Peptide methionine sulfoxide reductase from Escherichia coli and Mycobacterium tuberculosis protects bacteria against oxidative damage from reactive nitrogen intermediates

Increased catalytic efficiency following gene fusion of bifunctional methionine sulfoxide reductase enzymes from Shewanella oneidensis

Methionine sulfoxide reductase enzymes MsrA and MsrB have complementary stereospecificities that reduce the S and R stereoisomers of methionine sulfoxide (MetSO), respectively, and together function as critical antioxidant enzymes. In some pathogenic and metal-reducing bacteria, these genes are fused to form a bifunctional methionine sulfoxide reductase (i.e., MsrBA) enzyme. To investigate how gene fusion affects the substrate specificity and catalytic activities of Msr, we have cloned and expressed the MsrBA enzyme from Shewanella oneidensis, a metal-reducing bacterium and fish pathogen. For comparison, we also cloned and expressed the wild-type MsrA enzyme from S. oneidensis and a genetically engineered MsrB protein. MsrBA is able to completely reduce (i.e., repair) MetSO in the calcium regulatory protein calmodulin (CaM), while only partial repair is observed using both MsrA and MsrB enzymes together at 25 degrees C. A restoration of the normal protein fold is observed co-incident with the repair of MetSO in oxidized CaM (CaMox by MsrBA, as monitored by time-dependent increases in the anisotropy associated with the rigidly bound multiuse affinity probe 4',5'-bis(1,3,2-dithioarsolan-2-yl)fluorescein (FlAsH). Underlying the efficient repair of MetSO in CaMox is the coordinate activity of the two catalytic domains in the MsrBA fusion protein, which results in a 1 order of magnitude rate enhancement in comparison to those of the individual MsrA or MsrB enzyme alone. The coordinate binding of both domains of MsrBA permits the full repair of all MetSO in CaMox. The common expression of Msr fusion proteins in bacterial pathogens is consistent with an important role for this enzyme activity in the maintenance of protein function necessary for bacterial survival under highly oxidizing conditions associated with pathogenesis or bioremediation.

Mycobacterium tuberculosis and the environment within the phagosome

Once across the barrier of the epithelium, macrophages constitute the primary defense against microbial invasion. For most microbes, the acidic, hydrolytically competent environment of the phagolysosome is sufficient to kill them. Despite our understanding of the trafficking events that regulate phagosome maturation, our appreciation of the lumenal environment within the phagosome is only now becoming elucidated through real-time functional assays. The assays quantify pH change, phagosome/lysosome fusion, proteolysis, lipolysis, and beta-galactosidase activity. This information is particularly important for understanding pathogens that successfully parasitize the endosomal/lysosomal continuum. Mycobacterium tuberculosis infects macrophages through arresting the normal maturation process of the phagosome, retaining its vacuole at pH 6.4 with many of the characteristics of an early endosome. Current studies are focusing on the transcriptional response of the bacterium to the changing environment in the macrophage phagosome. Manipulation of these environmental cues, such as preventing the pH drop to pH 6.4 with concanamycin A, abrogates the majority of the transcriptional response in the bacterium, showing that pH is the dominant signal that the bacterium senses and responds to. These approaches represent our ongoing attempts to unravel the discourse that takes place between the pathogen and its host cell.

Molecular function of WhiB4/Rv3681c of Mycobacterium tuberculosis H37Rv: a [4Fe?4S] cluster co-ordinating protein disulphide reductase

The genome sequence of Mycobacterium tuberculosis H37Rv revealed the presence of seven whiB-like open reading frames. In spite of several genetic studies on whiB genes, the biochemical properties of WhiB proteins are poorly understood. All WhiB-like proteins have four conserved cysteine residues, out of which two are present in a CXXC motif. We report for the first time the detailed biochemical and biophysical properties of M. tuberculosis WhiB4/Rv3681c and demonstrate the functional relevance of four conserved cysteines and the CXXC motif. UV-visible absorption spectra of freshly purified mWhiB4 showed the presence of a [2Fe-2S] cluster, whereas the electron paramagnetic resonance (EPR) spectra of reconstituted protein showed the presence of a [4Fe-4S] cluster. The iron-sulphur cluster was redox sensitive but stably co-ordinated to the protein even in the presence of high concentration of chaotropic agents. Despite primary sequence divergence from thioredoxin family proteins, the apo mWhiB4 has properties similar to thioredoxins and functions as a protein disulphide reductase, whereas holo mWhiB4 is enzymatically inactive. Apart from the cysteine thiol of CXXC motif the distantly placed thiol pair also contributes equally to the enzymatic activity of mWhiB4. A functional model of mWhiB4 in redox signaling during oxidative stress in M. tuberculosis has been presented.

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.

Peroxiredoxin systems in mycobacteria

Like other actinomycetes Mycobacterium tuberculosis lacks glutathione and, consequently, the glutathione peroxidases that dominate the antioxidant defence of its mammalian hosts. The hydrogen peroxide metabolism of the pathogen has for long been recognised to depend on a heme-containing catalase/peroxidase. Clinical isolates lacking the catalase were virulent and proved to be resistant to the first line tuberculostatic isoniazid, because the enzyme is evidently required to activate this drug. The survival and virulence of such strains are attributed to the peroxiredoxin-type peroxidases alkyl hydroperoxide reductase (AhpC) and thioredoxin peroxidase (TPx). The most common AhpC reductant in bacteria, the disulfide reductase AhpF, is deleted in M. tuberculosis. Instead, AhpC can be reduced by AhpD, a CXXC-motif-containing protein, or by one of the mycobacterial thioredoxins, TrxC. TPx is reduced by thioredoxins B and C. Mycobacteria contain three more peroxiredoxins, the 1-Cys-Prx AhpE, Bcp and BcpB, whose function and reductants are still unknown.

A Mycobacterium marinum mel2 mutant is defective for growth in macrophages that produce reactive oxygen and reactive nitrogen species

Macrophages produce reactive oxygen species (ROS) and reactive nitrogen species (RNS) in response to bacterial infections. Mycobacteria are relatively resistant to ROS, but RNS inhibit growth of, and possibly even kill, mycobacteria in activated macrophages. We recently constructed a Mycobacterium marinum mel2 locus mutant, which is known to affect macrophage infection. We found previously that the mel2 locus confers resistance to ROS and RNS in laboratory medium, suggesting that this locus might play a similar role during growth in macrophages. Since J774A.1 murine macrophages produce high levels of ROS and RNS upon activation with gamma interferon (IFN-gamma), we examined the effects of IFN-gamma on ROS and RNS production by these cells as well as the effects on growth of M. marinum in these cells. We found that an M. marinum mutant with mutation of the first gene in the mel2 locus, melF, is defective for growth in IFN-gamma-plus-lipopolysaccharide-treated J774A.1 cells and that this defect is abrogated by the presence of either inhibitors of nitric oxide synthase or ROS scavengers. Furthermore, the M. marinum melF mutant displays a defect at late stages in the mouse footpad model of infection. These phenotypic characteristics could be complemented fully by the entire mel2 locus but only partially by the presence of melF alone, supporting data suggesting that this insertion mutation has polar effects on downstream genes in the mel2 locus. These observations demonstrate that the M. marinum mel2 locus plays a role in resistance to ROS and RNS produced by activated macrophages.

