Identification and characterization of a major cell wall-associated iron-regulated envelope protein (Irep-28) in Mycobacterium tuberculosis

Nucleoid-associated proteins of mycobacteria come with a distinctive flavor

In every bacterium, nucleoid-associated proteins (NAPs) play crucial roles in chromosome organization, replication, repair, gene expression, and other DNA transactions. Their central role in controlling the chromatin dynamics and transcription has been well-appreciated in several well-studied organisms. Here, we review the diversity, distribution, structure, and function of NAPs from the genus Mycobacterium. We highlight the progress made in our understanding of the effects of these proteins on various processes and in responding to environmental stimuli and stress of mycobacteria in their free-living as well as during distinctive intracellular lifestyles. We project them as potential drug targets and discuss future studies to bridge the information gap with NAPs from well-studied systems.

The role of transcriptional regulators in metal ion homeostasis of Mycobacterium tuberculosis

Metal ions are essential trace elements for all living organisms and play critical catalytic, structural, and allosteric roles in many enzymes and transcription factors. Mycobacterium tuberculosis (MTB), as an intracellular pathogen, is usually found in host macrophages, where the bacterium can survive and replicate. One of the reasons why Tuberculosis (TB) is so difficult to eradicate is the continuous adaptation of its pathogen. It is capable of adapting to a wide range of harsh environmental stresses, including metal ion toxicity in the host macrophages. Altering the concentration of metal ions is the common host strategy to limit MTB replication and persistence. This review mainly focuses on transcriptional regulatory proteins in MTB that are involved in the regulation of metal ions such as iron, copper and zinc. The aim is to offer novel insights and strategies for screening targets for TB treatment, as well as for the development and design of new therapeutic interventions.

Latent tuberculosis: interaction of virulence factors in Mycobacterium tuberculosis

Tuberculosis (TB) remains a prominent health concern worldwide. Besides extensive research and vaccinations available, attempts to control the pandemic are cumbersome due to the complex physiology of Mycobacterium tuberculosis (Mtb). Alongside the emergence of drug-resistant TB, latent TB has worsened the condition. The tubercle bacilli are unusually behaved and successful with its strategies to modulate genes to evade host immune system and persist within macrophages. Under latent/unfavorable conditions, Mtb conceals itself from immune system and modulates its genes. Among many intracellular modulated genes, important are those involved in cell entry, fatty acid degradation, mycolic acid synthesis, phagosome acidification inhibition, inhibition of phagosome-lysosome complex and chaperon protein modulation. Though the study on these genes date back to early times of TB, an insight on their inter-relation within and to newly evolved genes are still required. This review focuses on the findings and discussions on these genes, possible mechanism, credibility as target for novel drugs and repurposed drugs and their interaction that enables Mtb in survival, pathogenesis, resistance and latency.

Immunoreactive Proteins of Dormant Mycobacterium tuberculosis Cells

Abstract

The protein profile of dormant Mtb obtained after the gradual acidification of Mtb culture was studied to find antigenic proteins for humans that are expressed by M. tuberculosis (Mtb) cells in vitro under conditions close to the situation of persistence in vivo. According to 2D electrophoresis, a significant diversity of proteins in dormant cells was found. However, the representation of individual proteins in dormant versus active cells differed substantially. Immunoblotting in different protein fractions of dormant cells revealed ten proteins that are able to bind antibodies in pooled sera of TB patients. Two proteins (Rv2018 and Rv0341) are new immunogenics that were not previously found in other studies. Four proteins (Rv0341, Rv2018, Rv1509, Rv2986) with the maximal structural specificity for Mtb due to their unique extended domains were selected for further analysis. These proteins were expressed in E. coli cells and studied via enzyme-linked immunosorbent assay (ELISA) for the immunogenicity of individual sera of TB patients and healthy donors. All proteins were found to have the ability to react with individual sera of TB patients. In TB patients, 5–45% (depending on the particulate protein) have a titer that is higher than the average titers of healthy donors +SD; the most immunogenic was protein Rv2986. Thus, the application of phenotypically changed (dormant) Mtb cells makes it possible to identify a specific repertoire of immunodominant proteins that could be used in the construction of polypeptides that are useful for the serodiagnosis of active/latent TB.

