Induction of the soxRS regulon of Escherichia coli by glycolaldehyde

The ability of short-chain sugars to cause oxidative stress has been examined using glycolaldehyde as the simplest sugar. Short-chain sugars autoxidize in air, producing superoxide and alpha,beta-dicarbonyls. In Escherichia coli the soxRS regulon mediates an oxidative stress response, which protects the cell against both superoxide-generating agents and nitric oxide. In superoxide dismutase-deficient E. coli mutants, glycolaldehyde induces fumarase C and nitroreductase A, which are regulated as members of the soxRS regulon. A mutational defect in soxRS eliminates that induction. This establishes that glycolaldehyde can cause induction of this defensive regulon. This effect of glycolaldehyde was oxygen-dependent, was not shown by glyoxal, and was not seen in the superoxide dismutase-replete parental strain, and it was abolished by a cell-permeable SOD mimetic. All of these suggest that superoxide radicals produced by the oxidation of glycolaldehyde played a key role in the induction.     

Induction of the manganese-containing superoxide dismutase in Escherichia coli is independent of the oxidative stress (oxyR-controlled) regulon

The synthesis of manganese-superoxide dismutase in response to hydrogen peroxide and to paraquat was examined in strains of Escherichia coli with different mutations in the oxyR gene. Hydrogen peroxide treatment did not induce manganese-superoxide dismutase, but did induce the oxyR regulon. Paraquat induced this enzyme in a strain compromised in its ability to induce the defense response against oxidative stress (oxyR deletion) as well as in a strain that is constitutive and overexpresses the oxyR regulon. Catalase (HPI), but not manganese-superoxide dismutase, was over-expressed under anaerobic conditions in a strain harboring a constitutive oxyR mutation. The data clearly demonstrate that the induction of manganese-superoxide dismutase is independent of the oxyR-controlled regulon.     

The flavoenzyme ferredoxin (flavodoxin)-NADP(H) reductase modulates NADP(H) homeostasis during the soxRS response of Escherichia coli

Escherichia coli cells from strain fpr, deficient in the soxRS-induced ferredoxin (flavodoxin)-NADP(H) reductase (FPR), display abnormal sensitivity to the bactericidal effects of the superoxide-generating reagent methyl viologen (MV). Neither bacteriostatic effects nor inactivation of oxidant-sensitive hydrolyases could be detected in fpr cells exposed to MV. FPR inactivation did not affect the MV-driven soxRS response, whereas FPR overexpression led to enhanced stimulation of the regulon, with concomitant oxidation of the NADPH pool. Accumulation of a site-directed FPR mutant that uses NAD(H) instead of NADP(H) had no effect on soxRS induction and failed to protect fpr cells from MV toxicity, suggesting that FPR contributes to NADP(H) homeostasis in stressed bacteria.     

Hydrogen peroxide increases the activities of soxRS regulon enzymes and the levels of oxidized proteins and lipids in Escherichia coli

The effects of hydrogen peroxide treatments on Escherichia coli KS400 and AB1157 cells were assessed by monitoring the accumulation of oxidative damage products, carbonyl proteins and thiobarbituric acid-reactive substances (TBARS), as well as the activities of selected antioxidant enzymes. H(2)O(2) treatment stimulated increases in both TBARS and carbonyl protein levels in dose- and time-dependent manners in KS400 cells. The accumulation of TBARS was much more variable with H(2)O(2) treatment; TBARS content was significantly increased in response to 5 microM H(2)O(2), whereas a significant increase in carbonyl protein content occurred at 100 microM H(2)O(2). Similarly, treatment with 20 microM hydrogen peroxide for different lengths of time resulted in peak TBARS accumulation by 20 min, whereas carbonyl protein levels were significantly elevated only after 60 min. In AB1157 cells, treatment with 20 microM hydrogen peroxide for 20 min led to strong increases in both carbonyl protein and TBARS levels. This treatment also triggered increased activities of enzymes of the oxyR regulon (catalase, peroxidase, and glutathione reductase) in both strains. In the AB1157 strain, H(2)O(2) exposure also increased the activities of two enzymes of the soxRS regulon (superoxide dismutase and glucose-6-phosphate dehydrogenase) by 50-60%. The data show differential variability of lipids versus proteins to oxidative damage induced by H(2)O(2,) as well as strain-specific differences in the accumulation of damage products and the responses by antioxidant enzymes to H(2)O(2) stress.     

