Rapid kinetic microassay for catalase activity

Catalase is a commonly assayed enzyme found in many bacteria and eukaryotes. In this report, we examined the applicability of a kinetic microassay to quantify catalase from two different sources. The assay was found to be linear over a wide range (0.1-1.0 units), but was limited at high values (>1 unit) by oxygen evolution. Nonetheless, the microassay allows simultaneous evaluation of many samples (up to 96) in a short time (<5 min) and is thus well-suited to applications, such as high-throughput screening, where many parallel assays are required.

Can ageing-related degenerative diseases be ameliorated through administration of vitamin C at pharmacological levels?

Man, with other primates, lost the ability to synthesize vitamin C through an inactivating mutation of the gene encoding gulonolactone oxidase (GULO) millions of years ago. Though the consequences of this prehistoric loss must have been favorable (and thus selected for) at the population level, the inability to produce vitamin C may have serious health implications for modern humans, especially for those conditions in which antioxidants (like vitamin C) have been implicated as potential therapeutic agents. Two general types of recent findings regarding vitamin C have made re-evaluation of this important nutrient imperative. First, vitamin C is now known to be involved in several novel physiological phenomena including stem cell differentiation and respiratory development, which likely require pharmacological levels of vitamin C. Secondly, the growing recognition that many ageing-related diseases, including heart disease, neural degeneration and cancer, may have a contributing oxidative damage factor that might be reduced by dietary antioxidants such as vitamin C. In this paper, we hypothesize that high serum-level vitamin C provides important, broad-ranging therapeutic benefits in treating ageing-related degenerative diseases. This hypothesis can be readily tested using traditional and newly-developed genetically-engineered animal models.

Ascorbic acid synthesis due to L-gulono-1,4-lactone oxidase expression enhances NO production in endothelial cells

As a primary antioxidant, ascorbic acid (AA) provides beneficial effects for vascular health mitigating oxidative stress and endothelial dysfunction. However, the association of intracellular AA with NO production occurring inside the endothelial cells remains unclear. In the present study, we addressed this issue by increasing intracellular AA directly through de novo synthesis. To restore AA synthesis pathway, bovine aortic endothelial cells were transfected with the plasmid vector encoding L-gulono-1,4-lactone oxidase (GULO, EC 1.1.3.8), the missing enzyme converting L-gulono-1,4-lactone (GUL) to AA. Functional expression of GULO was verified by Western blotting and in vitro enzyme activity assay. GULO expression alone did not lead to AA synthesis but the supply of GUL resulted in a marked increase of intracellular AA. When the cells were stimulated with calcium ionophore, A23187, NO production was more active in the GULO-expressing cells supplied with GUL, in comparison with the cells without GULO expression or without GUL supply, indicating that intracellular AA regulated NO production. Enhancement of NO production by intracellular AA was further verified in aortic endothelial cells obtained from eNOS knockout mice that were cotransfected with eNOS and GULO constructs. GULO-dependent AA synthesis also elevated intracellular tetrahydrobiopterin content, implicating that this essential cofactor of endothelial nitric oxide synthase (eNOS) might mediate the AA effect. The present study strongly suggests that intracellular AA plays critical roles in vascular physiology through enhancing endothelial NO production.

Stationary phase expression of the arginine biosynthetic operon argCBH in Escherichia coli

BACKGROUND

Arginine biosynthesis in Escherichia coli is elevated in response to nutrient limitation, stress or arginine restriction. Though control of the pathway in response to arginine limitation is largely modulated by the ArgR repressor, other factors may be involved in increased stationary phase and stress expression.

RESULTS

In this study, we report that expression of the argCBH operon is induced in stationary phase cultures and is reduced in strains possessing a mutation in rpoS, which encodes an alternative sigma factor. Using strains carrying defined argR, and rpoS mutations, we evaluated the relative contributions of these two regulators to the expression of argH using operon-lacZ fusions. While ArgR was the main factor responsible for modulating expression of argCBH, RpoS was also required for full expression of this biosynthetic operon at low arginine concentrations (below 60 microM L-arginine), a level at which growth of an arginine auxotroph was limited by arginine. When the argCBH operon was fully de-repressed (arginine limited), levels of expression were only one third of those observed in deltaargR mutants, indicating that the argCBH operon is partially repressed by ArgR even in the absence of arginine. In addition, argCBH expression was 30-fold higher in deltaargR mutants relative to levels found in wild type, fully-repressed strains, and this expression was independent of RpoS.

