Role of Escherichia coli rpoS and associated genes in defense against oxidative damage

Wavelength synergistic effects in continuous flow-through water disinfection systems

The past decade's development of UV LEDs has fueled significant research in water disinfection, with widespread debate surrounding the potential synergies of multiple UV wavelengths. This study analyses the use of three UV sources (265, 275, and 310 nm) on the inactivation of Escherichia coli bacteria in two water matrixes. At maximum intensity in wastewater, individual inactivation experiments in a single pass set-up (Flow rate = 2 L min-1, Residence time = 0.75 s) confirmed the 265 nm light source to be the most effective (2.2 ± 0.2 log units), while the 310 nm led to the lowest inactivation rate (0.0003 ± 7.03× 10-5 log units). When a combination of the three wavelengths was used, an average log reduction of 4.4 ± 0.2 was observed in wastewater. For combinations of 265 and 275 nm, the average log reductions were similar to the sum of individual log reductions. For combinations involving the use of 310 nm, a potential synergistic effect was investigated by the use of robust statistical analysis techniques. It is concluded that combinations of 310 nm with 265 nm or 275 nm devices, in sequential and simultaneous mode, present a significant synergy at both intensities due to the emission spectra of the selected LEDs, ensuring the possibility of two inactivation mechanisms. Finally, the electrical energy per order of inactivation found the three-wavelength combination to be the most energy efficient (0.39 ± 0.05, 0.36 ± 0.01 kWh m-3, at 50% and 100% dose, respectively, in wastewater) among the synergistic combinations.

Kosakonia radicincitans with hypervirulent lON genes causes human bloodstream infections

Kosakonia radicincitans is a species within the new genus Kosakonia, which is typically a plant pathogen, with rare reports of human infection. The number of human infections may be underestimated because this new genus is under-represented among diagnostic tools. This report describes a case of bloodstream infection caused by K. radicincitans. The pathogen was identified by matrix-assisted laser desorption/ionization-TOF mass spectrometry and 16S rRNA gene sequencing. The hypervirulent human pathogenicity gene LON, which has not been described before, was detected in the bacterial genome by gene annotation. Thus, this discovery provides a new reference for studying the pathogenic mechanism of this rare pathogen.

Transcriptomic Response Analysis of Escherichia coli to Palladium Stress

Palladium (Pd), due to its unique catalytic properties, is an industrially important heavy metal especially in the form of nanoparticles. It has a wide range of applications from automobile catalytic converters to the pharmaceutical production of morphine. Bacteria have been used to biologically produce Pd nanoparticles as a new environmentally friendly alternative to the currently used energy-intensive and toxic physicochemical methods. Heavy metals, including Pd, are toxic to bacterial cells and cause general and oxidative stress that hinders the use of bacteria to produce Pd nanoparticles efficiently. In this study, we show in detail the Pd stress-related effects on E. coli. Pd stress effects were measured as changes in the transcriptome through RNA-Seq after 10 min of exposure to 100 μM sodium tetrachloropalladate (II). We found that 709 out of 3,898 genes were differentially expressed, with 58% of them being up-regulated and 42% of them being down-regulated. Pd was found to induce several common heavy metal stress-related effects but interestingly, Pd causes unique effects too. Our data suggests that Pd disrupts the homeostasis of Fe, Zn, and Cu cellular pools. In addition, the expression of inorganic ion transporters in E. coli was found to be massively modulated due to Pd intoxication, with 17 out of 31 systems being affected. Moreover, the expression of several carbohydrate, amino acid, and nucleotide transport and metabolism genes was vastly changed. These results bring us one step closer to the generation of genetically engineered E. coli strains with enhanced capabilities for Pd nanoparticles synthesis.

Involvement of CspC in response to diverse environmental stressors in Escherichia coli

The ability of Escherichia coli surviving a cold shock lies mainly with the induction of a few Csps termed as 'Major cold shock proteins'. Regardless of high sequence similarity among the nine homologous members, CspC appears to be functionally diverse in conferring the cell adaptability to various stresses based on fundamental properties of the protein including nucleic acid binding, nucleic acid melting and regulatory activity. Spanning three different stress regulons of acid, oxidative and heat, CspC regulates gene expression and transcript stability of stress proteins and bestows upon the cell tolerance to lethal-inducing agents ultimately helping it adapt to severe environmental assaults. While its exact role in cellular physiology is still to be detailed, understanding the transcriptional and translational control will likely provide insights into the mechanistic role of CspC under stress conditions. To this end, we review the knowledge on stress protein regulation by CspC and highlight its activity in response to stressors thereby elucidating its role as a major Csp player in response to one too many environmental triggers. The knowledge presented here could see various downstream applications in engineering microbes for industrial, agricultural and research applications in order to achieve high product efficiency and to aid bacteria cope with environmentally harsh conditions.

Photocatalytic TiO2-Based Nanostructured Materials for Microbial Inactivation

Pathogenic microorganisms can spread throughout the world population, as the current COVID-19 pandemic has dramatically demonstrated. In this scenario, a protection against pathogens and other microorganisms can come from the use of photoactive materials as antimicrobial agents able to hinder, or at least limit, their spreading by means of photocatalytically assisted processes activated by light—possibly sunlight—promoting the formation of reactive oxygen species (ROS) that can kill microorganisms in different matrices such as water or different surfaces without affecting human health. In this review, we focus the attention on TiO2 nanoparticle-based antimicrobial materials, intending to provide an overview of the most promising synthetic techniques, toward possible large-scale production, critically review the capability of such materials to promote pathogen (i.e., bacteria, virus, and fungi) inactivation, and, finally, take a look at selected technological applications.

Modeling Salmonella Typhimurium Inactivation in Dry-Fermented Sausages: Previous Habituation in the Food Matrix Undermines UV-C Decontamination Efficacy

The effects of previous Salmonella Typhimurium habituation to an Italian-style salami concerning pathogen resistance against ultraviolet-C light (UV-C) treatment were modeled in order to establish treatment feasibility for the decontamination of dry-fermented sausage. S. Typhimurium following 24 h habituation in fermented sausage (habituated cells) or non-habituation (non-habituated cells) were exposed to increasing UV-C radiation treatment times. The Weibull model was the best fit for describing S. Typhimurium UV-C inactivation. Heterogeneity in UV-C treatment susceptibilities within the S. Typhimurium population was observed, revealing intrinsic persistence in a sub-population. UV-C radiation up to 1.50 J/cm2 was a feasible treatment for dry-fermented sausage decontamination, as the matrices retained instrumental color and lipid oxidation physiochemical characteristics. However, habituation in the sausage matrix led to a 14-fold increase in the UV-C dose required to achieve the first logarithm reduction (δ value) in S. Typhimurium population. The results indicate that, although UV-C radiation might be considered an efficient method for dry-fermented sausage decontamination, effective doses should be reconsidered in order to reach desirable food safety parameters while preserving matrix quality.