Iron uptake and transport by the carboxymycobactin-mycobactin siderophore machinery of Mycobacterium tuberculosis is dependent on the iron-regulated protein HupB

Iron-starved Mycobacterium tuberculosis utilises the carboxymycobactin-mycobactin siderophore machinery to acquire iron. These two siderophores have high affinity for ferric iron and can withdraw the metal ion from insoluble iron hydroxides and iron-binding proteins. We first reported HupB, a multi-functional mycobacterial protein to be associated with iron acquisition in M. tuberculosis. This 28 kDa cell wall protein, up regulated upon iron limitation functions as a transcriptional activator of mycobactin biosynthesis and is essential for the pathogen to survive inside macrophages. The focus of this study is to understand the role of HupB in iron uptake and transport by the carboxmycobactin-mycobactin siderophore machinery in M. tuberculosis. Experimental approaches included radiolabelled iron uptake studies by viable organisms and protein-ligand binding studies using the purified HupB and the two siderophores. Uptake of ⁵⁵Fe-carboxymycobactin by wild type M. tuberculosis (WT M.tb.H37Rv) and not by the hupB KO mutant (M.tb.ΔhupB) showed that HupB is necessary for the uptake of ferri-carboxymycobactin. Additionally, the radiolabel recovery was high in HupB-incorporated liposomes upon addition of the labelled siderophore. Bioinformatic and experimental studies using spectrofluorimetry, CD analysis and surface plasmon resonance not only confirmed the binding of HupB with ferri-carboxymycobactin and ferri-mycobactin but also with free iron. In conclusion, HupB is established as a ferri- carboxymycobactin receptor and by virtue of its property to bind ferric iron, functions as a transporter of the ferric iron from the extracellular siderophore to mycobactin within the cell envelope.

Metabolic adaptation to glycolysis is a basic defense mechanism of macrophages for Mycobacterium tuberculosis infection

Macrophages are major components of tuberculosis (TB) granulomas and are responsible for host defenses against the intracellular pathogen, Mycobacterium tuberculosis. We herein showed the strong expression of hypoxia-inducible factor-1α (HIF-1α) in TB granulomas and more rapid death of HIF-1α-conditional knockout mice than wild-type (WT) mice after M. tuberculosis infection. Although interferon-γ (IFN-γ) is a critical host-protective cytokine against intracellular pathogens, HIF-1-deficient macrophages permitted M. tuberculosis growth even after activation with IFN-γ. These results prompted us to investigate the role of HIF-1α in host defenses against infection. We found that the expression of lactate dehydrogenase-A (LDH-A) was controlled by HIF-1α in M. tuberculosis-infected macrophages IFN-γ independently. LDH-A is an enzyme that converts pyruvate to lactate and we found that the intracellular level of pyruvate in HIF-1α-deficient bone marrow-derived macrophages (BMDMs) was significantly higher than in WT BMDMs. Intracellular bacillus replication was enhanced by an increase in intracellular pyruvate concentrations, which were decreased by LDH-A. Mycobacteria in phagosomes took up exogenous pyruvate more efficiently than glucose, and used it as the feasible carbon source for intracellular growth. These results demonstrate that HIF-1α prevents the hijacking of pyruvate in macrophages, making it a fundamental host-protective mechanism against M. tuberculosis.

Significance of a histone-like protein with its native structure for the diagnosis of asymptomatic tuberculosis

Tuberculosis causes the highest mortality among all single infections. Asymptomatic tuberculosis, afflicting one third of the global human population, is the major source as 5–10% of asymptomatic cases develop active tuberculosis during their lifetime. Thus it is one of important issues to develop diagnostic tools for accurately detecting asymptomatic infection. Mycobacterial DNA-binding protein 1 (MDP1) is a major protein in persistent Mycobacterium tuberculosis and has potential for diagnostic use in detecting asymptomatic infection. However, a previous ELISA-based study revealed a specificity problem; IgGs against MDP1 were detected in both M. tuberculosis-infected and uninfected individuals. Although the tertiary structures of an antigen are known to influence antibody recognition, the MDP1 structural details have not yet been investigated. The N-terminal half of MDP1, homologous to bacterial histone-like protein HU, is predicted to be responsible for DNA-binding, while the C-terminal half is assumed as totally intrinsically disordered regions. To clarify the relationship between the MDP1 tertiary structure and IgG recognition, we refined the purification method, which allow us to obtain a recombinant protein with the predicted structure. Furthermore, we showed that an IgG-ELISA using MDP1 purified by our refined method is indeed useful in the detection of asymptomatic tuberculosis.