CONCLUSION

The results of this study indicate that both derepression and positive control by RpoS are required for full control of arginine biosynthesis in stationary phase cultures of E. coli.

Stationary phase-induction of G-->T mutations in Escherichia coli

A series of Escherichia coli mutants, constructed originally by Cupples and Miller [C.G. Cupples, J.H. Miller, A set of lacZ mutations in Escherichia coli that allow rapid detection of each of the six base substitutions, Proc. Natl. Acad. Sci. U.S.A. 86 (1989) 5345-5349], provides a unique system for quantifying base-change mutations, and the repair processes that limit their establishment, in bacteria under selective and non-selective conditions. We focussed on one strain in which a T-->G replacement inactivates the lacZ gene. Reversions of this strain can occur through oxidation of G, leading to G-->T transversions. We show that spontaneous reversions occurred both in lactose (selective) and glucose (non-selective) medium. The number of revertants per viable cell was much greater in medium containing lactose or both sugars than glucose alone. In glucose medium, the rate of reversion was highest below 0.6% glucose and strongly inhibited at and above that level. Evidence that reversions occurred through G-->T transversions in both lactose and glucose media came from two observations: by sequence analysis of a series of revertants and by comparing the reversion rates in strains possessing and lacking the mutM gene (encoding formamidopyrimidine DNA glycosylase, FPG). However, the rate of reversion was stimulated by reducing O2 to 1% and inhibited or delayed by increasing O2 to 90%. In mutM- cells grown on glucose medium, the proportion of revertants increased over a 5-day period. In contrast, in mutM+ cells, revertants appeared primarily during the first 2-3 days after plating; few new revertants appeared in the following days. These data imply that base excision repair initiated by FPG was less effective in the first 2 days and more effective later in stationary phase.

RpoS-regulated genes of Escherichia coli identified by random lacZ fusion mutagenesis

RpoS is a conserved alternative sigma factor that regulates the expression of many stress response genes in Escherichia coli. The RpoS regulon is large but has not yet been completely characterized. In this study, we report the identification of over 100 RpoS-dependent fusions in a genetic screen based on the differential expression of an operon-lacZ fusion bank in rpoS mutant and wild-type backgrounds. Forty-eight independent gene fusions were identified, including several in well-characterized RpoS-regulated genes, such as osmY, katE, and otsA. Many of the other fusions mapped to genes of unknown function or to genes that were not previously known to be under RpoS control. Based on the homology to other known bacterial genes, some of the RpoS-regulated genes of unknown functions are likely important in nutrient scavenging.

Microarray analysis of RpoS-mediated gene expression in Escherichia coli K-12

The alternative sigma factor RpoS controls the expression of many stationary-phase genes in Escherichia coli and other bacteria. Though the RpoS regulon is a large, conserved system that is critical for adaptation to nutrient deprivation and other stresses, it remains incompletely characterized. In this study, we have used oligonucleotide arrays to delineate the transcriptome that is controlled by RpoS during entry into stationary phase of cultures growing in rich medium. The expression of known RpoS-dependent genes was confirmed to be regulated by RpoS, thus validating the use of microarrays for expression analysis. The total number of positively regulated stationary-phase genes was found to be greater than 100. More than 45 new genes were identified as positively controlled by RpoS. Surprisingly, a similar number of genes were found to be negatively regulated by RpoS, and these included almost all genes required for flagellum biosynthesis, genes encoding enzymes of the TCA cycle, and a physically contiguous group of genes located in the Rac prophage region. Negative regulation by RpoS is thus much more extensive than has previously been recognized, and is likely to be an important contributing factor to the competitive growth advantage of rpoS mutants reported in previous studies.

