The regulation and role of the periplasmic copper, zinc superoxide dismutase of Escherichia coli

The discovery of superoxide dismutase (CuZnSOD) within the periplasms of se veral Gram-negative pathogens suggested that this enzyme evolved to protect cells from exogenous sources of superoxide, such as the oxidative burst of phagocytes. However, its presence in some non-pathogenic bacteria implies that there may be a role for this SOD during normal growth conditions. We f ound that sodC, the gene that encodes the periplasmic SOD of Escherichia co li, is repressed anaerobically by Fnr and is among the many antioxidant gen es that are induced in stationary phase by RpoS. Surprisingly, the entry of wild-type E. coil into stationary phase is accompanied by a several-hourlo ng period of acute sensitivity to hydrogen peroxide. Induction of the RpoS regulon helps to diminish that sensitivity. While mutants of E. coli and Sa lmonella typhimurium that lacked CuZnSOD were not detectably sensitive to e xogenous superoxide, both were killed more rapidly than their parent strain s by exogenous hydrogen peroxide in early stationary phase. This sensitivit y required prior growth in air. Evidently, periplasmic superoxide is genera ted during stationary phase by endogenous metabolism and, if it is not scav enged by CuZnSOD, it causes an unknown lesion that augments or accelerates the damage done by peroxide. The molecular details await elucidation.