Effect of O antigen ligase gene mutation on oxidative stress resistance and pathogenicity of NMEC strain RS218

Escherichia coli is one of the primary causes of bacterial sepsis and meningitis in newborns. E. coli RS218, a prototype strain of neonatal meningitis E. coli (NMEC), is often used in research on the pathogenesis of NMEC. Phagocytes are crucial sentinels of immunity, and their antibacterial ability is largely determined by the capability to produce large amounts of ROS. The capacity of bacteria to endure oxidative pressure affects their colonization in the host. Here, we systematically screened the genes that plays key roles in the tolerance of the model of E. coli RS218 to peroxygen environment using a Tn5 mutant library. As a result, a gene encoding O antigen polymerase (O antigen ligase) that contains the Wzy_C superfamily domain (herein designated as Ocw) was identified in E. coli RS218. Furthermore, we constructed an isogenic deletion mutant of ocw gene and its complementary strain in E. coli. Our results revealed that ocw affects the lipopolysaccharide synthesis, ROS tolerance, and survival of E. coli in the host environment. The discovery of ocw provides important clues for better understanding the function of O-antigen.

Salmonella enterica serovar Typhimurium has three transketolase enzymes contributing to the pentose phosphate pathway

The genus Salmonella is responsible for many illnesses in humans and other vertebrate animals. We report here that Salmonella enterica serovar Typhimurium harbors three transketolases that support the non-oxidative branch of the pentose phosphate pathway. BLAST analysis identified two genes, STM14_2885 and STM14_2886, that together encode a putative transketolase (TktC) with 46-47% similarity to the known TktA and TktB isoforms. Assessing the mRNA and protein expression for each of the three transketolases, we determined that all are expressed in WT cells and regulated to varying extents by the alternative sigma factor RpoS. Enzyme assays with lysates from WT and transketolase-knockout strains established that TktA is responsible for >88% of the transketolase activity in WT cells. We purified recombinant forms of each isoenzyme to assess the kinetics for canonical transketolase reactions. TktA and TktB had comparable values for V_max (539-1362 μm NADH consumed/s), K_m (80-739 μm), and catalytic efficiency (1.02 × 10⁸-1.06 × 10⁹ m^-1/s) for each substrate tested. The recombinant form of TktC had lower K_m values (23-120 μm), whereas the V_max (7.8-16 μm NADH consumed/s) and catalytic efficiency (5.58 × 10⁶ to 6.07 × 10⁸ m^-1/s) were 10-100-fold lower. Using a murine model of Salmonella infection, we showed that a strain lacking all three transketolases is avirulent in C57BL/6 mice. These data provide evidence that S Typhimurium possesses three transketolases that contribute to pathogenesis.

Role of extracytoplasmic function sigma factor PG1660 (RpoE) in the oxidative stress resistance regulatory network of Porphyromonas gingivalis

In Porphyromonas gingivalis, the protein PG1660, composed of 174 amino acids, is annotated as an extracytoplasmic function (ECF) sigma factor (RpoE homologue-σ24). Because PG1660 can modulate several virulence factors and responds to environmental signals in P. gingivalis, its genetic properties were evaluated. PG1660 is co-transcribed with its downstream gene PG1659, and the transcription start site was identified as adenine residue 54-nucleotides upstream of the ATG translation start codon. In addition to binding its own promoter, using the purified rPG1660 and RNAP core enzyme from Escherichia coli with the PG1660 promoter DNA as template, the function of PG1660 as a sigma factor was demonstrated in an in vitro transcription assay. Transcriptome analyses of a P. gingivalis PG1660-defective isogenic mutant revealed that under oxidative stress conditions 176 genes including genes involved in secondary metabolism were downregulated more than two-fold compared with the parental strain. The rPG1660 protein also showed the ability to bind to the promoters of the highly downregulated genes in the PG1660-deficient mutant. As the ECF sigma factor PG0162 has a 29% identity with PG1660 and can modulate its expression, the cross-talk between their regulatory networks was explored. The expression profile of the PG0162PG1660-deficient mutant (P. gingivalis FLL356) revealed that the type IX secretion system genes and several virulence genes were downregulated under hydrogen peroxide stress conditions. Taken together, we have confirmed that PG1660 can function as a sigma factor, and plays an important regulatory role in the oxidative stress and virulence regulatory network of P. gingivalis.

Importance of stress-response genes to the survival of airborne Escherichia coli under different levels of relative humidity

Other than the needs for infection control to investigate the survival and inactivation of airborne bacterial pathogens, there has been a growing interest in exploring bacterial communities in the air and the effect of environmental variables on them. However, the innate biological mechanism influencing the bacterial viability is still unclear. In this study, a mutant-based approach, using Escherichia coli as a model, was used to prove the concept that common stress-response genes are important for airborne survival of bacteria. Mutants with a single gene knockout that are known to respond to general stress (rpoS) and oxidative stress (oxyR, soxR) were selected in the study. Low relative humidity (RH), 30–40% was more detrimental to the bacteria than high RH, >90%. The log reduction of ∆rpoS was always higher than that of the parental strain at all RH levels but the ∆oxyR had a higher log reduction than the parental strain at intermediate RH only. ∆soxR had the same viability compared to the parental strain at all RH levels. The results hint that although different types and levels of stress are produced under different RH conditions, stress-response genes always play a role in the bacterial viability. This study is the first reporting the association between stress-response genes and viability of airborne bacteria.

Effect of extracytoplasmic function sigma factors on autoaggregation, hemagglutination, and cell surface properties of Porphyromonas gingivalis

Porphyromonas gingivalis is a bacterium frequently isolated from chronic periodontal lesions and is involved in the development of chronic periodontitis. To colonize the gingival crevice, P. gingivalis has to adapt to environmental stresses. Microbial gene expression is regulated by transcription factors such as those in two-component systems and extracytoplasmic function (ECF) sigma factors. ECF sigma factors are involved in the regulation of environmental stress response genes; however, the roles of individual ECF sigma factors are largely unknown. The purpose of this study was to investigate the functions, including autoaggregation, hemagglutination, gingipain activity, susceptibility to antimicrobial agents, and surface structure formation, of P. gingivalis ECF sigma factors encoded by SigP (PGN_0274), SigCH (PGN_0319), PGN_0450, PGN_0970, and SigH (PGN_1740). Various physiological aspects of the sigP mutant were affected; autoaggregation was significantly decreased at 60 min (p < 0.001), hemagglutination activity was markedly reduced, and enzymatic activities of Kgp and Rgps were significantly decreased (p < 0.001). The other mutants also showed approximately 50% reduction in Rgps activity. Kgp activity was significantly reduced in the sigH mutant (p < 0.001). No significant differences in susceptibilities to tetracycline and ofloxacin were observed in the mutants compared to those of the wild-type strain. However, the sigP mutant displayed an increased susceptibility to ampicillin, whereas the PGN_0450 and sigH mutants showed reduced susceptibility. Transmission electron microscopy images revealed increased levels of outer membrane vesicles formed at the cell surfaces of the sigP mutant. These results indicate that SigP is important for bacterial surface-associated activities, including gingipain activity, autoaggregation, hemagglutination, vesicle formation, and antimicrobial susceptibility.

