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J. Bacteriol., 01 1997, 382-388, Vol 179, No. 2
B Gonzalez-Flecha and B Demple
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).
Copyright © 1997, American Society for Microbiology
Homeostatic regulation of intracellular hydrogen peroxide concentration in aerobically growing Escherichia coli
Department of Molecular and Cellular Toxicology, Harvard School of Public Health, Boston, Massachusetts 02115, USA.
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