Regulation of bacterial gene expression in response to oxidative stress

Staphylococcal strains adapted to epigallocathechin gallate (EGCG) show reduced susceptibility to vancomycin, oxacillin and ampicillin, increased heat tolerance, and altered cell morphology

Epigallocathechin gallate (EGCG) possesses many beneficial properties, such as anticarcinogenicity, antiatherogenicity, as well as antioxidant and antibacterial activities. However, the bacterial response to sublethal concentrations of EGCG has not been studied. Here we investigated whether short exposure of staphylococci strains to sublethal doses of EGCG can lead to adaptation and cross-resistance. Two-hour exposure of five strains to 20 microg/ml of EGCG did not affect the growth rate but significantly elevated the resistance towards antibiotics targeting the bacterial cell wall. The magnitude of cross-resistance towards such antibiotics varied with the staphylococci strain, with Staphylococcus aureus Newman exhibiting the highest magnitude of cross-resistance, showing a 2, 4 and 8-fold increase in resistance towards vancomycin, oxacillin and ampicillin respectively. All EGCG-adapted strains were also more heat tolerant than their control counterparts as derived from the Weibull model. Adaptation to EGCG led to a moderate increase in heat resistance of the adapted strains S. epidermis ATCC 12228, S. aureus Newman, and S. aureus ATCC 29213, and an extremely pronounced increase for S. aureus ATCC 6538 and S. aureus RN4220. The shape of the survival curve also varied with the staphylococci strain. Transmission electron microscopy (TEM) analysis revealed suppressed separation of daughter cells in cultures exposed to EGCG, as evidenced by the pseudomulticellular appearance and by more than 2-fold increase in cell wall thickness. These observations raise concerns over the potential of EGCG utilization in therapy in that it may contribute to the development and enhancement of microbial resistance mechanisms.

DAMPs and inflammatory processes: the role of redox in the different outcomes

Inflammation is deeply entangled with redox modulation. Triggering of PRRs on inflammatory cells induces ROS generation. As a consequence, activated cells mount antioxidant responses to counteract the possible harmful effects of oxidation. Therefore, when repair is completed, homeostasis is restored. Here, we describe some recent results showing that an exuberant antioxidant response to pro-oxidant inflammatory stimuli modifies not only the intra- but also the extracellular redox and contributes to the outcome of the inflammatory process. In particular, the role of redox modulation in IL-1beta secretion, in B lymphocyte differentiation to plasma cells, and in tumor progression will be discussed, and the potential consequences of extracellular redox alterations on DAMP activity will be considered.

The Mycobacterium marinum mel2 locus displays similarity to bacterial bioluminescence systems and plays a role in defense against reactive oxygen and nitrogen species

BACKGROUND

Mycobacteria have developed a number of pathways that provide partial protection against both reactive oxygen species (ROS) and reactive nitrogen species (RNS). We recently identified a locus in Mycobacterium marinum, mel2, that plays a role during infection of macrophages. The molecular mechanism of mel2 action is not well understood.

RESULTS

To better understand the role of the M. marinum mel2 locus, we examined these genes for conserved motifs in silico. Striking similarities were observed between the mel2 locus and loci that encode bioluminescence in other bacterial species. Since bioluminescence systems can play a role in resistance to oxidative stress, we postulated that the mel2 locus might be important for mycobacterial resistance to ROS and RNS. We found that an M. marinum mutant in the first gene in this putative operon, melF, confers increased susceptibility to both ROS and RNS. This mutant is more susceptible to ROS and RNS together than either reactive species alone.

CONCLUSION

These observations support a role for the M. marinum mel2 locus in resistance to oxidative stress and provide additional evidence that bioluminescence systems may have evolved from oxidative defense mechanisms.

Zinc center as redox switch--new function for an old motif

Oxidative stress affects a wide variety of different cellular processes. Now, an increasing number of proteins have been identified that use the presence of reactive oxygen species or alterations in the cellular thiol-disulfide state as regulators of their protein function. This review focuses on two members of this growing group of redox-regulated proteins that utilize a cysteine-containing zinc center as the redox switch: Hsp33, the first molecular chaperone, whose ability to protect cells against stress-induced protein unfolding depends on the presence of reactive oxygen species and RsrA, the first anti-sigma factor that uses a cysteine-containing zinc center to sense and respond to cellular disulfide stress.