Inactivation of Transcriptional Regulators during Within-Household Evolution of Escherichia coli

We analyzed the within-household evolution of two household-associated Escherichia coli strains from pandemic clonal group ST131-H30, using isolates recovered from five individuals within two families, each of which had a distinct strain. Family 1's strain was represented by a urine isolate from the index patient (older sister) with recurrent cystitis and a blood isolate from her younger sister with fatal urosepsis. Family 2's strain was represented by a urine isolate from the index patient (father) with pyelonephritis and renal abscesses, blood and kidney drainage isolates from the daughter with emphysematous pyelonephritis, and urine and fecal isolates from the mother with cystitis. Collectively, the several variants of each family's strain had accumulated a total of 8 (family 1) and 39 (family 2) point mutations; no two isolates were identical. Of the 47 total mutations, 36 resulted in amino acid changes or truncation of coded proteins. Fourteen such mutations (39%) targeted genes encoding transcriptional regulators, and 9 (25%) involved DNA-binding transcription factors (TFs), which significantly exceeded the relative contribution of TF genes to the isolates' genomes (∼6%). At least one-half of the transcriptional regulator mutations were inactivating, based on phenotypic and/or transcriptional analysis. In particular, inactivating mutations in the global regulator LrhA (repressor of type 1 fimbriae and flagella) occurred in the blood isolates from both households and increased the virulence of E. coli strains in a murine sepsis model. The results indicate that E. coli undergoes adaptive evolution between and/or within hosts, generating subpopulations with distinctive phenotypes and virulence potential.IMPORTANCE The clonal evolution of bacterial strains associated with interhost transmission is poorly understood. We characterized the genome sequences of clonal descendants of two Escherichia coli strains, recovered at different time points from multiple individuals within two households who had different types of urinary tract infection. We found evidence that the E. coli strains underwent extensive mutational diversification between and within these individuals, driven disproportionately by inactivation of transcriptional regulators. In urosepsis isolates, the mutations observed in the global regulator LrhA increased bacterial virulence in a murine sepsis model. Our findings help in understanding the adaptive dynamics and strategies of E. coli during short-term natural evolution.

Role of oxidative stress in inactivation of Escherichia coli BW25113 by nanoscale zero-valent iron

An Escherichia coli BW25113 wildtype strain and mutant strains lacking genes that protect against oxidative stress were examined at different growth phases for susceptibility to zero-valent iron (nZVI). Viability of cells was determined by the plate count method. All mutant strains were more susceptible than the wild type strain to nZVI; however, susceptibility differed among the mutant strains. Consistent with the role of rpoS as a global stress regulator, an rpoS gene knockout mutant exhibited the greatest susceptibility to nZVI under the majority of conditions tested (except exponential and declining phases at longer exposure time). Mutants lacking genes encoding the inducible and constitutively expressed cytosolic superoxide dismutases, sodA and sodB, respectively, were more susceptible to nZVI than a mutant lacking the gene encoding sodC, a periplasmic superoxide dismutase. This suggests that nZVI induces oxidative stress inside the cells via superoxide generation. Quantitative polymerase chain reaction was used to examine the expression of katG, a gene encoding the catalase-peroxidase enzyme, in nZVI-treated E. coli at different growth phases. Results showed that nZVI repressed the expression of katG in all but lag phases.

Oxidative Stress

The ancestors of Escherichia coli and Salmonella ultimately evolved to thrive in air-saturated liquids, in which oxygen levels reach 210 μM at 37°C. However, in 1976 Brown and colleagues reported that some sensitivity persists: growth defects still become apparent when hyperoxia is imposed on cultures of E. coli. This residual vulnerability was important in that it raised the prospect that normal levels of oxygen might also injure bacteria, albeit at reduced rates that are not overtly toxic. The intent of this article is both to describe the threat that molecular oxygen poses for bacteria and to detail what we currently understand about the strategies by which E. coli and Salmonella defend themselves against it. E. coli mutants that lack either superoxide dismutases or catalases and peroxidases exhibit a variety of growth defects. These phenotypes constitute the best evidence that aerobic cells continually generate intracellular superoxide and hydrogen peroxide at potentially lethal doses. Superoxide has reduction potentials that allow it to serve in vitro as either a weak univalent reductant or a stronger univalent oxidant. The addition of micromolar hydrogen peroxide to lab media will immediately block the growth of most cells, and protracted exposure will result in the loss of viability. The need for inducible antioxidant systems seems especially obvious for enteric bacteria, which move quickly from the anaerobic gut to fully aerobic surface waters or even to ROS-perfused phagolysosomes. E. coli and Salmonella have provided two paradigmatic models of oxidative-stress responses: the SoxRS and OxyR systems.

Exploring new roles for the rpoS gene in the survival and virulence of the fire blight pathogen Erwinia amylovora

Erwinia amylovora causes fire blight in economically important plants of the family Rosaceae. This bacterial pathogen spends part of its life cycle coping with starvation and other fluctuating environmental conditions. In many Gram-negative bacteria, starvation and other stress responses are regulated by the sigma factor RpoS. We obtained an E. amylovora rpoS mutant to explore the role of this gene in starvation responses and its potential implication in other processes not yet studied in this pathogen. Results showed that E. amylovora needs rpoS to develop normal starvation survival and viable but nonculturable (VBNC) responses. Furthermore, this gene contributed to stationary phase cross-protection against oxidative, osmotic, and acid stresses and was essential for cross-protection against heat shock, but nonessential against acid shock. RpoS also mediated regulation of motility, exopolysaccharide synthesis, and virulence in immature loquats, but not in pear plantlets, and contributed to E. amylovora survival in nonhost tissues during incompatible interactions. Our results reveal some unique roles for the rpoS gene in E. amylovora and provide new knowledge on the regulation of different processes related to its ecology, including survival in different environments and virulence in immature fruits.

Bacterial sRNAs: regulation in stress

Bacteria are often exposed to a hostile environment and have developed a plethora of cellular processes in order to survive. A burgeoning list of small non-coding RNAs (sRNAs) has been identified and reported to orchestrate crucial stress responses in bacteria. Among them, cis-encoded sRNA, trans-encoded sRNA, and 5'-untranslated regions (UTRs) of the protein coding sequence are influential in the bacterial response to environmental cues, such as fluctuation of temperature and pH as well as other stress conditions. This review summarizes the role of bacterial sRNAs in modulating selected stress conditions and highlights the alliance between stress response and clustered regularly interspaced short palindromic repeats (CRISPR) in bacterial defense.