Quantification of RNA damage by reverse transcription polymerase chain reactions

RNA damages, such as those generated by nucleic acid-modifying agents, occur randomly in RNA and present challenging problems to organisms. It has been unclear how RNA function would be affected by many forms of RNA damage and how cells are protected against the damage. Elucidation of these mechanisms has been hampered by the lack of sensitive and efficient methodologies detecting damages randomly occurring in RNA, especially for the damage of a specific RNA. In this work, we have developed a method using reverse transcription polymerase chain reactions (RT-PCRs) to determine the level of damage of a specific RNA. The level of damage of the Escherichia coli 16S rRNA caused by oxidative stress was examined. When RNA is treated by H(2)O(2) in vitro, the normalized level of long cDNA is inversely dependent on the dosage of H(2)O(2) as determined by gel-based assay or real-time PCR. Long cDNA was also produced at reduced levels using RNA prepared from H(2)O(2)-treated E. coli cultures compared with RNA from control cultures. Remarkably, the level of cDNA reduction caused by H(2)O(2) treatment depends on the length of cDNA examined, suggesting random occurrences of damage in RNA templates. Approximately 40% of the reduction in cDNA can be detected in each kilobase of RNA from E. coli cultures treated with 0.5 mM H(2)O(2). This method is able to detect any type of damage in RNA-causing termination of reverse transcription and works on specific RNA of interest with high sensitivity.

Methionine sulfoxide reductases: relevance to aging and protection against oxidative stress

Proteins are subject to modification by reactive oxygen species (ROS), and oxidation of specific amino acid residues can impair their biological function, leading to an alteration in cellular homeostasis. Methionine is among the amino acids the most susceptible to oxidation by almost all forms of ROS, resulting in both S and R diasteroisomeric forms of methionine sulfoxide. These modifications can be repaired specifically by the peptide methionine sulfoxide reductase A and B enzymes (MsrA and MsrB), respectively. MsrA has been detected in several organisms going from prokaryotes to eukaryotes. MsrA is tightly implicated in protection against oxidative stress and in protein maintenance, which is critical in the aging process. Several studies have shown that overexpression of MsrA led to an increased resistance against oxidative stress, while MsrA null mutants are more sensitive toward oxidative stress. Since oxidative damage is a key factor in aging, overexpression of MsrA in some organisms led to an increased life span whereas deletion of the gene led to the opposite. MsrA could also be involved, by regulating the function and/or expression of target proteins, in ROS-mediated signal transduction. In fact, changes in gene expression, including certain oxidative stress-response genes, have been observed when MsrA is overexpressed. This review elaborates on the current knowledge in the implication of the Msr system in protection against oxidative stress and aging.

Tuberculosis chemotherapy: the influence of bacillary stress and damage response pathways on drug efficacy

The global tuberculosis (TB) control effort is focused on interrupting transmission of the causative agent, Mycobacterium tuberculosis, through chemotherapeutic intervention in active infectious disease. The insufficiency of this approach is manifest in the inexorable annual increase in TB infection and mortality rates and the emergence of multidrug-resistant isolates. Critically, the limited efficacy of the current frontline anti-TB drug combination suggests that heterogeneity of host and bacillary physiologies might impair drug activity. This review explores the possibility that strategies enabling adaptation of M. tuberculosis to hostile in vivo conditions might contribute to the subversion of anti-TB chemotherapy. In particular, evidence that infecting bacilli are exposed to environmental and host immune-mediated DNA-damaging insults suggests a role for error-prone DNA repair synthesis in the generation of chromosomally encoded antibiotic resistance mutations. The failure of frontline anti-TB drugs to sterilize a population of susceptible bacilli is independent of genetic resistance, however, and instead implies the operation of alternative tolerance mechanisms. Specifically, it is proposed that the emergence of persister subpopulations might depend on the switch to an altered metabolic state mediated by the stringent response alarmone, (p)ppGpp, possibly involving some or all of the many toxin-antitoxin modules identified in the M. tuberculosis genome.

Defenses against oxidative stress in Neisseria gonorrhoeae: a system tailored for a challenging environment

Neisseria gonorrhoeae is a host-adapted pathogen that colonizes primarily the human genitourinary tract. This bacterium encounters reactive oxygen and reactive nitrogen species as a consequence of localized inflammatory responses in the urethra of males and endocervix of females and also of the activity of commensal lactobacilli in the vaginal flora. This review describes recent advances in the understanding of defense systems against oxidative stress in N. gonorrhoeae and shows that while some of its defenses have similarities to the paradigm established with Escherichia coli, there are also some key differences. These differences include the presence of a defense system against superoxide based on manganese ions and a glutathione-dependent system for defense against nitric oxide which is under the control of a novel MerR-like transcriptional regulator. An understanding of the defenses against oxidative stress in N. gonorrhoeae and their regulation may provide new insights into the ways in which this bacterium survives challenges from polymorphonuclear leukocytes and urogenital epithelial cells.

Dihydrolipoamide acyltransferase is critical for Mycobacterium tuberculosis pathogenesis

Mycobacterium tuberculosis has evolved to persist in host macrophages, where it faces a nutrient-poor environment and is exposed to oxidative and nitrosative stress. To defend itself against oxidative/nitrosative stress, M. tuberculosis expresses an NADH-dependent peroxidase and peroxynitrite reductase that is encoded by ahpC, ahpD, lpd, and dlaT. In addition to its central role in the peroxynitrite reductase complex, dlaT (Rv2215) also encodes the E2 component of pyruvate dehydrogenase. Here we demonstrate that inactivation of dlaT in the chromosome of H37Rv resulted in a mutant (H37RvDeltadlaT) that displayed phenotypes associated with DlaT's role in metabolism and in defense against nitrosative stress. The H37RvDeltadlaT strain showed retarded growth in vitro and was highly susceptible to killing by acidified sodium nitrite. Mouse macrophages readily killed intracellular H37RvDeltadlaT organisms, and in mice dlaT was required for full virulence.

Gene structure, localization and role in oxidative stress of methionine sulfoxide reductase A (MSRA) in the monkey retina

MSRA (EC 1.8.4.6) is a member of the methionine sulfoxide reductase family that can reduce methionine sulfoxide (MetO) in proteins. This repair function has been shown to protect cells against oxidative damage. In this study we have assembled the complete gene structure of msrA and identified the presence of two distinct putative promoters that generate three different transcripts. These transcripts were cloned by 5'RACE and code for three MSRA isoforms with different N-termini. The different forms of MSRA target to distinct intracellular regions. The main MSRA transcript (msrA1) had been previously shown to target the mitochondria. MsrA2 and 3 originate from a second promoter and target the cytosol and nuclei. In the monkey retina msrA message was detected mainly in the macular RPE-choroid region while its activity was measured mainly in the soluble fractions of fractionated neural retina and RPE-choroid. The MSRA protein is found throughout the retina but is especially abundant at the photoreceptor synapses, ganglion and Müller cells. Interestingly, MSRA was not detected in the mitochondria of the photoreceptor inner segments. The RPE in the peripheral retina shows very low levels of expression but the RPE in the macular region is strongly labeled. Targeted silencing of msrA message rendered cultured RPE cells more sensitive to oxidative damage suggesting a role for MSRA in RPE protection against oxidative stress. Collectively these data suggest MSRA may play an important role in protecting macular RPE from oxidative damage.

