|Abstract:||Escherichia coli resides in the lumen of the gut, where it encounters both biotic and abiotic sources of hydrogen peroxide stress. Hydrogen peroxide (H2O2) diffuses into cells, where it reacts with iron via Fenton chemistry. Three cellular targets are damaged: DNA, mononuclear iron enzymes, and [4Fe-4S] cluster enzymes. E. coli uses the transcription factor OxyR to sense and respond to H2O2 stress. When the intracellular H2O2 levels rise to 0.2 μM, OxyR is activated via the formation of a disulfide bond, and it induces the transcription of the OxyR regulon. The induced genes include those that scavenge H2O2, reduce the intracellular iron level, and repair the H2O2-mediated damage.
Dps, a mini ferritin, is induced by OxyR and it sequesters the free-iron pool, thereby reducing Fenton chemistry. However, this sequestration also causes secondary problems because cells require iron to metallate the iron-dependent enzymes in different biosynthetic pathways. Therefore, OxyR induces the chaperones ClpS and ClpA, which are a part of the Clp family of proteases. Together with the ClpX chaperone and the ClpP protease, ClpSA helps increase the intracellular iron levels, enabling the repair of the [4Fe-4S] enzyme isopropylmalate isomerase. Thus, H2O2-stressed cells maintain a delicate balance of the intracellular iron pools, keeping them low enough to minimize DNA damage but high enough to repair the damaged iron-dependent enzymes.
It seemed plausible that the prolonged induction of the OxyR regulon would cause other secondary problems. Indeed, the constitutive expression of OxyR—even in the absence of H2O2 stress—results in growth defects, which seem to arise due to defects in several amino acid biosynthetic pathways and the tricarboxylic acid cycle. Interestingly, these defects do not seem to be due to iron starvation, as evidenced by the lower activities not only of the iron-dependent enzyme fumarase A but also of the iron-independent enzyme isocitrate dehydrogenase. These cells are also unable to transition smoothly out of prolonged stationary phase. Single-gene deletions of various members of the OxyR regulon did not identify the gene involved, suggesting that several genes may be contributing to the growth defect, although the mechanism remains unclear.
Alternatively, it is possible that the correct gene has not yet been tested.