Positive selection for loss of RpoS function in Escherichia coli

Though RpoS, an alternative sigma factor, is required for survival and adaptation of Escherichia coli under stress conditions, many strains have acquired independent mutations in the rpoS gene. The reasons for this apparent selective loss and the nature of the selective agent are not well understood. In this study, we found that some wild type strains grow poorly in succinate minimal media compared with isogenic strains carrying defined RpoS null mutations. Using an rpoS+ strain harboring an operon lacZ fusion to the highly-RpoS dependent osmY promoter as an indicator strain, we tested if this differential growth characteristic could be used to selectively isolate mutants that have lost RpoS function. All isolated (Suc+) mutants exhibited attenuated beta-galactosidase expression on indicator media suggesting a loss in either RpoS or osmY promoter function. Because all Suc+ mutants were also defective in catalase activity, an OsmY-independent, RpoS-regulated function, it was likely that RpoS activity was affected. To confirm this, we sequenced PCR-amplified products containing the rpoS gene from 20 independent mutants using chromosomal DNA as a template. Sequencing and alignment analyses confirmed that all isolated mutants possessed mutated alleles of the rpoS gene. Types of mutations detected included single or multiple base deletions, insertions, and transversions. No transition mutations were identified. All identified point mutations could, under selection for restoration of beta-galactosidase, revert to rpoS+. Revertible mutation of the rpoS gene can thus function as a genetic switch that controls expression of the regulon at the population level. These results may also help to explain why independent laboratory strains have acquired mutations in this important regulatory gene.

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.

Controlled expression of an rpoS antisense RNA can inhibit RpoS function in Escherichia coli

We show that an inducible rpoS antisense RNA complementary to the rpoS message can inhibit expression of RpoS in both exponential and stationary phases and can attenuate expression of the rpoS regulon in Escherichia coli. Plasmids containing rpoS antisense DNA expressed under the control of the T7lac promoter and T7 RNA polymerase were constructed, and expression of the rpoS antisense RNA was optimized in the pET expression system. rpoS antisense RNA levels could be manipulated to effectively control the expression of RpoS and RpoS-dependent genes. RpoS expression was inhibited by the expression of rpoS antisense RNA in both exponential and stationary phases in E. coli. RpoS-dependent catalase HPII was also downregulated, as determined by catalase activity assays and with native polyacrylamide gels stained for catalase. Induced RpoS antisense expression also reduced the level of RpoS-dependent glycogen synthesis. These results demonstrate that controlled expression of antisense RNA can be used to attenuate expression of a regulator required for the expression of host adaptation functions and may offer a basis for designing effective antimicrobial agents.

Functional rescue of vitamin C synthesis deficiency in human cells using adenoviral-based expression of murine l-gulono-γ-lactone oxidase

l-Gulono-gamma-lactone oxidase (GULO) is a critical enzyme present in most mammalian species that is required for the terminal step in vitamin C biosynthesis. Primates are absolutely dependent on exogenously supplied dietary vitamin C due to inactivation of the Gulo gene by mutation over 40 million years ago. In this study, we report the cloning and expression of the murine l-gulono-gamma-lactone oxidase cDNA and gene. The cDNA (2.3 kb) encodes an open reading frame of 440 amino acids that shows high homology to the rat l-gulono-gamma-lactone oxidase (>94%). The Gulo gene is 22 kb long and contains 12 exons. The 11 introns range in size from 479 to 5641 bp. Northern blot analysis revealed high expression of Gulo transcript in the liver. To investigate whether metabolic loss of vitamin C biosynthesis in human cells can be corrected by heterologous expression of GULO, we constructed a first-generation adenoviral vector expressing the murine GULO cDNA under the transcriptional control of the murine cytomegalovirus (MCMV) early promoter. Low rescue efficiency of Gulo-expressing adenoviral constructs and reduced viral growth in HEK293 cells were observed, suggesting that overexpression of Gulo may be inhibitory to cell growth. Placement of a removable stuffer fragment flanked by lox sites between the MCMV promoter and the Gulo gene resulted in efficient vector rescue and normal viral replication in parental HEK293 cells and high-level expression of Gulo in HEK293 cells expressing Cre recombinase. Cells infected with Gulo-expressing vectors overexpressed an FAD-containing protein that corresponded in size to that predicted for recombinant GULO protein and expressed a functional enzyme as measured by the conversion of l-gulono-gamma-lactone to ascorbic acid in cell-free extracts. The cloning of the murine Gulo cDNA and the construction of Gulo-expressing adenoviral vectors are vital steps toward determining the role of vitamin C in basic metabolism and in disease.