Nitric Oxide in Salmonella and Escherichia coli Infections

This review discusses the role that nitric oxide (NO) and its congeners play on various stages in the pathophysiology of Escherichia coli and Salmonella infections, with special emphasis on the regulatory pathways that lead to high NO synthesis, the role of reactive nitrogen species (RNS) in host resistance, and the bacterial molecular targets and defense mechanisms that protect enteric bacteria against the nitrosative stress encountered in diverse host anatomical sites. In general, NO can react directly with prosthetic groups containing transition metal centers, with other radicals, or with sulfhydryl groups in the presence of an electron acceptor. Binding to iron complexes is probably the best characterized direct reaction of NO in biological systems. The targets of RNS are numerous. RNS can facilitate oxidative modifications including lipid peroxidation, hydroxylation, and DNA base and protein oxidation. In addition, RNS can inflict nitrosative stress through the nitrosation of amines and sulfhydryls. Numerous vital bacterial molecules can be targeted by NO. It is therefore not surprising that enteropathogenic bacteria are armed with a number of sensors to coordinate the protective response to nitrosative stress, along with an assortment of antinitrosative defenses that detoxify, repair, or avoid the deleterious effects of RNS encountered within the host. NO and NO-derived RNS play important roles in innate immunity to Salmonella and E. coli. Enzymatic NO production by NO synthases can be enhanced by microbial and other inflammatory stimuli and it exerts direct antimicrobial actions as well as immunomodulatory and vasoregulatory effects.

Role for nucleotide excision repair in virulence of Mycobacterium tuberculosis

Mutations in Mycobacterium tuberculosis uvrB result in severe sensitivity to acidified nitrite, a source of nitric oxide (6). In this study, we show that a uvrB mutant is exquisitely sensitive to UV light but not to several sources of reactive oxygen species in vitro. Furthermore, a uvrB mutant was attenuated in mice as judged by an extension of life span. Attenuation in mice was partially reversed by genetic inactivation of nitric oxide synthase 2 (iNOS) and almost completely reversed in mice lacking both iNOS and phagocyte oxidase. Thus, a gene predicted to encode a key element of DNA repair is required for resistance of M. tuberculosis to both reactive nitrogen and reactive oxygen species in mice.

A high-molecular-mass surface protein (Lsp) and methionine sulfoxide reductase B (MsrB) contribute to the ecological performance of Lactobacillus reuteri in the murine gut

Members of the genus Lactobacillus are common inhabitants of the gut, yet little is known about the traits that contribute to their ecological performance in gastrointestinal ecosystems. Lactobacillus reuteri 100-23 persists in the gut of the reconstituted Lactobacillus-free mouse after a single oral inoculation. Recently, three genes of this strain that were specifically induced (in vivo induced) in the murine gut were identified (38). We report here the detection of a gene of L. reuteri 100-23 that encodes a high-molecular-mass surface protein (Lsp) that shows homology to proteins involved in the adherence of other bacteria to epithelial cells and in biofilm formation. The three in vivo-induced genes and lsp of L. reuteri 100-23 were inactivated by insertional mutagenesis in order to study their biological importance in the murine gastrointestinal tract. Competition experiments showed that mutation of lsp and a gene encoding methionine sulfoxide reductase (MsrB) reduced ecological performance. Mutation of lsp impaired the adherence of the bacteria to the epithelium of the mouse forestomach and altered colonization dynamics. Homologues of lsp and msrB are present in the genomes of several strains of Lactobacillus and may play an important role in the maintenance of these bacteria in gut ecosystems.

Methionine sulphoxide reductase is an important antioxidant enzyme in the gastric pathogen Helicobacter pylori

The ability of Helicobacter pylori to colonize the stomach requires that it combat oxidative stress responses imposed by the host. The role of methionine sulfoxide reductase (Msr), a methionine repair enzyme, in H. pylori stress resistance was evaluated by a mutant analysis approach. An msr mutant strain lacked immunologically detectable sulphoxide reductase protein and also showed no enzyme activity when provided with oxidized methionines as substrates. The mutant strain showed diminished growth compared to the parent strain in the presence of chemical oxidants, and showed rapid viability loss when exposed to oxidizing conditions. The stress resistance and enzyme activity could be recovered by complementing the mutant with a functional copy of the msr gene. Upon fractionation of parent strain and the complemented mutant cells into membranes and cytoplasmic proteins, most of the immunologically detectable Msr was localized to the membrane, and this fraction contained all of the Msr activity. Qualitative detection of the whole cell protein pattern using 2,4-dinitro phenyl hydrazine (DNPH) showed a far greater number of oxidized protein species in the mutant than in the parent strain when the cells were subjected to oxygen, peroxide or s-nitrosoglutathione (GSNO) induced stress. Importantly, no oxidized proteins were discerned in either strain upon incubation in anaerobic conditions. A mutant strain that synthesized a truncated Msr (corresponding to the MsrA domain) was slightly more resistant to oxidative stress than the msr strain. Mouse colonization studies showed Msr is an important colonization factor, especially for effective longer-term (14 and 21 days) colonization. Complementation of the mutant msr strain by chromosomal insertion of a functional gene restored mouse colonization ability.

Transcriptional responses of Escherichia coli to S-nitrosoglutathione under defined chemostat conditions reveal major changes in methionine biosynthesis

Nitric oxide and nitrosating agents exert powerful antimicrobial effects and are central to host defense and signal transduction. Nitric oxide and S-nitrosothiols can be metabolized by bacteria, but only a few enzymes have been shown to be important in responses to such stresses. Glycerol-limited chemostat cultures in defined medium of Escherichia coli MG1655 were used to provide bacteria in defined physiological states before applying nitrosative stress by addition of S-nitrosoglutathione (GSNO). Exposure to 200 microm GSNO for 5 min was sufficient to elicit an adaptive response as judged by the development of NO-insensitive respiration. Transcriptome profiling experiments were used to investigate the transcriptional basis of the observed adaptation to the presence of GSNO. In aerobic cultures, only 17 genes were significantly up-regulated, including genes known to be involved in NO tolerance, particularly hmp (encoding the NO-consuming flavohemoglobin Hmp) and norV (encoding flavorubredoxin). Significantly, none of the up-regulated genes were members of the Fur regulon. Six genes involved in methionine biosynthesis or regulation were significantly up-regulated; metN, metI, and metR were shown to be important for GSNO tolerance, because mutants in these genes exhibited GSNO growth sensitivity. Furthermore, exogenous methionine abrogated the toxicity of GSNO supporting the hypothesis that GSNO nitrosates homocysteine, thereby withdrawing this intermediate from the methionine biosynthetic pathway. Anaerobically, 10 genes showed significant up-regulation, of which norV, hcp, metR, and metB were also up-regulated aerobically. The data presented here reveal new genes important for nitrosative stress tolerance and demonstrate that methionine biosynthesis is a casualty of nitrosative stress.