Growth conditions influence UVB sensitivity and oxidative damage in an estuarine bacterial isolate

The dose-dependent variation of oxidative cellular damage imposed by UVB exposure in a representative estuarine bacterial strain, Pseudomonas sp. NT5I1.2B, was studied at different growth phases (mid-exponential, late-exponential, and stationary), growth temperatures (15 °C and 25 °C) and growth media (nutrient-rich Tryptic Soy Broth [TSB] and nutrient-poor M9). Survival and markers of oxidative damage (lipid peroxidation, protein carbonylation, DNA strand breakage, and DNA-protein cross-links) were monitored during exposure to increasing UVB doses (0-60 kJ m(-2)). Oxidative damage did not follow a clear linear dose-dependent pattern, particularly at high UVB doses (>10 kJ m(-2)), suggesting a dynamic interaction between damage induction and repair during irradiation and/or saturation of oxidative damage. Survival of stationary phase cells generally exceeded that of exponential phase cells by up to 33.5 times; the latter displayed enhanced levels of DNA-protein cross-links (up to 15.6-fold) and protein carbonylation (up to 6.0-fold). Survival of mid-exponential phase cells was generally higher at 15 °C than at 25 °C (up to 6.6-fold), which was accompanied by lower levels of DNA strand breaks (up to 4000-fold), suggesting a temperature effect on reactive oxygen species (ROS) generation and/or ROS interaction with cellular targets. Survival under medium-high UVB doses (>10 kJ m(-2)) was generally higher (up to 5.4-fold) in cells grown in TSB than in M9. These results highlight the influence of growth conditions preceding irradiation on the extent of oxidative damage induced by UVB exposure in bacteria.

RpoS contributes to phagocyte oxidase-mediated stress resistance during urinary tract infection by Escherichia coli CFT073

Uropathogenic Escherichia coli (UPEC) is the most common causative agent of community-acquired urinary tract infection (UTI). In order to cause UTI, UPEC must endure stresses ranging from nutrient limitation to host immune components. RpoS (σ^S), the general stress response sigma factor, directs gene expression under a variety of inhibitory conditions. Our study of rpoS in UPEC strain CFT073 began after we discovered an rpoS-frameshift mutation in one of our laboratory stocks of “wild-type” CFT073. We demonstrate that an rpoS-deletion mutation in CFT073 leads to a colonization defect during UTI of CBA/J mice at 48 hours postinfection (hpi). There is no difference between the growth rates of CFT073 and CFT073 rpoS in urine. This indicates that rpoS is needed for replication and survival in the host rather than being needed to address limitations imposed by urine nutrients. Consistent with previous observations in E. coli K-12, CFT073 rpoS is more sensitive to oxidative stress than the wild type. We demonstrate that peroxide levels are elevated in voided urine from CFT073-infected mice compared to urine from mock-infected mice, which supports the notion that oxidative stress is generated by the host in response to UPEC. In mice that lack phagocyte oxidase, the enzyme complex expressed by phagocytes that produces superoxide, the competitive defect of CFT073 rpoS in bladder colonization is lost. These results demonstrate that σ^S is important for UPEC survival under conditions of phagocyte oxidase-generated stress during UTI. Though σ^S affects the pathogenesis of other bacterial species, this is the first work that directly implicates σ^S as important for UPEC pathogenesis.

UPEC must cope with a variety of stressful conditions in the urinary tract during infection. RpoS (σ^S), the general stress response sigma factor, is known to direct the expression of many genes under a variety of stressful conditions in laboratory-adapted E. coli K-12. Here, we show that σ^S is needed by the model UPEC strain CFT073 to cope with oxidative stress provided by phagocytes during infection. These findings represent the first report that implicates σ^S in the fitness of UPEC during infection and support the idea of the need for a better understanding of the effects of this global regulator of gene expression during UTI.

Adenylate cyclase and the cyclic AMP receptor protein modulate stress resistance and virulence capacity of uropathogenic Escherichia coli

In many bacteria, the second messenger cyclic AMP (cAMP) interacts with the transcription factor cAMP receptor protein (CRP), forming active cAMP-CRP complexes that can control a multitude of cellular activities, including expanded carbon source utilization, stress response pathways, and virulence. Here, we assessed the role of cAMP-CRP as a regulator of stress resistance and virulence in uropathogenic Escherichia coli (UPEC), the principal cause of urinary tract infections worldwide. Deletion of genes encoding either CRP or CyaA, the enzyme responsible for cAMP synthesis, attenuates the ability of UPEC to colonize the bladder in a mouse infection model, dependent on intact innate host defenses. UPEC mutants lacking cAMP-CRP grow normally in the presence of glucose but are unable to utilize alternate carbon sources like amino acids, the primary nutrients available to UPEC within the urinary tract. Relative to the wild-type UPEC isolate, the cyaA and crp deletion mutants are sensitive to nitrosative stress and the superoxide generator methyl viologen but remarkably resistant to hydrogen peroxide (H(2)O(2)) and acid stress. In the mutant strains, H(2)O(2) resistance correlates with elevated catalase activity attributable in part to enhanced translation of the alternate sigma factor RpoS. Acid resistance was promoted by both RpoS-independent and RpoS-dependent mechanisms, including expression of the RpoS-regulated DNA-binding ferritin-like protein Dps. We conclude that balanced input from many cAMP-CRP-responsive elements, including RpoS, is critical to the ability of UPEC to handle the nutrient limitations and severe environmental stresses present within the mammalian urinary tract.

Regulators of oxidative stress response genes in Escherichia coli and their functional conservation in bacteria

Oxidative stress, through the production of reactive oxygen species, is a natural consequence of aerobic metabolism. Escherichia coli has several major regulators activated during oxidative stress, including OxyR, SoxRS, and RpoS. OxyR and SoxR undergo conformation changes when oxidized in the presence of hydrogen peroxide and superoxide radicals, respectively, and subsequently control the expression of cognate genes. In contrast, the RpoS regulon is induced by an increase in RpoS levels. Current knowledge regarding the activation and function of these regulators and their dependent genes in E. coli during oxidative stress forms the scope of this review. Despite the enormous genomic diversity of bacteria, oxidative stress response regulators in E. coli are functionally conserved in a wide range of bacterial groups, possibly reflecting positive selection of these regulators. SoxRS and RpoS homologs are present and respond to oxidative stress in Proteobacteria, and OxyR homologs are present and function in H(2)O(2) resistance in a range of bacteria, from gammaproteobacteria to Actinobacteria. Bacteria have developed complex, adapted gene regulatory responses to oxidative stress, perhaps due to the prevalence of reactive oxygen species produced endogenously through metabolism or due to the necessity of aerotolerance mechanisms in anaerobic bacteria exposed to oxygen.