G-Quadruplex-Forming DNA Aptamers Inhibit the DNA-Binding Function of HupB and Mycobacterium tuberculosis Entry into Host Cells

The entry and survival of Mycobacterium tuberculosis (Mtb) within host cells is orchestrated partly by an essential histone-like protein HupB (Rv2986c). Despite being an essential drug target, the lack of structural information has impeded the development of inhibitors targeting the indispensable and multifunctional C-terminal domain (CTD) of HupB. To bypass the requirement for structural information in the classical drug discovery route, we generated a panel of DNA aptamers against HupB protein through systemic evolution of ligands by exponential (SELEX) enrichment. Two G-quadruplex-forming high-affinity aptamers (HupB-4T and HupB-13T) were identified, each of which bound two distinct sites on full-length HupB, with an estimated K_D of ∼1.72 μM and ∼0.17 μM, respectively, for the high-affinity sites. While HupB-4T robustly inhibited DNA-binding activity of HupB in vitro, both the aptamers recognized surface-located HupB and significantly blocked Mtb entry into THP-1 monocytic cells (p < 0.0001). In summary, DNA aptamers generated in this study block DNA-binding activity of HupB, inhibit virulent Mtb infection in host cells, and demonstrate aptamers to be inhibitors of HupB functions. This study also illustrates the utility of SELEX in developing inhibitors against essential targets for whom structural information is not available.

Altered Mycobacterium tuberculosis Cell Wall Metabolism and Physiology Associated With RpoB Mutation H526D

Background:Mycobacterium tuberculosis (Mtb) rpoB mutations are associated with global metabolic remodeling. However, the net effects of rpoB mutations on Mtb physiology, metabolism and function are not completely understood. Based on previous work, we hypothesized that changes in the expression of cell wall molecules in Mtb mutant RpoB 526D lead to changes in cell wall permeability and to altered resistance to environmental stresses and drugs. Methods: The phenotypes of a fully drug-susceptible clinical strain of Mtb and its paired rifampin-monoresistant, RpoB H526D mutant progeny strain were compared. Results: The rpoB mutant showed altered colony morphology, bacillary length and cell wall thickness, which were associated with increased cell wall permeability and susceptibility to the cell wall detergent sodium dodecyl sulfate (SDS) after exposure to nutrient starvation. Relative to the isogenic rifampin-susceptible strain, the RpoB H526D mutant showed altered bacterial cellular metabolic activity and an eightfold increase in susceptibility to the cell-wall acting drug vancomycin. Conclusion: Our data suggest that RpoB mutation H526D is associated with altered cell wall physiology and resistance to cell wall-related stress. These findings are expected to contribute to an improved understanding of the pathogenesis of drug-resistant M. tuberculosis infections.

Acetylation by Eis and Deacetylation by Rv1151c of Mycobacterium tuberculosis HupB: Biochemical and Structural Insight

Bacterial nucleoid-associated proteins (NAPs) are critical to genome integrity and chromosome maintenance. Post-translational modifications of bacterial NAPs appear to function similarly to their better studied mammalian counterparts. The histone-like NAP HupB from Mycobacterium tuberculosis (Mtb) was previously observed to be acetylated by the acetyltransferase Eis, leading to genome reorganization. We report biochemical and structural aspects of acetylation of HupB by Eis. We also found that the SirT-family NAD⁺-dependent deacetylase Rv1151c from Mtb deacetylated HupB in vitro and characterized the deacetylation kinetics. We propose that activities of Eis and Rv1151c could regulate the acetylation status of HupB to remodel the mycobacterial chromosome in response to environmental changes.