Transcriptional induction of the conserved alternative sigma factor RpoS in Escherichia coli is dependent on BarA, a probable two-component regulator

The stationary phase expression of many conserved, adaptive bacterial proteins is dependent on RpoS, a second vegetative sigma factor. The regulation of RpoS itself, however, is complex and not fully understood, particularly at the level of transcription. In this report, we show that the observed hydrogen peroxide sensitivity of a mutant defective in expression of barA, a bacterial virulence factor, can be explained by a reduction in catalase activity, an RpoS-controlled function. Levels of katE mRNA, encoding the major catalase of Escherichia coli, were much lower in the barA mutant, suggesting that BarA is required for the expression of this RpoS-regulated gene. Expression of another RpoS-regulated gene, osmY, was also found to be severely reduced in the barA mutant. Employing Western analyses with anti-RpoS antisera and Northern analyses using probes specific for rpoS, we found that BarA is required for the exponential phase induction of RpoS itself. Operon lacZ fusion expression studies and Northern analyses indicate that BarA itself is maximally expressed in early exponential phase cultures immediately preceding the transcriptional induction of RpoS. Results of primer extension studies indicate that exponential phase expression from the rpoSp1 promoter is reduced by more than 85% in a barA mutant but could be efficiently complemented by a plasmid-borne copy of barA in trans. These results suggest that regulatory signals that are operant in exponentially growing cultures play an important role in effecting stationary phase gene expression.

Expression of the Escherichia coli NRZ nitrate reductase is highly growth phase dependent and is controlled by RpoS, the alternative vegetative sigma factor

In the absence of oxygen, many bacteria preferentially use nitrate as a terminal electron acceptor for anaerobic respiration. In Escherichia coli, there are two membrane-bound, differentially regulated nitrate reductases. While the physiological basis for this metabolic redundancy is not completely understood, during exponential growth, synthesis of NRA is greatly induced by anaerobiosis plus nitrate, whereas NRZ is expressed at a low level that is not influenced by anaerobiosis or nitrate. In the course of identifying genes controlled by the stationary phase regulatory factor RpoS (sigmas), we found that the expression of NRZ is induced during entry into stationary phase and highly dependent on this alternative sigma factor. Expression studies, using operon fusions and nitrate reductase assays, revealed that the NRZ operon is controlled mainly at the level of transcription and is induced 10-fold at the onset of stationary phase in rich media. Consistent with previous reports of RpoS expression, the RpoS dependency of NRZ in minimal media was very high (several hundredfold). We also observed a fivefold stationary phase induction of NRZ in an rpoS background, indicating that other regulatory factors, besides RpoS, are probably involved in transcriptional control of NRZ. The RpoS dependence of NRZ expression was confirmed by Northern analyses using RNA extracted from wild-type and rpoS- strains sampled in exponential and stationary phase. In toto, these data indicate that RpoS-mediated regulation of NRZ may be an important physiological adaptation that allows the cell to use nitrate under stress-associated conditions.

Endonuclease III and endonuclease IV protect Escherichia coli from the lethal and mutagenic effects of near-UV irradiation

In contrast to the DNA damage caused by far-UV (λ < 290 nm), near-UV (290 < λ < 400 nm) induced DNA damage is partially oxygen dependent, suggesting the involvement of reactive oxygen species. To test the hypothesis that enzymes that protect cells from oxidative DNA damage are also involved in preventing near-UV mediated DNA damage, isogenic strains deficient in one or more of exonuclease III (xthA), endonuclease IV (nfo), and endonuclease III (nth) were exposed to increasing levels of far-UV and near-UV. All strains, with the exception of the nth single mutant, were found to be hypersensitive to the lethal effects of near-UV relative to a wild-type strain. A triple mutant strain (nth nfo xthA) exhibited the greatest sensitivity to near-UV-mediated lethality. The triple mutant was more sensitive than the nfo xthA double mutant to the lethal effects of near-UV, but not far-UV. A forward mutation assay also revealed a significantly increased sensitivity for the triple mutant compared to the nfo xthA deficient strain in the presence of near-UV. However, the triple mutant was no more sensitive to the mutagenic effects of far-UV than a nfo xthA double mutant. These data suggest that exonuclease III, endonuclease IV, and endonuclease III are important in protection against near-UV-induced DNA damage.Key words: near-UV, UVA, DNA damage, DNA repair.