Stationary-Phase Gene Regulation in Escherichia coli §

In their stressful natural environments, bacteria often are in stationary phase and use their limited resources for maintenance and stress survival. Underlying this activity is the general stress response, which in Escherichia coli depends on the σS (RpoS) subunit of RNA polymerase. σS is closely related to the vegetative sigma factor σ70 (RpoD), and these two sigmas recognize similar but not identical promoter sequences. During the postexponential phase and entry into stationary phase, σS is induced by a fine-tuned combination of transcriptional, translational, and proteolytic control. In addition, regulatory "short-cuts" to high cellular σS levels, which mainly rely on the rapid inhibition of σS proteolysis, are triggered by sudden starvation for various nutrients and other stressful shift conditons. σS directly or indirectly activates more than 500 genes. Additional signal input is integrated by σS cooperating with various transcription factors in complex cascades and feedforward loops. Target gene products have stress-protective functions, redirect metabolism, affect cell envelope and cell shape, are involved in biofilm formation or pathogenesis, or can increased stationary phase and stress-induced mutagenesis. This review summarizes these diverse functions and the amazingly complex regulation of σS. At the molecular level, these processes are integrated with the partitioning of global transcription space by sigma factor competition for RNA polymerase core enzyme and signaling by nucleotide second messengers that include cAMP, (p)ppGpp, and c-di-GMP. Physiologically, σS is the key player in choosing between a lifestyle associated with postexponential growth based on nutrient scavenging and motility and a lifestyle focused on maintenance, strong stress resistance, and increased adhesiveness. Finally, research with other proteobacteria is beginning to reveal how evolution has further adapted function and regulation of σS to specific environmental niches.

Susceptibility of a Vibrio alginolyticus rpoS mutant to environmental stresses and its expression of OMPs

Vibrio alginolyticus, one of the most important opportunistic pathogens, can be detected in human being and marine animals. Like other bacteria, V. alginolyticus is able to adapt to a variety of stressful environmental changes. The alternate sigma factor RpoS, which is a regulator during stationary phase, plays an important role in surviving under these stressful situations in many bacteria. Sequence analysis reveals a 990 bp open reading frame which is predicted to encode a 330-amino-acid protein with 68% to 96% overall identity to other reported sequences. To study the function of rpoS, the rpoS gene of V. alginolyticus VIB283 was cloned and an rpoS mutant was constructed by homologous recombination. Comparison of the study result of the wild type and the mutant showed that the mutant was more sensitive to stress conditions such as high osmolarity, oxidative stress, heat shock, and long-term starvation and that the LD(50) of the mutant strain to the zebra fish was about 2.8 times as that of the control strain. In addition, the SDS-PAGE analysis indicated that the outer membrane proteins (OMPs) existed great differences.

Genetic diversity among offspring from archived Salmonella enterica ssp. enterica serovar typhimurium (Demerec Collection): in search of survival strategies

Extensive phenotypic and genomic diversity was detected among offspring of Salmonella enterica ssp. enterica serovar Typhimurium LT2 (nonmutator) and LT7 (mutator, mutL) strains after decades of storage in sealed nutrient agar stabs. In addition to numerous losses in carbon and nitrogen metabolism, the acquired new metabolites indicated that alternate pathways were established. Particularly striking was the array of phage types when this phenotype was expected to be a stable feature. Evidence is presented regarding the role of mutator gene mutL(-) in the establishment of diversity as well as the ability of cells to return to mutL(+) genetic stabilization. Mutations included deletions, duplications, frameshifts, inversions and transpositions. In competition tests, survivors were more fit than were wild type. Because survival strategies continue to intrigue microbiologists, observations are compared with those of others who have addressed related questions. A brief genealogy of the archived strains is also recorded.

Porphyromonas gingivalis mutant defective in a putative extracytoplasmic function sigma factor shows a mutator phenotype

INTRODUCTION

Porphyromonas gingivalis is implicated as a major pathogen in the development and progression of chronic periodontitis. P. gingivalis must possess the ability to tolerate stress signals outside the cytoplasmic membrane by transcriptional activation of genes encoding proteins involved in defense or repair processes. Some bacteria utilize a distinct subfamily of sigma factors to regulate extracytoplasmic function (hence termed the ECF subfamily).

METHODS

To elucidate their role in P. gingivalis, a chromosomal mutant carrying a disruption of an ECF sigma factor PG1318-encoding gene was constructed. Hemagglutination and proteolytic activities were measured in the PG1318-defective mutant. Reverse transcription-polymerase chain reaction (RT-PCR) analysis and southern blot analysis were used to assess transcription of kgp in the PG1318-defective mutant. Frequency of spontaneous mutation that conferred resistance to l-trifluoromethionine was measured in the PG1318-defective mutant.

RESULTS

The PG1318-defective mutant formed non-pigmented colonies on blood agar plates at a relatively high frequency. Arginine-specific and lysine-specific proteinase activities of the non-pigmented variants were remarkably decreased compared with those of the parent strain and the pigmented variants. RT-PCR analysis showed that kgp was not transcribed in some non-pigmented variants and southern blot analysis revealed that there was a deletion in their kgp region. Frequency of mutation conferring resistance to l-trifluoromethionine was significantly higher in the PG1318-defective mutant than in the wild-type.

CONCLUSION

These results suggest that PG1318 plays a role in the regulation of mutation frequency in the bacterium.

Functionally undefined gene, yggE, alleviates oxidative stress generated by monoamine oxidase in recombinant Escherichia coli

Real-time PCR analysis showed that yggE gene was about two and three times up-regulated in Escherichia coli cells exposed to UVA irradiation and thermal elevation, respectively, suggesting that this gene is responsive to physiological stress. The yggE gene was introduced into E. coli BL21 cells, together with a monoamine oxidase (MAO) gene as a model source for oxidative stress generation. The distribution of independently isolated transformants (two dozen isolates) was examined in terms of MAO activity and cell vitality. In the case of control strain expressing MAO alone, the largest number of transformants existed in the low range of MAO activity less than 2 units mg(-1) and the number significantly decreased at increased MAO activity. On the other hand, the distribution of MAO/YggE-coexpressing transformants shifted to higher MAO activity with frequent appearance in the activity range of 4-8 units mg(-1). The yggE gene product therefore has a possible function for alleviating the stress generated in the cells.

Comparative genomic hybridization and physiological characterization of environmental isolates indicate that significant (eco-)physiological properties are highly conserved in the species Escherichia coli

Escherichia coli, the common inhabitant of the mammalian intestine, exhibits considerable intraspecies genomic variation, which has been suggested to reflect adaptation to different ecological niches. Also, regulatory trade-offs, e.g. between catabolic versatility and stress protection, are thought to result in significant physiological differences between strains. For these reasons, the relevance of experimental observations made for 'domesticated' E. coli strains with regard to the behaviour of this species in its natural environments is often questioned and doubts are frequently raised on the status of E. coli as a defined species. The variability of important (eco-)physiological functions, such as carbon substrate uptake and breakdown capabilities, as well as stress defence mechanisms, in the genomes of commensal and pathogenic E. coli strains were therefore investigated. Furthermore, (eco-)physiological properties of environmental strains were compared to standard laboratory strain K-12 MG1655. Catabolic, stress protection, and carbon- and energy source transport operons showed a very low intraspecies variability in 57 commensal and pathogenic E. coli. Environmental isolates adapted to glucose-limited growth in a similar way as E. coli MG1655, namely by increasing their catabolic flexibility and by inducing high-affinity substrate uptake systems. The results obtained indicate that significant (eco-)physiological properties are highly conserved in the natural population of E. coli. This questions the proposed dominant role of horizontal gene transfer for niche adaptation.