Histone methyltransferase SUV39H1 participates in host defense by methylating mycobacterial histone-like protein HupB

Host cell defense against an invading pathogen depends upon various multifactorial mechanisms, several of which remain undiscovered. Here, we report a novel defense mechanism against mycobacterial infection that utilizes the histone methyltransferase, SUV39H1. Normally, a part of the host chromatin, SUV39H1, was also found to be associated with the mycobacterial bacilli during infection. Its binding to bacilli was accompanied by trimethylation of the mycobacterial histone-like protein, HupB, which in turn reduced the cell adhesion capability of the bacilli. Importantly, SUV39H1-mediated methylation of HupB reduced the mycobacterial survival inside the host cell. This was also true in mice infection experiments. In addition, the ability of mycobacteria to form biofilms, a survival strategy of the bacteria dependent upon cell-cell adhesion, was dramatically reduced in the presence of SUV39H1. Thus, this novel defense mechanism against mycobacteria represents a surrogate function of the epigenetic modulator, SUV39H1, and operates by interfering with their cell-cell adhesion ability.

Mycobacterial DNA-binding protein 1 is critical for long term survival of Mycobacterium smegmatis and simultaneously coordinates cellular functions

Bacteria can proliferate perpetually without ageing, but they also face conditions where they must persist. Mycobacteria can survive for a long period. This state appears during mycobacterial diseases such as tuberculosis and leprosy, which are chronic and develop after long-term persistent infections. However, the fundamental mechanisms of the long-term living of mycobacteria are unknown. Every Mycobacterium species expresses Mycobacterial DNA-binding protein 1 (MDP1), a histone-like nucleoid associated protein. Mycobacterium smegmatis is a saprophytic fast grower and used as a model of mycobacterial persistence, since it shares the characteristics of the long-term survival observed in pathogenic mycobacteria. Here we show that MDP1-deficient M. smegmatis dies more rapidly than the parental strain after entering stationary phase. Proteomic analyses revealed 21 upregulated proteins with more than 3-fold in MDP1-deficient strain, including DnaA, a replication initiator, NDH, a NADH dehydrogenase that catalyzes downhill electron transfer, Fas1, a critical fatty acid synthase, and antioxidants such as AhpC and KatG. Biochemical analyses showed elevated levels of DNA and ATP syntheses, a decreased NADH/NAD⁺ ratio, and a loss of resistance to oxidative stress in the MDP1-knockout strain. This study suggests the importance of MDP1-dependent simultaneous control of the cellular functions in the long-term survival of mycobacteria.

Iron Homeostasis in Mycobacterium tuberculosis: Mechanistic Insights into Siderophore-Mediated Iron Uptake

Mycobacterium tuberculosis requires iron for normal growth but faces a limitation of the metal ion due to its low solubility at biological pH and the withholding of iron by the mammalian host. The pathogen expresses the Fe(3+)-specific siderophores mycobactin and carboxymycobactin to chelate the metal ion from insoluble iron and the host proteins transferrin, lactoferrin, and ferritin. Siderophore-mediated iron uptake is essential for the survival of M. tuberculosis, as knockout mutants, which were defective in siderophore synthesis or uptake, failed to survive in low-iron medium and inside macrophages. But as excess iron is toxic due to its catalytic role in the generation of free radicals, regulation of iron uptake is necessary to maintain optimal levels of intracellular iron. The focus of this review is to present a comprehensive overview of iron homeostasis in M. tuberculosis that is discussed in the context of mycobactin biosynthesis, transport of iron across the mycobacterial cell envelope, and storage of excess iron. The clinical significance of the serum iron status and the expression of the iron-regulated protein HupB in tuberculosis (TB) patients is presented here, highlighting the potential of HupB as a marker, notably in extrapulmonary TB cases.

Mechanisms of Phenotypic Rifampicin Tolerance in Mycobacterium tuberculosis Beijing Genotype Strain B0/W148 Revealed by Proteomics

The "successful" Russian clone B0/W148 of Mycobacterium tuberculosis Beijing is well-known for its capacity to develop antibiotic resistance. During treatment, resistant mutants can occur that have inheritable resistance to specific antibiotics. Next to mutations, M. tuberculosis has several mechanisms that increase their tolerance to a variety of antibiotics. Insights in the phenotypic mechanisms that contribute to drug tolerance will increase our understanding of how antibiotic resistance develops in M. tuberculosis. In this study, we examined the (phospho)proteome dynamics in M. tuberculosis Beijing strain B0/W148 when exposed to a high dose of rifampicin; one of the most potent first-line antibiotics. A total of 2,534 proteins and 191 phosphorylation sites were identified, and revealed the differential regulation of DosR regulon proteins, which are necessary for the development of a dormant phenotype that is less susceptible to antibiotics. By examining independent phenotypic markers of dormancy, we show that persisters of in vitro rifampicin exposure entered a metabolically hypoactive state, which yields rifampicin and other antibiotics largely ineffective. These new insights in the role of protein regulation and post-translational modifications during the initial phase of rifampicin treatment reveal a shortcoming in the antituberculosis regimen that is administered to 8-9 million individuals annually.

