Lethal oxidative damage and mutagenesis are generated by iron in delta fur mutants of Escherichia coli: protective role of superoxide dismutase

Abstract

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.