S-nitroso proteome of Mycobacterium tuberculosis: Enzymes of intermediary metabolism and antioxidant defense

The immune response to Mycobacterium tuberculosis (Mtb) includes expression of nitric oxide (NO) synthase (NOS)2, whose products can kill Mtb in vitro with a molar potency greater than that of many conventional antitubercular agents. However, the targets of reactive nitrogen intermediates (RNIs) in Mtb are unknown. One major action of RNIs is protein S-nitrosylation. Here, we describe, to our knowledge, the first proteomic analysis of S-nitrosylation in a whole organism after treating Mtb with bactericidal concentrations of RNIs. The 29 S-nitroso proteins identified are all enzymes, mostly serving intermediary metabolism, lipid metabolism, and/or antioxidant defense. Many are essential or implicated in virulence, including defense against RNIs. For each of two target enzymes tested, lipoamide dehydrogenase and mycobacterial proteasome ATPase, S-nitrosylation caused enzyme inhibition. Moreover, endogenously biotinylated proteins were driven into mixed disulfide complexes. Targeting of metabolic enzymes and antioxidant defenses by means of protein S-nitrosylation and mixed disulfide bonding may contribute to the antimycobacterial actions of RNIs.

Macrophage-derived peroxynitrite diffusion and toxicity to Trypanosoma cruzi

We studied the capacity of macrophage-derived peroxynitrite to diffuse into and exert cytotoxicity against Trypanosoma cruzi, the causative agent of Chagas' disease. In two types of macrophage-T. cruzi co-cultures, one with a fixed separation distance between source and target cells, and another involving cell-to-cell interactions, peroxynitrite resulted in significant oxidation of intracellular dihydrorhodamine and inhibition of [(3)H]thymidine incorporation in T. cruzi, which were not observed by superoxide or nitric oxide alone. The effects were attenuated in the presence of bicarbonate, in agreement with the extracellular consumption of peroxynitrite by its fast reaction with CO(2). However, studies using different T. cruzi densities, which allow to modify average diffusion distances of exogenously added peroxynitrite to target cells, indicate that at distances <5 microm, the diffusion process outcompetes the reaction with CO(2) and that the levels of peroxynitrite formed by macrophages would be sufficient to cause toxicity to T. cruzi during cell-to-cell contact and/or inside the phagosome.

Methionine sulfoxide reductases in prokaryotes

In living organisms, most methionine residues exposed to reactive oxygen species (ROS) are converted to methionine sulfoxides. This reaction can lead to structural modifications and/or inactivation of proteins. Recent years have brought a wealth of new information on methionine sulfoxide reductase A (MsrA) and B (MsrB) which makes methionine oxidation a reversible process. Homologs of msrA and msrB genes have been identified in most living organisms and their evolution throughout different species led to different genetic organization and different copy number per organism. While MsrA and MsrB had been the focus of multiple biochemical investigations, our understanding of their physiological role in vivo remains scarce. Yet, the recent identification of a direct link between protein targeting and MsrA/MsrB repair offers a best illustration of the physiological importance of this pathway. Repeatedly identified as a potential "virulence factor", contribution of msrA to pathogenicity is also discussed. It remains, however, unclear whether reduced virulence results from overall viability loss or relates to specific oxidized virulence factors left unrepaired. We speculate that a major issue towards assessing the in vivo role of the MsrA/MsrB repair pathway in the next future will be to decipher the interrelations, if any, between MsrA/MsrB-mediated repair and chaperone-assisted folding and/or protease-assisted degradation.

Methionine oxidation and aging

It is well established that many amino acid residues of proteins are susceptible to oxidation by various forms of reactive oxygen species (ROS), and that oxidatively modified proteins accumulate during aging, oxidative stress, and in a number of age-related diseases. Methionine residues and cysteine residues of proteins are particularly sensitive to oxidation by ROS. However, unlike oxidation of other amino acid residues, the oxidation of these sulfur amino acids is reversible. Oxidation of methionine residues leads to the formation of both R- and S-stereoisomers of methionine sulfoxide (MetO) and most cells contain stereospecific methionine sulfoxide reductases (Msr's) that catalyze the thioredoxin-dependent reduction of MetO residues back to methionine residues. We summarize here results of studies, by many workers, showing that the MetO content of proteins increases with age in a number of different aging models, including replicative senescence and erythrocyte aging, but not in mouse tissues during aging. The change in levels of MetO may reflect alterations in any one or more of many different mechanisms, including (i) an increase in the rate of ROS generation; (ii) a decrease in the antioxidant capacity; (iii) a decrease in proteolytic activities that preferentially degrade oxidized proteins; or (iv) a decrease in the ability to convert MetO residues back to Met residues, due either to a direct loss of Msr enzyme levels or indirectly to a loss in the availability of the reducing equivalents (thioredoxin, thioredoxin reductase, NADPH generation) involved. The importance of Msr activity is highlighted by the fact that aging is associated with a loss of Msr activities in a number of animal tissues, and mutations in mice leading to a decrease in the Msr levels lead to a decrease in the maximum life span, whereas overexpression of Msr leads to a dramatic increase in the maximum life span.

Methionine sulfoxide reductases: history and cellular role in protecting against oxidative damage

An enzyme that can reduce methionine sulfoxide in proteins was first discovered in Escherichia coli about 25 years ago. It is now apparent that there is a family of enzymes, referred to as methionine sulfoxide reductases (Msr), and in recent years there has been considerable interest in one of the members of the Msr family, MsrA. This enzyme has been shown to protect cells against oxidative damage, which suggests a possible role in a large number of age-related diseases. This review summarizes the history of the discovery of MsrA, properties of the enzyme and its role in protecting cells against oxidative damage. Other members of the Msr family that differ in substrate specificity and localization are described as well as a possible role for the Msr system in drug metabolism. The concept that the Msr system can be used to develop novel drugs that could be catalytic anti-oxidants is discussed.

Antimicrobial reactive oxygen and nitrogen species: concepts and controversies

Phagocyte-derived reactive oxygen and nitrogen species are of crucial importance for host resistance to microbial pathogens. Decades of research have provided a detailed understanding of the regulation, generation and actions of these molecular mediators, as well as their roles in resisting infection. However, differences of opinion remain with regard to their host specificity, cell biology, sources and interactions with one another or with myeloperoxidase and granule proteases. More than a century after Metchnikoff first described phagocytosis, and more than four decades after the discovery of the burst of oxygen consumption that is associated with microbial killing, the seemingly elementary question of how phagocytes inhibit, kill and degrade microorganisms remains controversial. This review updates the reader on these concepts and the topical questions in the field.