Highly immunoreactive antibodies against the rHup-F2 fragment (aa 63-161) of the iron-regulated HupB protein of Mycobacterium tuberculosis and its potential for the serodiagnosis of extrapulmonary and recurrent tuberculosis

HupB is an iron-regulated protein in Mycobacterium tuberculosis that functions as a positive regulator of mycobactin biosynthesis. It is essential for the growth and survival of the pathogen inside macrophages. Previously, using the full-length rHupB of M. tuberculosis, we demonstrated high levels of anti-HupB antibodies in the serum of pulmonary tuberculosis (TB) and, interestingly, extrapulmonary TB patients with negligible levels in household contacts and healthy controls. Here, we used three antigenic fragments of HupB, namely the recombinant HupB-F1 (aa 1-71), HupB-F2 (aa 63-161) and HupB-F3 (aa 164-214), as antigens in enzyme-linked immunosorbent assay (ELISA) to screen serum from TB patients. HupB-F2 showed enhanced immunoreactivity with serum from patients with pulmonary TB (three groups consisting of new cases, defaulters and recurrent cases) and extrapulmonary TB, with negligible levels in normal healthy controls. The negative correlation of the anti-(HupB-F2) antibodies with serum iron was maximal, with a Pearson's correlation coefficient value of -0.415. The study, in addition to strengthening the diagnostic potential of HupB, reflected the superior performance of HupB-F2 as an antigen in screening pulmonary and extrapulmonary TB.

Transcriptional regulation of Mycobacterium tuberculosis hupB gene expression

The influence of iron levels on the transcription of the hupB gene in Mycobacterium tuberculosis is the focus of this study. Studies in our laboratory showed HupB to be co-expressed with the two siderophores in low-iron organisms. Mycobactin biosynthesis is repressed by the IdeR-Fe(2+) complex that binds the IdeR box in the mbtB promoter. Recently, we demonstrated the positive regulatory effect of HupB on mycobactin biosynthesis by demonstrating its binding to a 10 bp HupB box in the mbtB promoter. Earlier, we observed that HupB, expressed maximally in low-iron media (0.02 µg Fe ml(-1); 0.36 µM Fe) was still detectable at 8 µg Fe ml(-1) (144 µM Fe) when the siderophores were absent and complete repression was seen only at 12 µg Fe ml(-1) (216 µM Fe). In this study, we observed elevated levels of hupB transcripts in iron-limited organisms. IdeR, and not FurA, functioned as the iron regulator, by binding to two IdeR boxes in the hupB promoter. Interestingly, the 10 bp HupB box, first reported in the mbtB promoter, was identified in the hupB promoter. Using DNA footprinting and electrophoretic mobility shift assays, we demonstrated the functionality of the HupB box and the two IdeR boxes. The high hupB transcript levels expressed by the organism and the in vitro protein-DNA interaction studies led us to hypothesize the sequence of events occurring in response to changes in the intracellular iron concentration, emphasizing the roles played by IdeR and HupB in iron homeostasis.

Expression of antimicrobial drug tolerance by attached communities of Mycobacterium tuberculosis

There is an urgent need to improve methods used to screen antituberculosis drugs. An in vitro assay was developed to test drug treatment strategies that specifically target drug-tolerant Mycobacterium tuberculosis. The H37Rv strain of M. tuberculosis survived antimicrobial treatment as attached microbial communities when maintained in tissue culture media (RPMI-1640) with or without lysed human peripheral blood leukocytes. When cultured planktonically in the presence of Tween-80, bacilli failed to form microbial communities or reach logarithmic phase growth yet remained highly susceptible to antimicrobial drugs. In the absence of Tween, bacilli tolerated drug therapy by forming complex microbial communities attached to untreated well surfaces or to the extracellular matrix derived from lysed human leukocytes. Treatment of microbial communities with DNase I or Tween effectively dispersed bacilli and restored drug susceptibility. These data demonstrate that in vitro expression of drug tolerance by M. tuberculosis is linked to the establishment of attached microbial communities and that dispersion of bacilli targeting the extracellular matrix including DNA restores drug susceptibility. Modifications of this in vitro assay may prove beneficial in a high-throughput platform to screen new antituberculosis drugs especially those that target drug-tolerant bacilli.