DpsA protects the human pathogen Burkholderia pseudomallei against organic hydroperoxide

The human pathogen, Burkholderia pseudomalle, is able to survive and multiply in hostile environments such as within macrophages. In an attempt to understand its strategy to cope with oxidative stress, the physiological role and gene regulation of a nonspecific DNA-binding protein (DpsA) was investigated. Expression of dpsA increases in response to oxidative stress through increased transcription from the upstream katG (catalase-peroxidase) promoter, which is OxyR dependent. dpsA is also transcribed from its own promoter, which is activated by osmotic stress in an OxyR-independent manner. DpsA-deficient mutants are hypersensitive to tert-butyl hydroperoxide, while overexpression of DpsA leads to increased resistance to organic oxidants. B. pseudomallei DpsA can also protect Escherichia coli against organic hydroperoxide toxicity. The mechanism of DpsA-mediated resistance to organic hydroperoxides was shown to differ from that of alkyl hydroperoxide reductase.

Characterization of Escherichia coli null mutants for glutaredoxin 2

Three Escherichia coli glutaredoxins catalyze GSH-disulfide oxidoreductions, but the atypical 24-kDa glutaredoxin 2 (Grx2, grxB gene), in contrast to the 9-kDa glutaredoxin 1 (Grx1, grxA gene) and glutaredoxin 3 (Grx3, grxC gene), is not a hydrogen donor for ribonucleotide reductase. To improve the understanding of glutaredoxin function, a null mutant for grxB (grxB(-)) was constructed and combined with other mutations. Null mutants for grxB or all three glutaredoxin genes were viable in rich and minimal media with little changes in their growth properties. Expression of leaderless alkaline phosphatase showed that Grx1 and Grx2 (but not Grx3) contributed in the reduction of cytosolic protein disulfides. Moreover, Grx1 could catalyze disulfide formation in the oxidizing cytosol of combined null mutants for glutathione reductase and thioredoxin 1. grxB(-) cells were more sensitive to hydrogen peroxide and other oxidants and showed increased carbonylation of intracellular proteins, particularly in the stationary phase. Significant up-regulation of catalase activity was observed in null mutants for thioredoxin 1 and the three glutaredoxins, whereas up-regulation of glutaredoxin activity was observed in catalase-deficient strains with additional defects in the thioredoxin pathway. The expression of catalases is thus interconnected with the thioredoxin/glutaredoxin pathways in the antioxidant response.

ohr, Encoding an organic hydroperoxide reductase, is an in vivo-induced gene in Actinobacillus pleuropneumoniae

Actinobacillus pleuropneumoniae is the causative agent of porcine pleuropneumonia, a disease characterized by pulmonary necrosis and hemorrhage caused in part by neutrophil degranulation. In an effort to understand the pathogenesis of this disease, we have developed an in vivo expression technology (IVET) system to identify genes that are specifically up-regulated during infection. One of the genes that we have identified as being induced in vivo is ohr, encoding organic hydroperoxide reductase, an enzyme that could play a role in detoxification of organic hydroperoxides generated during infection. Among the 12 serotypes of A. pleuropneumoniae, ohr was found in only serotypes 1, 9, and 11. This distribution correlated with increased resistance to cumene hydroperoxide, an organic hydroperoxide, but not to hydrogen peroxide or to paraquat, a superoxide generator. Functional assays of Ohr activity demonstrated that A. pleuropneumoniae serotype 1 cultures, but not serotype 5 cultures, were able to degrade cumene hydroperoxide. In A. pleuropneumoniae serotype 1, expression of ohr was induced by cumene hydroperoxide, but not by either hydrogen peroxide or paraquat. In contrast, an ohr gene from serotype 1 cloned into A. pleuropneumoniae serotype 5 was not induced by cumene hydroperoxide or by other forms of oxidative stress, suggesting the presence of a serotype-specific positive regulator of ohr in A. pleuropneumoniae serotype 1.

Transcriptional responses to DNA damage

In Escherichia coli, DNA repair and protective responses are regulated at the transcriptional level. Regulatory mechanisms have evolved that allow cells to respond to DNA damage by mounting the appropriate responses. The regulatory proteins controlling these responses are activated when they recognize the presence of a specific DNA damaging agent, the production of specific DNA lesions, or the production of damage intermediates resulting from replication of lesions containing DNA. Transcription of the responses to DNA damage are induced when the activated regulatory proteins stimulate transcription of the genes they control by a variety of complex and unique molecular mechanisms.