TiO2 photocatalysis causes DNA damage via fenton reaction-generated hydroxyl radicals during the recovery period

Here, we show that resistance of Escherichia coli to TiO2 photocatalysis involves defenses against reactive oxygen species. Results support the idea that TiO2 photocatalysis generates damage which later becomes deleterious during recovery. We found this to be partly due to DNA attack via hydroxyl radicals generated by the Fenton reaction during recovery.

Causes and consequences of DNA repair activity modulation during stationary phase in Escherichia coli

Escherichia coli responds to nutrient exhaustion by entering a state commonly referred to as the stationary phase. Cells entering the stationary phase redirect metabolic circuits to scavenge any available nutrients and become resistant to different stresses. However, many DNA repair pathways are downregulated in stationary-phase cells, which results in increased mutation rates. DNA repair activity generally depends on consumption of energy and often requires de novo proteins synthesis. Consequently, unless stringently regulated during stationary phase, DNA repair activities may lead to an irreversible depletion of energy sources and, therefore to cell death. Most stationary phase morphological and physiological modifications are regulated by an alternative RNA polymerase sigma factor RpoS. However, nutrient availability, and the frequency and nature of stresses, are different in distinct environmental niches, which impose conflicting choices that result in selection of the loss or of the modification of RpoS function. Consequently, DNA repair activity, which is partially controlled by RpoS, is differently modulated in different environments. This results in the variable mutation rates among different E. coli ecotypes. Hence, the polymorphism of mutation rates in natural E. coli populations can be viewed as a byproduct of the selection for improved fitness.

Decline in ribosomal fidelity contributes to the accumulation and stabilization of the master stress response regulator sigmaS upon carbon starvation

The sigma(S) subunit of RNA polymerase is a master regulator of Escherichia coli that retards cellular senescence and bestows cells with general stress protective functions during growth arrest. We show that mutations and drugs triggering translational errors elevate sigma(S) levels and stability. Furthermore, mutations enhancing translational fidelity attenuate induction of the rpoS regulon and prevent stabilization of sigma(S) upon carbon starvation. Destabilization of sigma(S) by increased proofreading requires the presence of the sigma(S) recognition factor SprE (RssB) and the ClpXP protease. The data further suggest that sigma(S) becomes stabilized upon starvation as a result of ClpP sequestration and this sequestration is enhanced by oxidative modifications of aberrant proteins produced by erroneous translation. ClpP overproduction counteracted starvation-induced stabilization of sigma(S), whereas overproduction of a ClpXP substrate (ssrA-tagged GFP) stabilized sigma(S) in exponentially growing cells. We present a model for the sequence of events leading to the accumulation and activation of sigma(S) upon carbon starvation, which are linked to alterations in both ribosomal fidelity and efficiency.

Detection and quantification of superoxide formed within the periplasm of Escherichia coli

Many gram-negative bacteria harbor a copper/zinc-containing superoxide dismutase (CuZnSOD) in their periplasms. In pathogenic bacteria, one role of this enzyme may be to protect periplasmic biomolecules from superoxide that is released by host phagocytic cells. However, the enzyme is also present in many nonpathogens and/or free-living bacteria, including Escherichia coli. In this study we were able to detect superoxide being released into the medium from growing cultures of E. coli. Exponential-phase cells do not normally synthesize CuZnSOD, which is specifically induced in stationary phase. However, the engineered expression of CuZnSOD in growing cells eliminated superoxide release, confirming that this superoxide was formed within the periplasm. The rate of periplasmic superoxide production was surprisingly high and approximated the estimated rate of cytoplasmic superoxide formation when both were normalized to the volume of the compartment. The rate increased in proportion to oxygen concentration, suggesting that the superoxide is generated by the adventitious oxidation of an electron carrier. Mutations that eliminated menaquinone synthesis eradicated the superoxide formation, while mutations in genes encoding respiratory complexes affected it only insofar as they are likely to affect the redox state of menaquinone. We infer that the adventitious autoxidation of dihydromenaquinone in the cytoplasmic membrane releases a steady flux of superoxide into the periplasm of E. coli. This endogenous superoxide may create oxidative stress in that compartment and be a primary substrate of CuZnSOD.

Evidence that feedback inhibition of NAD kinase controls responses to oxidative stress

Formation of NADP+ from NAD+ is catalyzed by NAD kinase (NadK; EC 2.7.1.23). Evidence is presented that NadK is the only NAD kinase of Salmonella enterica and is essential for growth. NadK is inhibited allosterically by NADPH and NADH. Without effectors, NadK exists as an equilibrium mixture of dimers and tetramers (KD = 1.0 +/- 0.8 mM) but is converted entirely to tetramers in the presence of the inhibitor NADPH. Comparison of NadK kinetic parameters with pool sizes of NADH and NADPH suggests that NadK is substantially inhibited during normal growth and, thus, can increase its activity greatly in response to temporary drops in the pools of inhibitory NADH and NADPH. The primary inhibitor is NADPH during aerobic growth and NADH during anaerobic growth. A model is proposed in which variation of NadK activity is central to the adjustment of pyridine nucleotide pools in response to changes in aeration, oxidative stress, and UV irradiation. It is suggested that each of these environmental factors causes a decrease in the level of reduced pyridine nucleotides, activates NadK, and increases production of NADP(H) at the expense of NAD(H). Activation of NadK may constitute a defensive response that resists loss of protective NADPH.

Bacterial luciferase activity and the intracellular redox pool in Escherichia coli

In this study, we analyzed the activity of a bacterial luciferase (LuxAB of Vibrio fischeri) expressed under the control of a consensus-type promoter, lacUV5, in Escherichia coli, and found that activity declines abruptly upon entry into the stationary growth phase. Since this decline was reproducibly observed in strains cultured in various growth media, we refer to this phenomenon as ADLA (Abrupt Decline of Luciferase Activity) and define the time point when activity begins to decline as T (0). Because the levels of luciferase proteins (LuxA and LuxB) remained constant before and after T (0), ADLA cannot be due to the repression of luciferase gene expression. Further analyses suggested that a decline in the supply of intracellular reducing power for luciferase was responsible for ADLA. We also found that ADLA was alleviated or did not occur in several mutants deficient in nucleoid proteins, suggesting that ADLA is a genetically controlled process involved in intracellular redox flow.