Iron-regulated protein HupB of Mycobacterium tuberculosis positively regulates siderophore biosynthesis and is essential for growth in macrophages

Mycobacterium tuberculosis expresses the 28-kDa protein HupB (Rv2986c) and the Fe(3+)-specific high-affinity siderophores mycobactin and carboxymycobactin upon iron limitation. The objective of this study was to understand the functional role of HupB in iron acquisition. A hupB mutant strain of M. tuberculosis, subjected to growth in low-iron medium (0.02 μg Fe ml(-1)), showed a marked reduction of both siderophores with low transcript levels of the mbt genes encoding the MB biosynthetic machinery. Complementation of the mutant strain with hupB restored siderophore production to levels comparable to that of the wild type. We demonstrated the binding of HupB to the mbtB promoter by both electrophoretic mobility shift assays and DNA footprinting. The latter revealed the HupB binding site to be a 10-bp AT-rich region. While negative regulation of the mbt machinery by IdeR is known, this is the first report of positive regulation of the mbt operon by HupB. Interestingly, the mutant strain failed to survive inside macrophages, suggesting that HupB plays an important role in vivo.

Serum iron profile and ELISA-based detection of antibodies against the iron-regulated protein HupB of Mycobacterium tuberculosis in TB patients and household contacts in Hyderabad (Andhra Pradesh), India

BACKGROUND

HupB is a 28 kDa cell-wall-associated protein co-expressed with the siderophores mycobactin and carboxymycobactin in iron-limited Mycobacterium tuberculosis. HupB is expressed in vivo and anti-HupB antibodies are present in the serum of TB patients.

METHODS

The aims of this study were to evaluate the serodiagnostic potential of HupB and to correlate levels of anti-HupB antibodies with the serum iron status in TB patients, household contacts and normal healthy controls.

RESULTS

TB patients from Hyderabad (India) showed high levels of anti-HupB antibodies compared with household contacts and normal healthy controls. Interestingly, the levels were maximal in extrapulmonary TB patients, with a two-fold higher titre than pulmonary TB patients. Serum iron levels, total iron-binding capacity (TIBC) and percent saturation of serum transferrin were low in subjects with active TB, whilst serum ferritin was notably high in pulmonary TB patients compared with normal controls.

CONCLUSIONS

There is a strong negative correlation between serum iron levels and TIBC with the titre of anti-HupB antibodies in subjects with active TB. This study reflects the usefulness of screening for anti-HupB antibodies for diagnosis of pulmonary and extrapulmonary TB in this endemic region.

T-cell recognition of iron-regulated culture filtrate proteins of Mycobacterium tuberculosis in tuberculosis patients and endemic normal controls

BACKGROUND

Culture filtrate proteins (CFPs) of Mycobacterium tuberculosis are potential vaccine candidates.

OBJECTIVE

The aim was to study the influence of iron levels on CFPs and assess the immuno-protective potential of defined antigenic fractions from high (8 μg Fe/mL) and low iron (0.02 μg Fe / mL) cultures of M. tuberculosis.

MATERIALS AND METHODS

The CFPs of M. tuberculosis from high (CFP-high) and low (CFP-low) iron conditions were first compared to identify iron-regulated proteins and then fractionated to obtain ten antigen pools (CF-Ags H1- H5 and L1-L5) that were used to assess the immune response of TB patients and normal healthy controls.

RESULTS

Iron limitation resulted in the up-regulation of two novel iron-regulated low-molecular-weight proteins Irp-1 (in CF-Ag L4) and Irp-2 (in CF-Ag L5) and repression of two ESAT proteins (identified with monoclonal antibody HYB 76.8). The median stimulation indices (SIs) against most of the CF-Ags were high in pulmonary TB patients. The CF-Ags L1 and L2 showed statistically significant SI (P values of 0.0027 and 0.0029 respectively); the % case recognition was high with these antigens as well as with L4 ( P = 0.0275). IFN-γ in response to these CF-Ags was significantly high in the endemic normals; maximal expression was seen with CF-Ag L5 (median value of 233 pg mL -1 ) that was higher than the corresponding H5 (140 pg mL -1 ) and H3 and L3 (205 and 206 pg mL -1 respectively).