Methionine sulfoxide reductase A is important for lens cell viability and resistance to oxidative stress

Age-related cataract, an opacity of the eye lens, is the leading cause of visual impairment in the elderly, the etiology of which is related to oxidative stress damage. Oxidation of methionine to methionine sulfoxide is a major oxidative stress product that reaches levels as high as 60% in cataract while being essentially absent from clear lenses. Methionine oxidation results in loss of protein function that can be reversed through the action of methionine sulfoxide reductase A (MsrA), which is implicated in oxidative stress protection and is an essential regulator of longevity in species ranging from Escherichia coli to mice. To establish a role for MsrA in lens protection against oxidative stress, we have examined the levels and spatial expression patterns of MsrA in the human lens and have tested the ability of MsrA to protect lens cells directly against oxidative stress. In the present report, we establish that MsrA is present throughout the human lens, where it is likely to defend lens cells and their components against methionine oxidation. We demonstrate that overexpression of MsrA protects lens cells against oxidative stress damage, whereas silencing of the MsrA gene renders lens cells more sensitive to oxidative stress damage. We also provide evidence that MsrA is important for lens cell function in the absence of exogenous stress. Collectively, these data implicate MsrA as a key player in lens cell viability and resistance to oxidative stress, a major factor in the etiology of age-related cataract.

Methionine sulfoxide reductase A (MsrA) deficiency affects the survival of Mycobacterium smegmatis within macrophages

Methionine sulfoxide reductase A (MsrA) is an antioxidant repair enzyme which reduces oxidized methionine to methionine. Since oxidation of methionine in proteins impairs their function, an absence of MsrA leads to abnormalities in different organisms, including alterations in the adherence patterns and in vivo survival of certain pathogenic bacteria. To understand the role of MsrA in intracellular survival of bacteria, we disrupted the gene encoding MsrA in Mycobacterium smegmatis through homologous recombination. The msrA mutant strain of M. smegmatis exhibited significantly reduced intracellular survival in murine J774A.1 macrophages compared to the survival of its wild-type counterpart. Furthermore, immunofluorescence and immunoblotting of phagosomes containing M. smegmatis strains revealed that the phagosomes with the msrA mutant strain acquired both p67(phox) of phagocyte NADPH oxidase and inducible nitric oxide synthase much earlier than the phagosomes with the wild-type strain. In addition, the msrA mutant strain of M. smegmatis was observed to be more sensitive to hydroperoxides than the wild-type strain was in vitro. These results suggest that MsrA plays an important role in both extracellular and intracellular survival of M. smegmatis.

The immunological aspects of latency in tuberculosis

A unique feature of Mycobacterium tuberculosis is its ability to exist in the granuloma of an asymptomatic host in a latent state that can subsequently reactivate to cause active disease. The latent state of infection poses a major obstacle to eradicating tuberculosis. In latent tuberculosis, the host immune response is capable of controlling the infection and yet falls short of eradicating the pathogen. That the host immune response contributes to the maintenance of latent tuberculous infection is supported by the observation that certain immunodeficient states, including those associated with the human immunodeficiency virus and tumor necrosis factor neutralization therapy, are associated with increased risks for developing reactivation disease. Latent tuberculosis is the product of a complex set of interactions between M. tuberculosis and the host immune response. The molecular basis for the persistence phenotype of M. Tuberculosis and the pertinent host immune mechanisms that contribute to the maintenance of tuberculous latency are just beginning to be understood. This review discusses the interactions between M. tuberculosis and the macrophage, the primary host cell that the tubercle bacillus parasitizes.

Multiple thioredoxin-mediated routes to detoxify hydroperoxides in Mycobacterium tuberculosis

Drug resistance and virulence of Mycobacterium tuberculosis are in part related to the pathogen's antioxidant defense systems. KatG(-) strains are resistant to the first line tuberculostatic isoniazid but need to compensate their catalase deficiency by alternative peroxidase systems to stay virulent. So far, only NADH-driven and AhpD-mediated hydroperoxide reduction by AhpC has been implicated as such virulence-determining mechanism. We here report on two novel pathways which underscore the importance of the thioredoxin system for antioxidant defense in M. tuberculosis: (i) NADPH-driven hydroperoxide reduction by AhpC that is mediated by thioredoxin reductase and thioredoxin C and (ii) hydroperoxide reduction by the atypical peroxiredoxin TPx that equally depends on thioredoxin reductase but can use both, thioredoxin B and C. Kinetic analyses with different hydroperoxides including peroxynitrite qualify the redox cascade comprising thioredoxin reductase, thioredoxin C, and TPx as the most efficient system to protect M. tuberculosis against oxidative and nitrosative stress in situ.

Prominent roles of the NorR and Fur regulators in the Escherichia coli transcriptional response to reactive nitrogen species

We examined the genomewide transcriptional responses of Escherichia coli treated with nitrosylated glutathione or the nitric oxide (NO)-generator acidified sodium nitrite (NaNO(2)) during aerobic growth. These assays showed that NorR, a homolog of NO-responsive transcription factors in Ralstonia eutrophus, and Fur, the global repressor of ferric ion uptake, are major regulators of the response to reactive nitrogen species. In contrast, SoxR and OxyR, regulators of the E. coli defenses against superoxide-generating compounds and hydrogen peroxide, respectively, have minor roles. Moreover, additional regulators of the E. coli response to reactive nitrogen species remain to be identified because several of the induced genes were regulated normally in norR, fur, soxRS, and oxyR mutant strains. We propose that the E. coli transcriptional response to reactive nitrogen species is a composite response mediated by the modification of multiple transcription factors containing iron or redox-active cysteines, some specifically designed to sense NO and its derivatives and others that are collaterally activated by the reactive nitrogen species.

The Role of the Flavodiiron Proteins in Microbial Nitric Oxide Detoxification

The flavodiiron proteins (first named as A-type flavoproteins) constitute a large superfamily of enzymes, widespread among anaerobic and facultative anaerobic prokaryotes, from both the Archaea and Bacteria domains. Noticeably, genes encoding for homologous enzymes are also present in the genomes of some pathogenic and anaerobic amitochondriate protozoa. The fingerprint of this enzyme family is the conservation of a two-domain structural core, built by a metallo-beta-lactamase-like domain, at the N-terminal region, harbouring a non-heme diiron site, and a flavodoxin-like domain, containing one FMN moiety. These enzymes have a significant nitric oxide reductase activity, and there is increasing evidence that they are involved in microbial resistance to nitric oxide. In this review, we will discuss available data for this novel family of enzymes, including their physicochemical properties, structural and phylogenetic analyses, enzymatic properties and the molecular genetic approaches so far used to tackle their function.

The proteasome of Mycobacterium tuberculosis is required for resistance to nitric oxide

The production of nitric oxide and other reactive nitrogen intermediates (RNI) by macrophages helps to control infection by Mycobacterium tuberculosis (Mtb). However, the protection is imperfect and infection persists. To identify genes that Mtb requires to resist RNI, we screened 10,100 Mtb transposon mutants for hypersusceptibility to acidified nitrite. We found 12 mutants with insertions in seven genes representing six pathways, including the repair of DNA (uvrB) and the synthesis of a flavin cofactor (fbiC). Five mutants had insertions in proteasome-associated genes. An Mtb mutant deficient in a presumptive proteasomal adenosine triphosphatase was attenuated in mice, and exposure to proteasomal protease inhibitors markedly sensitized wild-type Mtb to RNI. Thus, the mycobacterial proteasome serves as a defense against oxidative or nitrosative stress.

