylenediaminetetraacetic acid (EDTA) protected cells from the effects of H₂O₂, although EDTA was only partially effective. Pretreatment of PQ300 with the antioxidant ascorbic acid or the hydroxyl radical scavenger dimethyl sulfoxide also diminished the SOS response, whereas mannitol and glucose were ineffective. The results show that the net effect of H₂O_2-induced DNA damage is influenced by the balance of oxidative and antioxidative factors and, furthermore, can be modulated by constituents of the extracellular milieu. © 1993 Wiley-Liss, Inc.