CONCLUSIONS

CF-Ags L5, H3 and L3 showed immuno-protective potential in this geographical location.

A novel mechanism of growth phase-dependent tolerance to isoniazid in mycobacteria

Tuberculosis remains one of the most deadly infectious diseases worldwide and is a leading public health problem. Although isoniazid (INH) is a key drug for the treatment of tuberculosis, tolerance to INH necessitates prolonged treatment, which is a concern for effective tuberculosis chemotherapy. INH is a prodrug that is activated by the mycobacterial enzyme, KatG. Here, we show that mycobacterial DNA-binding protein 1 (MDP1), which is a histone-like protein conserved in mycobacteria, negatively regulates katG transcription and leads to phenotypic tolerance to INH in mycobacteria. Mycobacterium smegmatis deficient for MDP1 exhibited increased expression of KatG and showed enhanced INH activation compared with the wild-type strain. Expression of MDP1 was increased in the stationary phase and conferred growth phase-dependent tolerance to INH in M. smegmatis. Regulation of KatG expression is conserved between M. smegmatis and Mycobacterium tuberculosis complex. Artificial reduction of MDP1 in Mycobacterium bovis BCG was shown to lead to increased KatG expression and susceptibility to INH. These data suggest a mechanism by which phenotypic tolerance to INH is acquired in mycobacteria.

Global effects of inactivation of the pyruvate kinase gene in the Mycobacterium tuberculosis complex

To better understand the global effects of "natural" lesions in genes involved in the pyruvate metabolism in Mycobacterium bovis, null mutations were made in the Mycobacterium tuberculosis H37Rv ald and pykA genes to mimic the M. bovis situation. Like M. bovis, the M. tuberculosis DeltapykA mutant yielded dysgonic colonies on solid medium lacking pyruvate, whereas colony morphology was eugonic on pyruvate-containing medium. Global effects of the loss of the pykA gene, possibly underlying colony morphology, were investigated by using proteomics on cultures grown in the same conditions. The levels of Icd2 increased and those of Icl and PckA decreased in the DeltapykA knockout. Proteomics suggested that the synthesis of enzymes involved in fatty acid and lipid biosynthesis were decreased, whereas those involved in beta-oxidation were increased in the M. tuberculosis DeltapykA mutant, as confirmed by direct assays for these activities. Thus, the loss of pykA from M. tuberculosis results in fatty acids being used principally for energy production, in contrast to the situation in the host when carbon from fatty acids is conserved through the glyoxylate cycle and gluconeogenesis; when an active pykA gene was introduced into M. bovis, the opposite effects occurred. Proteins involved in oxidative stress-AhpC, KatG, and SodA-showed increased synthesis in the DeltapykA mutant, and iron-regulated proteins were also affected. Ald levels were decreased in the DeltapykA knockout, explaining why an M. tuberculosis DeltapykA Deltaald double mutant showed little additional phenotypic effect. Overall, these data show that the loss of the pykA gene has powerful, global effects on proteins associated with central metabolism.

Experimental tuberculosis: the role of comparative pathology in the discovery of improved tuberculosis treatment strategies

The use of laboratory animals is critical to the discovery and in vivo pre-clinical testing of new drugs and drug combinations for use in humans. M. tuberculosis infection of mice, rats, guinea pigs, rabbits and non-human primates are the most commonly used animal models of human tuberculosis. While granulomatous inflammation characterizes the most fundamental host response to M. tuberculosis aerosol infection in humans and animals, there are important species differences in pulmonary and extra-pulmonary lesion morphology which may influence responses to drug therapy. Lesions that progress to necrosis or cavitation are common, unfavorable host responses in naturally occurring tuberculosis of humans, but are not seen consistently in experimental infections in most animal model species. The importance of these unique lesion morphologies is that they represent irreversible tissue damage that can harbor persistent bacilli which are difficult to treat with standard therapies. Understanding the differences in host response to experimental tuberculosis infections may aid in selecting the most appropriate animal models to test drugs that have been rationally designed to have specific mechanisms of action in vivo. A better understanding of lesion pathogenesis across species may also aid in the identification of novel therapeutic targets or strategies that can be used alone or in combination with more conventional tuberculosis treatments in humans.