Oxidation of methionine residues of proteins: biological consequences

Most reactive oxygen species (ROS) can oxidize methionine (Met) residues of proteins to methionine sulfoxide (MetO). However, unlike the ROS-dependent oxidation of other amino acid residues of proteins (except cysteine residues), the oxidation of Met residues is readily reversed by the action of methionine sulfoxide reductase (Msr) that catalyzes the thioredoxin-dependent reduction of MetO residues of proteins back to Met. We summarize here results of studies showing that the cyclic interconversion of Met and MetO residues of proteins is involved in several different biological processes: (a) It is the basis of an important antioxidant mechanism for the scavenging of ROS. (b) It is likely involved in the regulation of enzyme activities. (c) It is involved in cell signaling. (d) It can target proteins for proteolytic degradation. Furthermore, a loss in the ability to catalyze the reduction of protein MetO to Met residues leads to a decrease in the maximum life span, whereas overexpression of this activity leads to an increase in the life span of animals. In addition, a decrease in Msr activities in brain tissues is associated with the development of Alzheimer's disease.

Latent tuberculosis: mechanisms of host and bacillus that contribute to persistent infection

Most people infected with Mycobacterium tuberculosis contain the initial infection and develop latent tuberculosis. This state is characterised by evidence of an immune response against the bacterium (a positive tuberculin skin test) but no signs of active infection. It can be maintained for the lifetime of the infected person. However, reactivation of latent infection occurs in about 10% of infected individuals, leading to active and contagious tuberculosis. An estimated 2 billion people worldwide are infected with M tuberculosis--an enormous reservoir of potential tuberculosis cases. The establishment and reactivation of latent infection depend on several factors, related to both host and bacterium. Elucidation of the host immune mechanisms that control the initial infection and prevent reactivation has begun. The bacillus is well adapted to the human host and has a range of evasion mechanisms that contribute to its ability to avoid elimination by the immune system and establish a persistent infection. We discuss here current understanding of both host and bacterial factors that contribute to latent and reactivation tuberculosis.

Immune evasion by Mycobacterium tuberculosis: living with the enemy

Mycobacterium tuberculosis is successful as a pathogen because of its ability to persist in an immunocompetent host. This bacterium lives within the macrophage, a cell whose function is the elimination of microbes. Recent advances have improved our understanding of how M. tuberculosis evades two major antimicrobial mechanisms of macrophages: phagolysosome fusion and the production of toxic reactive nitrogen intermediates. M. tuberculosis also modulates antigen presentation to prevent the detection of infected macrophages by CD4(+) T cells.

Structure of Mycobacterium tuberculosis methionine sulfoxide reductase A in complex with protein-bound methionine

Peptide methionine sulfoxide reductase (MsrA) repairs oxidative damage to methionine residues arising from reactive oxygen species and reactive nitrogen intermediates. MsrA activity is found in a wide variety of organisms, and it is implicated as one of the primary defenses against oxidative stress. Disruption of the gene encoding MsrA in several pathogenic bacteria responsible for infections in humans results in the loss of their ability to colonize host cells. Here, we present the X-ray crystal structure of MsrA from the pathogenic bacterium Mycobacterium tuberculosis refined to 1.5 A resolution. In contrast to the three catalytic cysteine residues found in previously characterized MsrA structures, M. tuberculosis MsrA represents a class containing only two functional cysteine residues. The structure reveals a methionine residue of one MsrA molecule bound at the active site of a neighboring molecule in the crystal lattice and thus serves as an excellent model for protein-bound methionine sulfoxide recognition and repair.

Selenium-deficient diet enhances protein oxidation and affects methionine sulfoxide reductase (MsrB) protein level in certain mouse tissues

Mammals contain two methionine sulfoxide (MetO) reductases, MsrA and MsrB, that catalyze the thioredoxin-dependent reduction of the S-MetO and R-MetO derivatives, respectively, to methionine. The major mammalian MsrB is a selenoprotein (except in the heart). Here, we show that there is a loss of MsrB activity in the MsrA-/- mouse that correlates with parallel losses in the levels of MsrB mRNA and MsrB protein, suggesting that MsrA might have a role in MsrB transcription. Moreover, mice that were grown on a selenium-deficient (SD) diet showed a substantial decrease in the levels of MsrB-catalytic activity, MsrB protein, and MsrB mRNA in liver and kidney tissues of both WT and MsrA-/- mouse strains. Whereas no significant protein-MetO could be detected in tissue proteins of young mature mice grown on a selenium-adequate diet, growth on the SD diet led to substantial accumulations of MetO in proteins and also of protein carbonyl derivatives in the liver, kidney, cerebrum, and cerebellum, respectively. In addition, accumulation of protein-MetO derivatives increased with age in tissues of mice fed with a selenium-adequate diet. It should be pointed out that even though the total Msr level is at least 2-fold higher in WT than in MsrA-/- mice, SD diet causes an equal elevation of protein-MetO (except in brain cerebellum) and carbonyl levels in both strains, suggesting involvement of other selenoproteins in regulation of the level of cellular protein-MetO accumulation. Furthermore, the development of the "tip-toe" walking behavior previously observed in the MsrA-/- mice occurred earlier when they were fed with the SD diet.

Aerobic growth deficient Haemophilus influenzae mutants are non-virulent: implications on metabolism

We investigated aerobic metabolism in Haemophilus influenzae to better understand its essential physiological growth pathways. We describe the isolation and characterization of transposon insertions leading to knockout mutations in lpdA, encoding dihydrolipoamide dehydrogenase. H. influenzae Rd lpdA::Tn10d-cat mutants were unable to grow aerobically and an H. influenzae type b lpdA::Tn10d-cat mutant was significantly attenuated in an infant rat infection model. Since LpdA is a functional subunit of both pyruvate dehydrogenase (aceEF) and alpha-ketoglutarate dehydrogenase (sucAB) the phenotype of the lpdA mutant was further explored by creating separate knockout mutants in the sucAB and aceEF loci. DeltaaceEF and deltasucAB mutants were both significantly attenuated in virulence in the infant rat, but only the sucAB mutant was able to grow aerobically. We therefore conclude that the ability for aerobic growth is critical for invasive disease, and furthermore that a TCA cycle enzyme, alpha-ketoglutarate dehydrogenase, appears to contribute a key metabolic function in vivo, but is not required for growth under laboratory conditions.

Reversible methionine sulfoxidation of Mycobacterium tuberculosis small heat shock protein Hsp16.3 and its possible role in scavenging oxidants

Mycobacterium tuberculosis (TB) small heat shock protein Hsp16.3 was found to be a major membrane protein that is most predominantly expressed under oxidative stress and is localized to the thickened cell envelope. Gene knock-out studies indicate that the Hsp16.3 protein is required for TB to grow in its host macrophage cells. The physiological function of Hsp16.3 has not yet revealed. Our analyses via mass spectrometry, conformation-dependent trypsin digestion, nondenaturing pore gradient electrophoresis, ANS-binding fluorescence measurements, and circular dichroism demonstrate that the three and only the three methionine residues (cysteine and tryptophan residues, which can also be readily oxidized by such oxidant as H(2)O(2), are absent in Hsp16.3) can be readily sulfoxidized with H(2)O(2) treatment in vitro, and the methionine sulfoxide can be effectively reduced back to the methionine form. Interconversion between the methionine and methioninesulfoxide has been confirmed by selective oxidation and reduction. The sulfoxidation leads to a small degree of conformational change, which in turn results in a significant decrease of the chaperone-like activity. Data presented in this report strongly implicate that reversible sulfoxidation/desulfoxidation of methionine residues may occur in Hsp16.3, which serves as a way to scavenger reactive oxygen or nitrogen species abundantly present in macrophage cells, thus protecting the plasma membrane and other components of M. tuberculosis allowing their survival in such bacteriocidal hosts.

