During Oxidative Stress the Clp Proteins of Ensure that Iron Pools Remain Sufficient To Reactivate Oxidized Metalloenzymes.

Hydrogen peroxide (HO) is formed in natural environments by both biotic and abiotic processes. It easily enters the cytoplasms of microorganisms, where it can disrupt growth by inactivating iron-dependent enzymes. It also reacts with the intracellular iron pool, generating hydroxyl radicals that can lethally damage DNA. Therefore, virtually all bacteria possess HO-responsive transcription factors that control defensive regulons. These typically include catalases and peroxidases that scavenge HO Another common component is the miniferritin Dps, which sequesters loose iron and thereby suppresses hydroxyl-radical formation. In this study, we determined that also induces the ClpS and ClpA proteins of the ClpSAP protease complex. Mutants that lack this protease, plus its partner, ClpXP protease, cannot grow when HO levels rise. The growth defect was traced to the inactivity of dehydratases in the pathway of branched-chain amino acid synthesis. These enzymes rely on a solvent-exposed [4Fe-4S] cluster that HO degrades. In a typical cell the cluster is continuously repaired, but in the mutant the repair process is defective. We determined that this disability is due to an excessively small iron pool, apparently due to the oversequestration of iron by Dps. Dps was previously identified as a substrate of both the ClpSAP and ClpXP proteases, and in their absence its levels are unusually high. The implication is that the stress response to HO has evolved to strike a careful balance, diminishing iron pools enough to protect the DNA but keeping them substantial enough that critical iron-dependent enzymes can be repaired. Hydrogen peroxide mediates the toxicity of phagocytes, lactic acid bacteria, redox-cycling antibiotics, and photochemistry. The underlying mechanisms all involve its reaction with iron atoms, whether in enzymes or on the surface of DNA. Accordingly, when bacteria perceive toxic HO, they activate defensive tactics that are focused on iron metabolism. In this study, we identify a conundrum: DNA is best protected by the removal of iron from the cytoplasm, but this action impairs the ability of the cell to reactivate its iron-dependent enzymes. The actions of the Clp proteins appear to hedge against the oversequestration of iron by the miniferritin Dps. This buffering effect is important, because seeks not just to survive HO but to grow in its presence.