Endonuclease III and endonuclease IV protect Escherichia coli from the lethal and mutagenic effects of near-UV irradiation

In contrast to the DNA damage caused by far-UV (lambda < 290 nm), near-UV (290 < lambda < 400 nm) induced DNA damage is partially oxygen dependent, suggesting the involvement of reactive oxygen species. To test the hypothesis that enzymes that protect cells from oxidative DNA damage are also involved in preventing near-UV mediated DNA damage, isogenic strains deficient in one or more of exonuclease III (xthA), endonuclease IV (nfo), and endonuclease III (nth) were exposed to increasing levels of far-UV and near-UV. All strains, with the exception of the nth single mutant, were found to be hypersensitive to the lethal effects of near-UV relative to a wild-type strain. A triple mutant strain (nth nfo xthA) exhibited the greatest sensitivity to near-UV-mediated lethality. The triple mutant was more sensitive than the nfo xthA double mutant to the lethal effects of near-UV, but not far-UV. A forward mutation assay also revealed a significantly increased sensitivity for the triple mutant compared to the nfo xthA deficient strain in the presence of near-UV. However, the triple mutant was no more sensitive to the mutagenic effects of far-UV than a nfo xthA double mutant. These data suggest that exonuclease III, endonuclease IV, and endonuclease III are important in protection against near-UV-induced DNA damage.

Identification of conserved, RpoS-dependent stationary-phase genes of Escherichia coli

During entry into stationary phase, many free-living, gram-negative bacteria express genes that impart cellular resistance to environmental stresses, such as oxidative stress and osmotic stress. Many genes that are required for stationary-phase adaptation are controlled by RpoS, a conserved alternative sigma factor, whose expression is, in turn, controlled by many factors. To better understand the numbers and types of genes dependent upon RpoS, we employed a genetic screen to isolate more than 100 independent RpoS-dependent gene fusions from a bank of several thousand mutants harboring random, independent promoter-lacZ operon fusion mutations. Dependence on RpoS varied from 2-fold to over 100-fold. The expression of all fusion mutations was normal in an rpoS/rpoS+ merodiploid (rpoS background transformed with an rpoS-containing plasmid). Surprisingly, the expression of many RpoS-dependent genes was growth phase dependent, albeit at lower levels, even in an rpoS background, suggesting that other growth-phase-dependent regulatory mechanisms, in addition to RpoS, may control postexponential gene expression. These results are consistent with the idea that many growth-phase-regulated functions in Escherichia coli do not require RpoS for expression. The identities of the 10 most highly RpoS-dependent fusions identified in this study were determined by DNA sequence analysis. Three of the mutations mapped to otsA, katE, ecnB, and osmY-genes that have been previously shown by others to be highly RpoS dependent. The six remaining highly-RpoS-dependent fusion mutations were located in other genes, namely, gabP, yhiUV, o371, o381, f186, and o215.

Near ultraviolet radiation (UVA and UVB) causes a formamidopyrimidine glycosylase-dependent increase in G to T transversions

In contrast to far-UV (< 290 nm) DNA damage, a large fraction of the DNA damage caused by near-UV is oxygen-dependent, suggesting the involvement of reactive oxygen species (ROS). The oxidized base 8-oxo-7,8-dihydroguanine (GO) is characteristic of ROS-induced DNA damage and is removed by Fapy (formamidopyrimidine) glycosylase. We have recently shown that Escherichia coli strains deficient in Fapy glycosylase (fpg) are hypersensitive to the lethal effects of UVA but not far-UV (UVC), suggesting lesions recognized by this enzyme may be important premutagenic or lethal lesions generated by near-UV radiation. In this study, we have found that while the far-UV-induced mutation rates of Fapy-deficient and wild-type strains were similar, near-UV (UVA and UVB) was hypermutagenic to a Fapy-deficient strain, causing a dose-dependent increase in induced mutation relative to wild type (up to five-fold at 200 kJ/m2). Using a plasmid back mutation assay, the predominant near-UV-induced mutations in both wild-type and Fapy-deficient strains were found to be C-->T transitions and G -->T transversions. The former is probably due to replicative bypass of pyrimidine dimers or (6-4) photoproducts that are known to be generated by near-UV, whereas the latter may be due to mispairing of GO lesions with adenine during replication. Consistent with this, the frequency of near-UV-induced G-->T transversions was 16-fold higher in a Fapy-deficient strain than a wild-type strain.