Genetic and physiologic characterization of ferric/cupric reductase constitutive mutants of Cryptococcus neoformans

Cryptococcus neoformans is a pathogenic yeast that causes meningitis in immunocompromised patients. Because iron acquisition is critical for growth of a pathogen in a host, we studied the regulation of the ferric reductase and ferrous uptake system of this organism. We isolated 18 mutants, representing four independent loci, with dysregulated ferric reductase. The mutant strains had >10-fold higher than wild-type WT reductase activity in the presence of iron. Two of the strains also had >7-fold higher than WT iron uptake in the presence of iron but were not markedly iron sensitive. Both were sensitive to the oxidative stresses associated with superoxide and hydrogen peroxide. One strain exhibited only 23% of the WT level of iron uptake in the absence of iron and grew poorly without iron supplementation of the medium, phenotypes consistent with an iron transport deficiency; it was sensitive to superoxide but not to hydrogen peroxide. The fourth strain had high reductase activity but normal iron uptake; it was not very sensitive to oxidative stress. We also demonstrated that the ferric reductase was regulated by copper and could act as a cupric reductase. Sensitivity to oxidants may be related to iron acquisition by a variety of mechanisms and may model the interaction of the yeast with the immune system.

Importance of glucose-6-phosphate dehydrogenase in the adaptive response to hydrogen peroxide in Saccharomyces cerevisiae

Glucose-6-phosphate dehydrogenase (G6PDH)-deficient cells of Saccharomyces cerevisiae showed increased susceptibility and were unable to induce adaptation to oxidative stress. Historically, mainly in human erythrocytes, it has been suggested and accepted that decreased cellular GSH, due to loss of the NADPH-dependent activity of glutathione reductase (GR), is responsible for the increased sensitivity to oxidative stress in G6PDH-deficient cells. In the present study we investigated whether the increased susceptibility and the inability to induce adaptation to H2O2 stress of G6PDH-deficient yeast is caused by incompleteness of glutathione recycling. We constructed G6PDH- and GR-deficient mutants and analysed their adaptive response to H2O2 stress. Although G6PDH-deficient cells contained comparable amounts of GSH and GR activity to wild-type cells, GSSG was not reduced efficiently, and intracellular GSSG levels and the ratio of GSSG to total glutathione (GSSG/tGSH) were higher in G6PDH-deficient cells than in wild-type. On the other hand, GR-deficient cells showed a susceptibility identical with that of wild-type cells and induced adaptation to H2O2 stress, even though the GSSG/tGSH ratio in GR-deficient cells was higher than in G6PDH-deficient cells. These results indicate that incompleteness of glutathione recycling alone is not sufficient to account for the increased sensitivity and inability to induce adaptation to H2O2 stress of G6PDH-deficient yeast cells. In S. cerevisiae, G6PDH appears to play other important roles in the adaptive response to H2O2 stress besides supplying NADPH to the GR reaction.

In vivo kinetics of a redox-regulated transcriptional switch

SoxR is a transcription activator governing a cellular response to superoxide and nitric oxide in Escherichia coli. SoxR protein is a homodimer, and each monomer has a redox-active [2Fe-2S] cluster. Oxidation and reduction of the [2Fe-2S] clusters can reversibly activate and inactivate SoxR transcriptional activity. Here, we use electron paramagnetic resonance spectroscopy to follow the redox-switching process of SoxR protein in vivo. SoxR [2Fe-2S] clusters were in the fully reduced state during normal aerobic growth, but were completely oxidized after only 2-min aerobic exposure of the cells to superoxide-generating agents such as paraquat. The oxidized SoxR [2Fe-2S] clusters were rapidly re-reduced in vivo once the oxidative stress was removed. The in vivo kinetics of SoxR [2Fe-2S] cluster oxidation and reduction exactly paralleled the increase and decrease of transcription of soxS, the target gene for SoxR. The kinetic analysis also revealed that an oxidative stress-linked decrease in soxS mRNA stability contributes to the rapid attainment of a new steady state after SoxR activation. Such a redox stress-related change in soxS mRNA stability may represent a new level of biological control.

Molecular definition and identification of new proteins of Mycobacterium leprae

This report describes N-terminal group analysis of six new proteins isolated from in vivo-grown Mycobacterium leprae, three of which correspond to products of the cysA, ahpC, and rpIL genes, which were recently defined through the M. leprae genome project and which encode a putative sulfate sulfurtransferase, an antioxidant enzyme, and the L7/L12 ribosomal protein, respectively.

The role of iron-sulfur clusters in in vivo hydroxyl radical production

The in vivo production of HO- requires iron ions, H2O2 and O2- or other oxidants but probably does not occur through the Haber-Weiss reaction. Instead oxidants, such as O2-, increase free iron by releasing Fe(II) from the iron-sulfur clusters of dehydratases and by interfering with the iron-sulfur clusters reassembly. Fe(II) then reduces H2O2, and in turn Fe(III) and the oxidized cluster are re-reduced by cellular reductants such as NADPH and glutathione. In this way, SOD cooperates with cellular reductants in keeping the iron-sulfur clusters intact and the rate of HO. production to a minimum. O2- and other oxidants can release iron from Fe(II)-containing enzymes as well as copper from thionein. The released Fe(III) and Cu(II) are then reduced to Fe(II) and Cu(I) and can then participate in the Fenton reaction. In mammalian cells oxidants are able to convert cytosolic aconitase into active IRE-BP, which increases the "free" iron concentration intracellularly both by decreasing the biosynthesis of ferritin and increasing biosynthesis of transferrin receptors. The biological role of the soxRS regulon of Escherichia coli, which is involved in the adaptation toward oxidative stress, is presumably to counteract the oxidative inactivation of the iron clusters and the subsequent release of iron with consequent increased rate of production of HO.