Novel stationary-phase-upregulated protein of Porphyromonas gingivalis influences production of superoxide dismutase, thiol peroxidase and thioredoxin

Porphyromonas gingivalis, an obligately anaerobic bacterium, is implicated as a major pathogen in the development and progression of chronic periodontitis. Although expression of several virulence factors of the bacterium has been found to be affected by environmental stress such as entrance into the stationary growth phase and heat, there is relatively little information on the mechanisms that may operate in the bacterium in response to environmental stress. In this study, a novel protein (UstA) was investigated that was initially identified following two-dimensional gel analysis. Expression of UstA was upregulated in stationary phase or by exposure to atmospheric oxygen. N-terminal sequencing and database analysis with the P. gingivalis genome sequence revealed that the UstA-encoding gene (ustA) was located upstream of a homologue of the usp gene encoding the universal stress protein on the chromosome. The ustA gene appeared to be transcribed in a monocistronic fashion, as revealed by primer extension and Northern blot analysis. To elucidate the role of UstA in the bacterium, chromosomal mutants carrying a disruption of the ustA gene were constructed. The ustA mutant grew slower than the wild-type parent strain in rich medium, resulting in a lower yield in stationary phase. Furthermore, in this mutant, expression levels of the P. gingivalis homologues of superoxide dismutase, thiol peroxidase and thioredoxin were markedly higher than those in the wild-type, especially in stationary phase. The ustA mutant was more resistant to diamide, a thiol-specific oxidant, than the wild-type. In addition, the ustA mutation suppressed hypersensitivities of the oxyR mutant to diamide, metronidazole and mitomycin C. These results suggest that UstA may play a significant role in oxidative stress responses in the bacterium.

Induction of oxidative stress by high hydrostatic pressure in Escherichia coli

Using leaderless alkaline phosphatase as a probe, it was demonstrated that pressure treatment induces endogenous intracellular oxidative stress in Escherichia coli MG1655. In stationary-phase cells, this oxidative stress increased with the applied pressure at least up to 400 MPa, which is well beyond the pressure at which the cells started to become inactivated (200 MPa). In exponential-phase cells, in contrast, oxidative stress increased with pressure treatment up to 150 MPa and then decreased again, together with the cell counts. Anaerobic incubation after pressure treatment significantly supported the recovery of MG1655, while mutants with increased intrinsic sensitivity toward oxidative stress (katE, katF, oxyR, sodAB, and soxS) were found to be more pressure sensitive than wild-type MG1655. Furthermore, mild pressure treatment strongly sensitized E. coli toward t-butylhydroperoxide and the superoxide generator plumbagin. Finally, previously described pressure-resistant mutants of E. coli MG1655 displayed enhanced resistance toward plumbagin. In one of these mutants, the induction of endogenous oxidative stress upon high hydrostatic pressure treatment was also investigated and found to be much lower than in MG1655. These results suggest that, at least under some conditions, the inactivation of E. coli by high hydrostatic pressure treatment is the consequence of a suicide mechanism involving the induction of an endogenous oxidative burst.

The Brucella abortus Cu,Zn superoxide dismutase is required for optimal resistance to oxidative killing by murine macrophages and wild-type virulence in experimentally infected mice

Two-dimensional gel electrophoretic analysis of cell lysates from Brucella abortus 2308 and the isogenic hfq mutant Hfq3 revealed that the RNA binding protein Hfq (also known as host factor I or HF-I) is required for the optimal stationary phase production of the periplasmic Cu,Zn superoxide dismutase SodC. An isogenic sodC mutant, designated MEK2, was constructed from B. abortus 2308 by gene replacement, and the sodC mutant exhibited much greater susceptibility to killing by O(2)(-) generated by pyrogallol and the xanthine oxidase reaction than the parental 2308 strain supporting a role for SodC in protecting this bacterium from O(2)(-) of exogenous origin. The B. abortus sodC mutant was also found to be much more sensitive to killing by cultured resident peritoneal macrophages from C57BL6J mice than 2308, and the attenuation displayed by MEK2 in cultured murine macrophages was enhanced when these phagocytes were treated with gamma interferon (IFN-gamma). The attenuation displayed by the B. abortus sodC mutant in both resting and IFN-gamma-activated macrophages was alleviated, however, when these host cells were treated with the NADPH oxidase inhibitor apocynin. Consistent with its increased susceptibility to killing by cultured murine macrophages, the B. abortus sodC mutant also displayed significant attenuation in experimentally infected C57BL6J mice compared to the parental strain. These experimental findings indicate that SodC protects B. abortus 2308 from the respiratory burst of host macrophages. They also suggest that reduced SodC levels may contribute to the attenuation displayed by the B. abortus hfq mutant Hfq3 in the mouse model.

Stationary-phase physiology

Bacteria enjoy an infinite capacity for reproduction as long as they reside in an environment supporting growth. However, their rapid growth and efficient metabolism ultimately results in depletion of growth-supporting substrates and the population of cells enters a phase defined as the stationary phase of growth. In this phase, their reproductive ability is gradually lost. The molecular mechanism underlying this cellular degeneration has not been fully deciphered. Still, recent analysis of the physiology and molecular biology of stationary-phase E. coli cells has revealed interesting similarities to the aging process of higher organisms. The similarities include increased oxidation of cellular constituents and its target specificity, the role of antioxidants and oxygen tension in determining life span, and an apparent trade-off between activities related to reproduction and survival.

Escherichia coli deltafur mutant displays low HPII catalase activity in stationary phase

Iron is among the most important micronutrients used by bacteria. As a partner of the Fenton reaction, however, iron potentiates oxygen toxicity. Strict regulation of iron metabolism, and its coupling with regulation of defenses against oxidative stress, is an essential factor for life in the presence of oxygen. In Escherichia coli, iron metabolism is regulated by the Fur protein. A Fur-deficient mutant, in stationary phase, displayed about 30y-fold lower HPII activity than the respective, Fur-proficient parental strain. Deletion of fur seems to affect HPII catalase specifically, since the mutant was capable of inducing HPI catalase when challenged with H(2)O(2). Low HPII catalase activity appears to be among the reasons for hydrogen peroxide hypersensitivity of the deltafur mutant.

Does UVB radiation induce SoxS gene expression in Escherichia coli cells?

The SoxRS regulon is induced when bacterial cells are exposed to redox-cycling agents such as menadione or paraquat. In this paper it is shown that a physical agent, such as ultraviolet radiation with a wavelength of 312 nm (UVB) can induce soxS gene expression. The results indicate that this induction involves the RpoS protein. Moreover, an unexpected increase of soxS gene expression independent of a functional soxR gene in UVB-irradiated cells has been verified. This increase could be explained by transcription of soxS gene in a rpoS-dependent pathway.

Competitiveness in root colonization by Pseudomonas putida requires the rpoS gene

The rpoS gene in Pseudomonas putida was essential for plant root colonization under competitive conditions from other microbes. The RpoS- mutant survived less well than the wild-type strain in culture medium, and unlike the wild-type, failed to colonize the roots in a peat matrix containing an established diverse microflora. The RpoS-deficient P. putida isolate was generated by insertion of a glucuronidase-npt cassette into the rpoS gene. The RpoS mutant had dose-dependent increased sensitivity to oxidative stress and produced Mn-superoxide dismutase activity earlier than the parent. While extracts from wild-type P. putida stationary-phase cells contained three isozymes of catalase (CatA, CatB, and CatC), the sigma38-deficient P. putida lacked CatB. These results are consistent with previous findings that CatB is induced in stationary-phase.