Identification and characterization of hydrogen peroxide-sensitive mutants of Escherichia coli: genes that require OxyR for expression

Escherichia coli produces an inducible set of proteins that protect the cell from exogenous peroxide stress. A subset of these genes is induced by hydrogen peroxide and is controlled at the transcriptional level by the OxyR protein. To identify additional genes involved in protection from hydrogen peroxide, a library of random transcriptional fusions of lambda(plac)Mu53 was screened for hydrogen peroxide sensitivity and 27 such mutants were identified. These fusions were transduced into nonlysogenic strains to ensure that the phenotypes observed were the result of a single mutation. The mutants were grouped into three classes based on the expression of the lacZ fusion during growth in oxyR+ and deltaoxyR backgrounds. The expression of the lacZ fusion in 8 mutants was independent of OxyR, 10 mutants required OxyR for expression, and 6 mutants showed reduced levels of expression in the presence of OxyR. OxyR dependence varied from 2- to 50-fold in these mutants. The OxyR-dependent phenotype was complemented by a plasmid-borne copy of oxyR gene in all mutants. Three mutants exhibited dual regulation by OxyR and RpoS. We sequenced the fusion junctions of several of these mutants and identified the genetic loci responsible for the hydrogen peroxide-sensitive (hps) phenotype. In this study, we report the identification of several genes that require OxyR for expression, including hemF (encoding coproporphyrinogen III oxidase), rcsC (encoding a sensor-regulator protein of capsular polysaccharide synthesis genes), and an open reading frame, f497, that is similar to arylsulfatase-encoding genes.

Role of Fapy glycosylase and UvrABC excinuclease in the repair of UVA (320-400 nm)-mediated DNA damage in Escherichia coli

In contrast to the damage caused by far-UV, the damage caused by UVA (320-400 nm) is largely oxygen dependent, suggesting near-UV-mediated DNA damage involves reactive oxygen species. The DNA repair enzymes that recognize oxidized bases may, therefore, be an important part of the cell's near-UV defense repertoire. To evaluate the relative importance of Fpg (Fapy) glycosylase (an enzyme known to remove oxidized bases) and the DNA damage-inducible UvrABC excinuclease in recovery from near-UV-induced stress, we have constructed fpg- and uvrA- derivatives of Escherichia coli and tested the response (survival) of these strains to both UVA and far-UV radiation. Relative to control strains, the fpg- derivatives were found to be consistently more sensitive to the lethal effects of UVA, but not far-UV radiation. In contrast, uvrA- mutants were more sensitive than control strains to both UVA and far-UV radiation. Thymine dimers, known to be produced by far-UV and corrected by UvrABC, were not generated by the UVA fluences used in this study, suggesting that some other UVA-induced lesion(s) is recognized and repaired by this excinuclease.

Regulation of hydroperoxidase (catalase) expression in Escherichia coli

As part of its adaptive response to oxidative stress, Escherichia coli produces two inducible hydroperoxidases called HPI and HPII. Upon exposure to sublethal levels of hydrogen peroxide, HPI expression is induced at the transcriptional level by OxyR, a member of the LysR family of autoregulators. OxyR, functioning as both a sensor and transducer, contains a critical redox-sensitive Cys residue that is oxidized by hydrogen peroxide. This is thought to induce a conformational change in the tertiary structure of the OxyR tetramer altering its DNA-binding specificity and resulting in an increase in the transcription of katG and several other OxyR-dependent genes. In contrast, synthesis of the HPII enzyme is not induced by hydrogen peroxide. Expression of both HPI and HPII is growth phase-dependent levels of HPI and HPII are 10-fold higher in stationary phase than exponential phase cultures. These growth phase-dependent increases are largely dependent on RpoS, a stationary phase specific sigma factor that is itself subject to complex transcriptional and post-transcriptional controls. Several metabolic signals have been proposed to activate the RpoS regulon including hyperosmolarity, weak acids, homoserine lactone and UDP-glucose. Since both HPI and HPII are members of the RpoS regulon, elucidation of the mechanism of regulation of RpoS should contribute to our general understanding of hydroperoxidase regulation.