Lack of expression of the global regulator OxyR in Haemophilus influenzae has a profound effect on growth phenotype

A pBR322-based library of chromosomal DNA from the nontypeable Haemophilus influenzae TN106 was screened for the expression of transferrin-binding activity in Escherichia coli. A recombinant clone expressing transferrin-binding activity contained a 3.7-kb fragment of nontypeable H. influenzae DNA. Nucleotide sequence analysis of this insert revealed the presence of two complete open reading frames encoding proteins of approximately 26 and 34 kDa. Mini-Tn10kan transposon mutagenesis at different sites within the open reading frame encoding the 34-kDa protein resulted in the abolition of transferrin-binding activity in the recombinant E. coli clone. The deduced amino acid sequence of the 34-kDa protein had 70% identity with the OxyR protein of E. coli; this latter macromolecule is a member of the LysR family of transcriptional activators. When a mutated H. influenzae oxyR gene was introduced into the chromosome of the wild-type H. influenzae strain by allelic exchange, the resulting oxyR mutant still exhibited wild-type levels of transferrin-binding activity but was unable to grow on media containing the heme precursor protoporphyrin IX (PPIX) in place of heme. This mutant also exhibited reduced growth around disks impregnated with heme sources. Supplementation of the PPIX-based growth media with catalase or sodium pyruvate resulted in normal growth of the H. influenzae oxyR mutant. Provision of the wild-type H. influenzae oxyR gene in trans also permitted the growth of this mutant on a PPIX-based medium. Exogenously supplied catalase restored the growth of this mutant with heme sources to nearly wild-type levels. These results indicate that expression of a wild-type OxyR protein by H. influenzae is essential to allow this organism to protect itself against oxidative stresses in vitro.

Disparate responses to oxidative stress in saprophytic and pathogenic mycobacteria

To persist in macrophages and in granulomatous caseous lesions, pathogenic mycobacteria must be equipped to withstand the action of toxic oxygen metabolites. In Gram-negative bacteria, the OxyR protein is a critical component of the oxidative stress response. OxyR is both a sensor of reactive oxygen species and a transcriptional activator, inducing expression of detoxifying enzymes such as catalase/hydroperoxidase and alkyl hydroperoxidase. We have characterized the responses of various mycobacteria to hydrogen peroxide both phenotypically and at the levels of gene and protein expression. Only the saprophytic Mycobacterium smegmatis induced a protective oxidative stress response analogous to the OxyR response of Gram-negative bacteria. Under similar conditions, the pathogenic mycobacteria exhibited a limited, nonprotective response, which in the case of Mycobacterium tuberculosis was restricted to induction of a single protein, KatG. We have also isolated DNA sequences homologous to oxyR and ahpC from M. tuberculosis and Mycobacterium avium. While the M. avium oxyR appears intact, the oxyR homologue of M. tuberculosis contains numerous deletions and frameshifts and is probably nonfunctional. Apparently the response of pathogenic mycobacteria to oxidative stress differs significantly from the inducible OxyR response of other bacteria.

Genetic map of Salmonella typhimurium, edition VIII

We present edition VIII of the genetic map of Salmonella typhimurium LT2. We list a total of 1,159 genes, 1,080 of which have been located on the circular chromosome and 29 of which are on pSLT, the 90-kb plasmid usually found in LT2 lines. The remaining 50 genes are not yet mapped. The coordinate system used in this edition is neither minutes of transfer time in conjugation crosses nor units representing "phage lengths" of DNA of the transducing phage P22, as used in earlier editions, but centisomes and kilobases based on physical analysis of the lengths of DNA segments between genes. Some of these lengths have been determined by digestion of DNA by rare-cutting endonucleases and separation of fragments by pulsed-field gel electrophoresis. Other lengths have been determined by analysis of DNA sequences in GenBank. We have constructed StySeq1, which incorporates all Salmonella DNA sequence data known to us. StySeq1 comprises over 548 kb of nonredundant chromosomal genomic sequences, representing 11.4% of the chromosome, which is estimated to be just over 4,800 kb in length. Most of these sequences were assigned locations on the chromosome, in some cases by analogy with mapped Escherichia coli sequences.