Influence des conditions de culture préalables et de la présence du gènerpoSpour la survie deSalmonella typhimuriumen eau de mer exposée à la lumière solaire

The effect of sunlight exposure on Salmonella typhimurium isogenic strains harboring an rpoS gene functional (rpoS+) or not functional (rpoS–) was investigated in microcosms of sterile sea water at 20 °C. The two strains rapidly lost their ability to produce colonies on solid culture media. The detrimental action of sunlight was more important when the salinity of sea water increased. The survival of stationary phase cells was influenced by RpoS. Bacteria grown in media with high salinity or osmolarity and transferred to sea water in stationary phase were more resistant to irradiation than those grown in media with low salinity. Prior growth under oxidative (0.2 mmol/L of H2O2) or amino acid starved (minimal medium) conditions did not modify the survival of either strain when they were exposed to sunlight. Bacteria were more resistant when cells were incubated in sea water in the dark prior to being exposed to sunlight. The resistance to sunlight irradiation was also greater in clones of both strains isolated from microcosms exposed to sunlight for 90 min, then further inoculated into sea water and reexposed to sunlight.Key words: Salmonella typhimurium, sea water, sunlight, rpoS, σs.

What is driving the acquisition of mutS and rpoS polymorphisms in Escherichia coli?

Pathogenic and commensal Escherichia coli isolates frequently contain defective alleles of the mutS and rpoS genes, located in a highly polymorphic segment of the chromosome. The environments leading to enrichment of rpoS mutations and the selective advantages of these mutants are becoming apparent. Unexpectedly, rpoS defects occur because of a basic design limitation in cellular regulation. Antagonistic pleiotropy results from the futile competition between different sigma factors associated with the RNA polymerase, and drives the elimination of RpoS (or sigma(S)) in environments requiring high levels of transcription that is dependent on RpoD (or sigma(D) or sigma(70)). Nutrient-limited environments provide an ideal breeding ground for rpoS mutations. By contrast, in other settings, increased stress resistance selects for restoration of rpoS function. Hence extensive polymorphism in the mutS-rpoS region is postulated to result from cycling between environments in which the functional or non-functional genes provide distinct fitness advantages.

Contrasting sensitivities of Escherichia coli aconitases A and B to oxidation and iron depletion

Superoxide damages dehydratases that contain catalytic [4Fe-4S](2+) clusters. Aconitases are members of that enzyme family, and previous work showed that most aconitase activity is lost when Escherichia coli is exposed to superoxide stress. More recently it was determined that E. coli synthesizes at least two isozymes of aconitase, AcnA and AcnB. Synthesis of AcnA, the less-abundant enzyme, is positively controlled by SoxS, a protein that is activated in the presence of superoxide-generating chemicals. We have determined that this arrangement exists because AcnA is resistant to superoxide in vivo. Surprisingly, purified AcnA is extremely sensitive to superoxide and other chemical oxidants unless it is combined with an uncharacterized factor that is present in cell extracts. In contrast, AcnB is highly sensitive to a variety of chemical oxidants in vivo, in extracts, and in its purified form. Thus, the induction of AcnA during oxidative stress provides a mechanism to circumvent a block in the tricarboxylic acid cycle. AcnA appears to be as catalytically competent as AcnB, so the retention of the latter as the primary housekeeping enzyme must provide some other advantage. We observed that the [4Fe-4S] cluster of AcnB is in dynamic equilibrium with the surrounding iron pool, so that AcnB is rapidly demetallated when intracellular iron pools drop. AcnA and other dehydratases do not show this trait. Demetallated AcnB is known to bind its cognate mRNA. The absence of AcnB activity also causes the accumulation and excretion of citrate, an iron chelator for which E. coli synthesizes a transport system. Thus, AcnB may be retained as the primary aconitase because the lability of its exposed cluster allows E. coli to sense and respond to iron depletion.

Polyamines enhance synthesis of the RNA polymerase sigma 38 subunit by suppression of an amber termination codon in the open reading frame

The mechanisms by which polyamines stimulate synthesis of the RNA polymerase sigma(38) subunit in Escherichia coli were studied. Polyamine stimulation was observed only in strains in which the 33rd codon of RpoS mRNA is a UAG termination codon instead of a CAG codon for glutamine in wild-type E. coli. Readthrough of the termination codon by Gln-tRNA(supE) was stimulated by polyamines. This stimulation was found to be caused by an increase in both the level of suppressor tRNA(supE) and the binding affinity of Gln-tRNA(supE) for ribosomes. The stimulatory effect was observed with a UAG termination codon but not with UGA and UAA codons. Readthrough of the UAG termination codon at the 270th amino acid position of RpoS mRNA was also stimulated by polyamines, indicating that polyamines stimulate readthrough of a UAG codon regardless of its location within the RpoS mRNA. When cell viability of an E. coli strain having a termination codon in the 33rd position of RpoS mRNA was compared using cells cultured with or without putrescine, it was higher in cells cultured with putrescine than in cells cultured without putrescine. The level of sigma(38) subunit in the cells cultured with putrescine was higher than that in cells cultured without putrescine on days 2, 4, and 8, but the level of sigma(70) subunit was almost the same in cells cultured with or without putrescine. These results confirm that elevated expression of the rpoS gene is important for cell viability at late stationary phase.

Involvement of adrenergic pathways in activation of catalase by myocardial ischemia-reperfusion

In situ rabbit hearts were subjected to 15 min of regional myocardial ischemia, and at various time points of reperfusion, antioxidant enzyme activity and mRNA expression were measured in ischemic and nonischemic myocardium. Catalase activity increased significantly in both ischemic and nonischemic myocardium, peaking at 1 h after reperfusion and then gradually returning to the control level. Northern blot analysis showed enhanced expression of catalase mRNA in both areas. There were no changes in redox status, because glutathione levels were not altered by ischemia-reperfusion (I/R). We also tested whether catalase activation in the heart results from signaling pathways that might influence not only the heart but also other organs. We found that catalase activity in the brain was increased after myocardial I/R and ischemic stress to the intestine was equipotent to myocardial I/R in catalase activation. We next sought to elucidate the possible involvement of the adrenergic system in catalase stimulation induced by ischemic stimuli. After pretreatment with the alpha-adrenergic receptor antagonist prazosin, I/R failed to increase catalase activity in the heart and brain. Intravenous norepinephrine increased catalase activity in the heart, brain, and liver. This study shows that brief I/R activates a signaling mechanism to induce catalase activation in multiple organs and the alpha-adrenergic system is involved as an intermediate pathway in this signal transmission.