Homeostatic regulation of intracellular hydrogen peroxide concentration in aerobically growing Escherichia coli

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Abstract
The exponential phase of aerobic growth is associated with risk of endogenous oxidative stress in which cells need to cope with an approximately 10-fold increase in the rate of H2O2 generation. We addressed this issue by studying the regulation of the intracellular concentration of H2O2 in aerobically growing Escherichia coli. Intracellular H2O2 was kept at an almost constant steady-state value of approximately 0.2 microM (variation, less than twofold) over a broad range of cell densities in rich medium. This regulation was achieved in part by a transient increase in the OxyR-dependent transcription of the catalase gene katG (monitored by using a katG::lacZ operon fusion) during exponential growth, directly correlated with the increased rate of H2O2 generation. The OxyR-regulated alkyl hydroperoxide reductase encoded by ahpFC did not detectably affect H2O2 or catalase activity levels. Induction of katG, ahpFC, and perhaps other genes prevented the accumulation of oxidatively modified lipids but may not have protected DNA: the spontaneous mutation rate was significantly increased in both wild-type and delta(oxy)R strains during exponential growth compared to that in these strains during lag or stationary phases. Strains lacking oxyR showed throughout growth an 8- to 10-fold-higher frequency of spontaneous mutation than was seen for wild-type bacteria. The ahpdelta5 allele also had a mutator effect half of that of delta(oxy)R in exponential and stationary phases and equal to that of deltaoxyR in lag phase, perhaps by affecting organic peroxide levels. These results show that oxyR-regulated catalase expression is not solely an emergency response of E. coli to environmental oxidative stress, but also that it mediates a homeostatic regulation of the H2O2 produced by normal aerobic metabolism. The activation of the oxyR regulon in this process occurs at much lower levels of H2O2 (approximately 10(-7)M) than those reported for oxyR activation by exogenous H2O2 (approximately 10(-5) M).