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Background  Acid tolerance in  Escherichia coli  O157:H7 contributes to persistence in its bovine host and is thought to promote passage through the gastric barrier of humans. Dps ( D NA-binding  p rotein in  s tarved cells) mutants of  E. coli  have reduced acid tolerance when compared to the parent strain although the role of Dps in acid tolerance is unclear. This study investigated the mechanism by which Dps contributes to acid tolerance in  E. coli  O157:H7.    Results  The results from this study showed that acid stress lead to damage of chromosomal DNA, which was accentuated in  dps  and  recA  mutants. The use of  Bal 31, which cleaves DNA at nicks and single-stranded regions, to analyze chromosomal DNA extracted from cells challenged at pH 2.0 provided  in vivo  evidence of acid damage to DNA. The DNA damage in a  recA  mutant further corroborated the hypothesis that acid stress leads to DNA strand breaks. Under  in vitro  assay conditions, Dps was shown to bind plasmid DNA directly and protect it from acid-induced strand breaks. Furthermore, the extraction of DNA from Dps-DNA complexes required a denaturing agent at low pH (2.2 and 3.6) but not at higher pH (>pH4.6). Low pH also restored the DNA-binding activity of heat-denatured Dps. Circular dichroism spectra revealed that at pH 3.6 and pH 2.2 Dps maintains or forms α-helices that are important for Dps-DNA complex formation.    Conclusion  Results from the present work showed that acid stress results in DNA damage that is more pronounced in  dps  and  recA  mutants. The contribution of RecA to acid tolerance indicated that DNA repair was important even when Dps was present. Dps protected DNA from acid damage by binding to DNA. Low pH appeared to strengthen the Dps-DNA association and the secondary structure of Dps retained or formed α-helices at low pH. Further investigation into the precise interplay between DNA protection and damage repair pathways during acid stress are underway to gain additional insight.

Acid stress damage of DNA is prevented by Dps binding in Escherichia coliO157:H7