Identification of Lactobacillus reuteri genes specifically induced in the mouse gastrointestinal tract

Lactobacilli are common inhabitants of the gastrointestinal tracts of mammals and have received considerable attention due to their putative health-promoting properties. Little is known about the traits that enhance the ability of these bacteria to inhabit the gastrointestinal tract. In this paper we describe the development and application of a strategy based on in vivo expression technology (IVET) that enables detection of Lactobacillus reuteri genes specifically induced in the murine gut. A plasmid-based system was constructed containing 'ermGT (which confers lincomycin resistance) as the primary reporter gene for selection of promoters active in the gastrointestinal tract of mice treated with lincomycin. A second reporter gene, 'bglM (beta-glucanase), allowed differentiation between constitutive and in vivo inducible promoters. The system was successfully tested in vitro and in vivo by using a constitutive promoter. Application of the IVET system with chromosomal DNA of L. reuteri 100-23 and reconstituted lactobacillus-free mice revealed three genes induced specifically during colonization. Two of the sequences showed homology to genes encoding xylose isomerase (xylA) and peptide methionine sulfoxide reductase (msrB), which are involved in nutrient acquisition and stress responses, respectively. The third locus showed homology to the gene encoding a protein whose function is not known. Our IVET system has the potential to identify genes of lactobacilli that have not previously been functionally characterized but which may be essential for growth of these bacteria in the gastrointestinal ecosystem.

New membrane-associated and soluble peptide methionine sulfoxide reductases in Escherichia coli

It is known that reactive oxygen species can oxidize methionine residues in proteins in a non-stereospecific manner, and cells have mechanisms to reverse this damage. MsrA and MsrB are members of the methionine sulfoxide family of enzymes that specifically reduce the S and R forms, respectively, of methionine sulfoxide in proteins. However, in Escherichia coli the level of MsrB activity is very low which suggested that there may be other enzymes capable of reducing the R epimer of methionine sulfoxide in proteins. Employing a msrA/B double mutant, a new peptide methionine sulfoxide reductase activity has been found associated with membrane vesicles from E. coli. Both the R and S forms of N-acetylmethionine sulfoxide, D-ala-met(o)-enkephalin and methionine sulfoxide, are reduced by this membrane associated activity. The reaction requires NADPH and may explain, in part, how the R form of methionine sulfoxide in proteins is reduced in E. coli. In addition, a new soluble Msr activity was also detected in the soluble extracts of the double mutant that specifically reduces the S epimer of met(o) in proteins.

Exogenous glutathione completes the defense against oxidative stress in Haemophilus influenzae

Since they are equipped with several strategies by which they evade the antimicrobial defense of host macrophages, it is surprising that members of the genus Haemophilus appear to be deficient in common antioxidant systems that are well established to protect prokaryotes against oxidative stress. Among others, no genetic evidence for glutathione (gamma-Glu-Cys-Gly) (GSH) biosynthesis or for alkyl hydroperoxide reduction (e.g., the Ahp system characteristic or enteric bacteria) is apparent from the Haemophilus influenzae Rd genome sequence, suggesting that the organism relies on alternative systems to maintain redox homeostasis or to reduce small alkyl hydroperoxides. In this report we address this apparent paradox for the nontypeable H. influenzae type strain NCTC 8143. Instead of biosynthesis, we could show that this strain acquires GSH by importing the thiol tripeptide from the growth medium. Although such GSH accumulation had no effect on growth rates, the presence of cellular GSH protected against methylglyoxal, tert-butyl hydroperoxide (t-BuOOH), and S-nitrosoglutathione toxicity and regulated the activity of certain antioxidant enzymes. H. influenzae NCTC 8143 extracts were shown to contain GSH-dependent peroxidase activity with t-BuOOH as the peroxide substrate. The GSH-mediated protection against t-BuOOH stress is most probably catalyzed by the product of open reading frame HI0572 (Prx/Grx), which we isolated from a genomic DNA fragment that confers wild-type resistance to t-BuOOH toxicity in the Ahp-negative Escherichia coli strain TA4315 and that introduces GSH-dependent alkyl hydroperoxide reductase activity into naturally GSH peroxidase-negative E. coli. Finally, we demonstrated that cysteine is an essential amino acid for growth and that cystine, GSH, glutathione amide, and cysteinylglycine can be catabolized in order to complement cysteine deficiency.

Functional complementation in yeast reveals a protective role of chloroplast 2-Cys peroxiredoxin against reactive nitrogen species

The importance of nitric oxide (NO) as a signaling molecule to various plant physiological and pathophysiological processes is becoming increasingly evident. However, little is known about how plants protect themselves from nitrosative and oxidative damage mediated by NO and NO-derived reactive nitrogen species (RNS). Peroxynitrite, the product of the reaction between NO and superoxide anion, is considered to play a central role in RNS-induced cytotoxicity, as a result of its potent ability to oxidize diverse biomolecules. Employing heterologous expression in bacteria and yeast, we investigated peroxynitrite-scavenging activity in plants of 2-Cys peroxiredoxin (2CPRX), originally identified as a hydroperoxide-reducing peroxidase that is ubiquitously distributed among organisms. The putative mature form of a chloroplast-localized 2CPRX from Arabidopsis thaliana was overproduced in Escherichia coli as an amino-terminally hexahistidine-tagged fusion protein. The purified recombinant 2CPRX, which was catalytically active as peroxidase, efficiently prevented the peroxynitrite-induced oxidation of a sensitive compound. We also examined in vivo the ability of the Arabidopsis 2CPRX to complement the 2CPRX deficiency of a Saccharomyces cerevisiae mutant. Functional expression in the mutant strain of the Arabidopsis 2CPRX not only increased cellular tolerance to hydrogen peroxide, but also complemented the hypersensitive growth defect induced by nitrite-mediated cytotoxicity. The complemented cells significantly enhanced the capacity to reduce RNS-mediated oxidative damages. The results presented here demonstrate a new role of plant 2CPRX as a critical determinant of the resistance to RNS, and support the existence of a plant enzymatic basis for RNS metabolism.

A methionine sulfoxide reductase in Escherichia coli that reduces the R enantiomer of methionine sulfoxide

It is known that Escherichia coli methionine mutants can grow on both enantiomers of methionine sulfoxide (met(o)), i.e., met-R-(o) or met-S-(o), indicating the presence of enzymes in E. coli that can reduce each of these enantiomers to methionine (met). Previous studies have identified two members of the methionine sulfoxide reductase (Msr) family of enzymes, MsrA and fSMsr, that could reduce free met-S-(o), but the reduction of free met-R-(o) to met has not been elucidated. One possible candidate is MsrB which is known to reduce met-R-(o) in proteins to met. However, free met-R-(o) is a very poor substrate for MsrB and the level of MsrB activity in E. coli extracts is very low. A new member of the Msr family (fRMsr) has been identified in E. coli extracts that reduces free met-R-(o) to met. Partial purification of FRMsr has been obtained using extracts from an MsrA/MsrB double mutant of E. coli.