Isolation and sequencing of gene fusions carried by lambda placMu specialized transducing phage

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Induction of resistance to hydrogen peroxide and radiation in Deinococcus radiodurans

Though bacteria of the radiation-resistant genus Deinococcus have a high resistance to the lethal and mutagenic effects of many DNA-damaging agents, the mechanisms involved in the response of these bacteria to oxidative stress are poorly understood. To investigate antioxidant enzyme responses in Deinococcus spp., the catalase activity produced by these bacteria was measured and the sensitivity of these bacteria to hydrogen peroxide was tested. Deinococcus spp. had higher levels of catalase and were more resistant to hydrogen peroxide than Escherichia coli K12. The high levels of catalase produced by Deinococcus radiodurans were, in part, regulated by growth phase. Cultures of D. radiodurans, when pretreated with sublethal levels of hydrogen peroxide, became relatively resistant to the lethal effects of hydrogen peroxide and exhibited higher levels of catalase than untreated control cultures. These pretreated cells were also resistant to lethality mediated by ultraviolet light and gamma-rays. These results suggest that Deinococcus spp. possess inducible defense mechanism(s) against the deleterious effects of oxidants and ionizing and ultraviolet radiation.

Induction of Escherichia coli hydroperoxidase I by acetate and other weak acids

Escherichia coli produces two independently regulated hydroperoxidases (catalases) that protect the cell from toxic concentrations of hydrogen peroxide. Hydroperoxidase I (HPI) is induced by hydrogen peroxide in an OxyR-dependent manner, while hydroperoxidase II (HPII) synthesis is regulated by an alternative sigma factor called RpoS (KatF). The activities of both hydroperoxidases increase as exponentially growing cells enter stationary phase. In this study, we examined the growth phase-dependent expression of HPI. Treatment of early-exponential-phase cells with spent culture supernatant resulted in induction of HPI synthesis. Extracellular levels of hydrogen peroxide, accumulating in the culture supernatant during late exponential phase, were found to be lower than the concentrations normally required to induce OxyR-dependent synthesis of HPI. This finding suggested that factors other than hydrogen peroxide may play a role in HPI expression. Weak acids such as acetate, which accumulate in culture supernatant and have been implicated in the regulation of HPII, caused a sixfold increase in HPI expression. Increases in HPI synthesis, mediated by weak acids and spent culture fluid supernatant, could be prevented by chloramphenicol, indicating that de novo protein synthesis is required for induction. Expression studies using a plasmid-borne lacZ transcriptional fusion to katG, the structural gene for HPI, indicated that growth phase-dependent regulation of HPI occurs primarily at the level of transcription and is dependent on RpoS. These results suggest that there may be a common regulatory mechanism of HPI and HPII expression in addition to previously described independent control mechanisms.

Regulation of katF and katE in Escherichia coli K-12 by weak acids

Chromosomal transcriptional and translational lacZ fusions to the katE (structural gene for the HPII hydroperoxidase) and katF (putative sigma factor required for katE expression) genes of Escherichia coli were isolated, and the regulation of these fusions was used to identify factors that control the expression of these two important antioxidant factors. While katE was found to be regulated primarily at the level of transcription (since induction patterns were similar for both transcriptional and translational fusions), katF expression was a function of both transcriptional and translational signals. The katE gene was induced 57-fold as cells entered the stationary phase, while katF was induced 23-fold. katF induction was coincident with katE induction and occurred at the onset of the stationary growth phase. Expression of both katE and katF could be induced by resuspending uninduced exponential-phase cells in spent culture supernatant recovered from stationary-phase cells. The component of stationary-phase culture supernatant responsible for induction of the katF regulon appeared to be acetate, since expression of both katE and katF fusions was induced when exponential-phase cells were exposed to this weak acid. Other weak acids, including propionate and benzoate, were also found to be effective inducers of expression of both katF and katE. Induction of katE and katF fusions was unaffected in merodiploid strains containing both mutant and wild-type alleles, indicating that expression of both genes is independent of the wild-type gene product. Examination of catalase zymograms prepared from cells exposed to various levels of acetate revealed that both HPI and HPII catalases are induced by this weak acid, suggesting that there is a common link in the regulation of these two enzymes.