D Touati

The Escherichia coli Fur protein, with its iron(II) cofactor, represses iron assimilation and manganese superoxide dismutase (MnSOD) genes, thus coupling iron metabolism to protection against oxygen toxicity. Iron assimilation is triggered by iron starvation in wild-type cells and is constitutive in fur mutants. We show that iron metabolism deregulation in fur mutants produces an iron overload, leading to oxidative stress and DNA damage including lethal and mutagenic lesions. fur recA mutants were not viable under aerobic conditions and died after a shift from anaerobiosis to aerobiosis. Reduction of the intracellular iron concentration by an iron chelator (ferrozine), by inhibition of ferric iron transport (tonB mutants), or by overexpression of the iron storage ferritin H-like (FTN) protein eliminated oxygen sensitivity. Hydroxyl radical scavengers dimethyl sulfoxide and thiourea also provided protection. Functional recombinational repair was necessary for protection, but SOS induction was not involved. Oxygen-dependent spontaneous mutagenesis was significantly increased in fur mutants. Similarly, SOD deficiency rendered sodA sodB recA mutants nonviable under aerobic conditions. Lethality was suppressed by tonB mutations but not by iron chelation or overexpression of FTN. Thus, superoxide-mediated iron reduction was responsible for oxygen sensitivity. Furthermore, overexpression of SOD partially protected fur recA mutants. We propose that a transient iron overload, which could potentially generate oxidative stress, occurs in wild-type cells on return to normal growth conditions following iron starvation, with the coupling between iron and MnSOD regulation helping the cells cope.

Escherichia coli double mutants (sodA sodB) completely lacking superoxide dismutase (SOD) have greatly enhanced mutation rates during aerobic growth. Single mutants lacking manganese SOD (MnSOD) but possessing iron SOD (FeSOD) have a smaller increase, and single mutants lacking FeSOD but possessing MnSOD do not show such an increase. The enhancement of mutagenesis is completely dependent on the presence of oxygen, and treatments that increase the flux of superoxide radicals produce even higher levels of mutagenesis. The presence of a plasmid overproducing either form of SOD reduces the level of mutagenesis to that of wild type, showing that the O2-dependent enhancement results from a lack of SOD. The enhancement of mutagenesis is RecA-independent, and a complete lack of SOD does not induce the SOS response during aerobic growth. However, the enhanced mutagenesis in aerobically grown sodA sodB mutants is largely dependent on functional exonuclease III, suggesting that the increased flux of superoxide radicals results in DNA lesions that can be acted on by this enzyme, leading to mutations.

We have investigated the mechanisms of killing of Escherichia coli by HOCl by identifying protective functions. HOCl challenges were performed on cultures arrested in stationary phase and in exponential phase. Resistance to HOCl in both cases was largely mediated by genes involved in resistance to hydrogen peroxide (H2O2). In stationary phase, a mutation in rpoS, which controls the expression of starvation genes including those which protect against oxidative stress, renders the cells hypersensitive to killing by HOCl. RpoS-regulated genes responsible for this sensitivity were dps, which encodes a DNA-binding protein, and, to a lesser extent, katE and katG, encoding catalases; all three are involved in resistance to H2O2. In exponential phase, induction of the oxyR regulon, an adaptive response to H2O2, protected against HOCl exposure, and the oxyR2 constitutive mutant is more resistant than the wild-type strain. The genes involved in this oxyR-dependent resistance have not yet been identified, but they differ from those primarily involved in resistance to H2O2, including katG, ahp, and dps. Pretreatment with HOCl conferred resistance to H2O2 in an OxyR-independent manner, suggesting a specific adaptive response to HOCl. fur mutants, which have an intracellular iron overload, were more sensitive to HOCl, supporting the generation of hydroxyl radicals upon HOCl exposure via a Fenton-type reaction. Mutations in recombinational repair genes (recA or recB) increased sensitivity to HOCl, indicative of DNA strand breaks. Sensitivity was visible in the wild type only at concentrations above 0.6 mg/liter, but it was observed at much lower concentrations in dps recA mutants.

Transcription of the sodA gene of Escherichia coli, which encodes manganese superoxide dismutase, is governed by six global regulators: the product of the soxRS locus (superoxide response) and mutated alleles of the soxQ locus (such as cfxB) act as activators; the products of the fur (ferric uptake regulation), arcA (aerobic regulation control), and fnr (fumarate nitrate reductase) genes and the integration host factor (IHF) negatively regulate sodA. The action of these effectors on the sodA promoter was investigated by using chromosomal sodA-lacZ operon fusions with intact or deleted promoters, different environmental conditions, and strains carrying different combinations of null mutations in the effector genes. The data allow us to assign target regions in the sodA promoter for activation by SoxRS and CfxB and for repression by Fur and ArcA. In aerobiosis, activation of sodA transcription by SoxRS was compatible with CfxB activation or Fur repression, whereas cfxB and fur controls were mutually exclusive. Repression by Fnr appeared, at least in part, to be ArcA dependent. IHF enhanced aerobic Fur repression, and in the absence of Fur, it enhanced anaerobic repression by ArcA. The DNA targets for Fur (encompassing the -35 region) and ArcA (from and downstream of the -35 region) appear to overlap, suggesting that Fur and ArcA repressions are mutually exclusive. Fur (in response to the iron pool) or ArcA, acting with Fnr and IHF (in response to the redox state of the cells), can block anaerobic sodA-lacZ expression with about equivalent efficiencies. The possible biological significance of this result is discussed.