Unravelling the biology of macrophage infection by gene expression profiling of intracellular Salmonella enterica

For intracellular pathogens such as Salmonellae, Mycobacteriae and Brucellae, infection requires adaptation to the intracellular environment of the phagocytic cell. The transition from extracellular to intravacuolar environment has been expected to involve a global modulation of bacterial gene expression, but the precise events have been difficult to determine. We now report the complete transcriptional profile of intracellular Salmonella enterica sv. Typhimurium following macrophage infection. During replication in murine macrophage-like J774-A.1 cells, 919 of 4451 S. Typhimurium genes showed significant changes in transcription. The expression profile identified alterations in numerous virulence and SOS response genes and revealed unexpected findings concerning the biology of the Salmonella-macrophage interaction. We observed that intracellular Salmonella are not starved for amino acids or iron (Fe2+), and that the intravacuolar environment is low in phosphate and magnesium but high in potassium. S. Typhimurium appears to be using the Entner-Douderoff pathway to use gluconate and related sugars as a carbon source within macrophages. Almost half the in vivo-regulated genes were of unknown function, suggesting that intracellular growth involves novel macrophage-associated functions. This is the first report that identifies the whole set of in vivo-regulated genes for any bacterial pathogen during infection of mammalian cells.

Modulation of inducible nitric oxide synthase expression by the attaching and effacing bacterial pathogen citrobacter rodentium in infected mice

Citrobacter rodentium belongs to the attaching and effacing family of enteric bacterial pathogens that includes both enteropathogenic and enterohemorrhagic Escherichia coli. These bacteria infect their hosts by colonizing the intestinal mucosal surface and intimately attaching to underlying epithelial cells. The abilities of these pathogens to exploit the cytoskeleton and signaling pathways of host cells are well documented, but their interactions with the host's antimicrobial defenses, such as inducible nitric oxide synthase (iNOS), are poorly understood. To address this issue, we infected mice with C. rodentium and found that iNOS mRNA expression in the colon significantly increased during infection. Immunostaining identified epithelial cells as the major source for immunoreactive iNOS. Finding that nitric oxide (NO) donors were bacteriostatic for C. rodentium in vitro, we examined whether iNOS expression contributed to host defense by infecting iNOS-deficient mice. Loss of iNOS expression caused a small but significant delay in bacterial clearance without affecting tissue pathology. Finally, immunofluorescence staining was used to determine if iNOS expression was localized to infected cells by staining for the C. rodentium virulence factor, translocated intimin receptor (Tir), as well as iNOS. Interestingly, while more than 85% of uninfected epithelial cells expressed iNOS, fewer than 15% of infected (Tir-positive) cells expressed detectable iNOS. These results demonstrate that both iNOS and intestinal epithelial cells play an active role in host defense during C. rodentium infection. However, the selective expression of iNOS by uninfected but not infected cells suggests that this pathogen has developed mechanisms to locally limit its exposure to host-derived NO.

Mouse methionine sulfoxide reductase B: effect of selenocysteine incorporation on its activity and expression of the seleno-containing enzyme in bacterial and mammalian cells

The mammalian methionine sulfoxide reductase B (MsrB) has been found to be a selenoprotein which can reduce R form of both free and protein-incorporated methionine sulfoxide to methionine. Together with MsrA, which reduces specifically the S form of methionine sulfoxide, the living cell can repair methionine-damaged proteins and salvage free methionine under oxidative stress conditions. Here, we report about the pivotal role of the selenocysteine residue in the protein putative active site by site-directed mutagenesis directed to the selenocysteine codon. Using the Escherichia coli SECIS (selenocysteine insertion sequence) element, needed for the recognition of the UGA codon as a selenocysteine codon in E. coli, we expressed the seleno-MsrB as a recombinant selenoprotein in E. coli. The recombinant seleno-MsrB has been shown to be much more active than the cysteine mutant, whereas the mutations to alanine and serine rendered the protein inactive. Although the yields of expression of the full-length N-terminus and C-terminus His-tagged seleno-MsrB were only 3% (of the total MsrB expressed), the C-terminus His-tagged protein enabled us to get a pure preparation of the seleno-MsrB. Using both recombinant selenoproteins, the N-terminus His-tagged and the C-terminus His-tagged proteins, we were able to determine the specific activities of the recombinant seleno-MsrB, which were found to be much higher than the cysteine mutant homologue. This finding confirmed our suggestion that the selenocysteine is essential for maintaining high reducing activity of MsrB. In addition, using radioactive selenium we were able to determine the in vivo presence of MsrB as a selenoprotein in mammalian cell cultures.

Importance of individuality in oxidative stress and aging

Tight linkage between aging and oxidative stress is indicated by the observations that reactive oxygen species generated under various conditions of oxidative stress are able to oxidize nucleic acids, proteins, and lipids and that aging is associated with the accumulation of oxidized forms of cellular constituents, and also by the fact that there is an inverse relationship between the maximum life span of organisms and the age-related accumulation of oxidative damage. Nevertheless, validity of the oxidative stress hypothesis of aging is questioned by (i) the failure to establish a causal relationship between aging and oxidative damage and (ii) lack of a consistent correlation between the accumulation of oxidative damage and aging. The present discussion is focused on the complexity of the aging process and suggests that discrepancies between various studies in this area are likely due to the fact that aging is not a single process and that the lack of consistent experimental results is partly explained by individual variations. Even so, there is overwhelming support for a dominant role of oxidative stress in the aging of some individuals.

Identification of surface proteins of Helicobacter pylori by selective biotinylation, affinity purification, and two-dimensional gel electrophoresis

Helicobacter pylori is a widespread human pathogen that can cause gastric ulcers and cancer. To identify surface proteins that may play a role in pathogen-host interactions and represent potential targets for the control of this infection, we selectively biotinylated intact H. pylori with the hydrophilic reagent sulfosuccinimidyl-6-(biotinamido)-hexanoate and purified the labeled proteins by membrane isolation, solubilization, and affinity chromatography. After separation of 82 biotinylated proteins on two-dimensional gels, 18 were identified with comparison to proteome data and peptide mass fingerprinting. Among the identified proteins, 9 have previously been shown to be surface-exposed, 7 are associated with virulence, and 11 are highly immunogenic in infected patients. In conclusion, this generally applicable combined proteome approach facilitates the rapid identification of promising targets for the control of H. pylori and might be applicable to numerous other human pathogens although larger biotinylation reagents might be required in some cases to prevent permeation of porin channels in the outer membrane.

The NorR protein of Escherichia coli activates expression of the flavorubredoxin gene norV in response to reactive nitrogen species

The Escherichia coli norVW genes encode a flavorubredoxin and NADH:(flavo)rubredoxin reductase, respectively, which are involved in nitric oxide detoxification under anaerobic growth conditions. Here it is shown that the norVW genes also have a role in protection against reactive nitrogen intermediates generated from nitroprusside. Transcription from the norV promoter is activated by the presence of nitroprusside in the growth medium; activation requires the product of a divergently transcribed regulatory gene, norR.