大肠杆菌鼠李糖调节子

大肠杆菌（Escherichia coil）L-鼠李糖（rha）调节子由三个功能相关的操纵子（operon）组成,位于大肠杆菌染色体基因组中。它编码大肠杆菌吸收和利用L-鼠李糖的蛋白,即一个鼠李糖运输蛋白（RhaT）、三个鼠李糖代谢酶（RhaB、RhaA、RhaD）以及两个调节蛋白（RhaS、RhaR）。三个操纵子均受到L-鼠李糖本身的诱导,同时以调控蛋白RhaS、RhaR和CRP（cAMP受体蛋白）为中介的正调控也参与调节。     

Osmoregulation of the maltose regulon in Escherichia coli.

The maltose regulon consists of four operons that direct the synthesis of proteins required for the transport and metabolism of maltose and maltodextrins. Expression of the mal genes is induced by maltose and maltodextrins and is dependent on a specific positive regulator, the MalT protein, as well as on the cyclic AMP-catabolite gene activator protein complex. In the absence of an exogenous inducer, expression of the mal regulon was greatly reduced when the osmolarity of the growth medium was high; maltose-induced expression was not affected, and malTc-dependent expression was only weakly affected. Mutants lacking MalK, a cytoplasmic membrane protein required for maltose transport, expressed the remaining mal genes at a high level, presumably because an internal inducer of the mal system accumulated; this expression was also strongly repressed at high osmolarity. The repression of mal regulon expression at high osmolarity was not caused by reduced expression of the malT, envZ, or crp gene or by changes in cellular cyclic AMP levels. In strains carrying mutations in genes encoding amylomaltase (malQ), maltodextrin phosphorylase (malP), amylase (malS), or glycogen (glg), malK mutations still led to elevated expression at low osmolarity. The repression at high osmolarity no longer occurred in malQ mutants, however, provided that glycogen was present.     

Hypochlorous acid activates the heat shock and soxRS systems of Escherichia coli.

A series of plasmids, containing fusions of different stress promoters to lux reporter genes, was used in an attempt to monitor the defense circuits activated upon exposure of Escherichia coli to sublethal doses of free chlorine. A significant level of activation was exhibited by promoters of three heat shock genes (grpE, dnaK, and lon), in an rpoH-dependent manner. The promoter of micF, a gene under the control of the soxRS regulon, was also strongly induced, but not in a soxR mutant. This induction was not affected by sodA and sodB mutations, implying that it did not involve oxygen radical activity. Free-chlorine activation of both heat shock and soxRS regulons required an exposure of less then I s in duration. The oxyR or the SOS regulons were apparently not induced by free chlorine (as judged by lack of activation of katG and recA, respectively), and neither was the universal stress (uspA) protein.     

Controlled induction of the RpoS regulon in Escherichia coli, using an RpoS-expressing plasmid

RpoS, an alternative sigma factor produced by many gram-negative bacteria, primarily controls genes that are expressed in stationary phase in response to nutrient deprivation. To test the idea that induction of RpoS in the exponential phase, when RpoS is not normally expressed, increases RpoS-dependent gene expression, we constructed a plasmid carrying the rpoS gene under the control of an IPTG (isopropyl-beta-D-thiogalactopyranoside)-inducible T7lac promoter. Northern and Western analyses revealed that levels of RpoS mRNA and protein, respectively, increased in response to the inducer IPTG. Assays of changes in RpoS-dependent functions (catalase activity and glycogen accumulation), confirmed that induced RpoS was functional in exponential phase and was sufficient for the expression of RpoS-dependent functions. Controlled expression of RpoS and RpoS-dependent genes by plasmid-encoded rpoS may thus offer a useful tool for the study of RpoS-dependent gene expression.