The mycobacterial DNA-binding protein 1 (MDP1) from Mycobacterium bovis BCG influences various growth characteristics

BACKGROUND

Pathogenic mycobacteria such as M. tuberculosis, M. bovis or M. leprae are characterised by their extremely slow growth rate which plays an important role in mycobacterial virulence and eradication of the bacteria. Various limiting factors influence the generation time of mycobacteria, and the mycobacterial DNA-binding protein 1 (MDP1) has also been implicated in growth regulation. Our strategy to investigate the role of MDP1 in mycobacterial growth consisted in the generation and characterisation of a M. bovis BCG derivative expressing a MDP1-antisense gene.

RESULTS

The expression rate of the MDP1 protein in the recombinant M. bovis BCG containing the MDP1-antisense plasmid was reduced by about 50% compared to the reference strain M. bovis BCG containing the empty vector. In comparison to this reference strain, the recombinant M. bovis BCG grew faster in broth culture and reached higher cell masses in stationary phase. Likewise its intracellular growth in mouse and human macrophages was ameliorated. Bacterial clumping in broth culture was reduced by the antisense plasmid. The antisense plasmid increased the susceptibility of the bacteria towards Ampicillin. 2-D protein gels of bacteria maintained under oxygen-poor conditions demonstrated a reduction in the number and the intensity of many protein spots in the antisense strain compared to the reference strain.

CONCLUSION

The MDP1 protein has a major impact on various growth characteristics of M. bovis BCG. It plays an important role in virulence-related traits such as aggregate formation and intracellular multiplication. Its impact on the protein expression in a low-oxygen atmosphere indicates a role in the adaptation to the hypoxic conditions present in the granuloma.

Lsr2 of Mycobacterium tuberculosis is a DNA-bridging protein

Lsr2 is a small, basic protein present in Mycobacterium and related actinomycetes. Recent studies suggest that Lsr2 is a regulatory protein involved in multiple cellular processes including cell wall biosynthesis and antibiotic resistance. However, the underlying molecular mechanisms remain unknown. In this article, we performed biochemical studies of Lsr2–DNA interactions and structure–function analysis of Lsr2. Analysis by atomic force microscopy revealed that Lsr2 has the ability to bridge distant DNA segments, suggesting that Lsr2 plays a role in the overall organization and compactness of the nucleoid. Mutational analysis identified critical residues and selection of dominant negative mutants demonstrated that both DNA binding and protein oligomerization are essential for the normal functions of Lsr2 in vivo. These results provide strong evidence that Lsr2 is a DNA bridging protein, which represents the first identification of such proteins in bacteria phylogenetically distant from the Enterobacteriaceae. DNA bridging by Lsr2 also provides a mechanism of transcriptional regulation by Lsr2.

Expression and characterization of an iron-regulated hemin-binding protein, HbpA, from Leptospira interrogans serovar Lai

In an earlier study, based on the ferric enterobactin receptor FepA of Escherichia coli, we identified and modeled a TonB-dependent outer membrane receptor protein (LB191) from the genome of Leptospira interrogans serovar Lai. Based on in silico analysis, we hypothesized that this protein was an iron-dependent hemin-binding protein. In this study, we provide experimental evidence to prove that this protein, termed HbpA (hemin-binding protein A), is indeed an iron-regulated hemin-binding protein. We cloned and expressed the full-length 81-kDa recombinant rHbpA protein and a truncated 55-kDa protein from L. interrogans serovar Lai, both of which bind hemin-agarose. Assay of hemin-associated peroxidase activity and spectrofluorimetric analysis provided confirmatory evidence of hemin binding by HbpA. Immunofluorescence studies by confocal microscopy and the microscopic agglutination test demonstrated the surface localization and the iron-regulated expression of HbpA in L. interrogans. Southern blot analysis confirmed our earlier observation that the hbpA gene was present only in some of the pathogenic serovars and was absent in Leptospira biflexa. Hemin-agarose affinity studies showed another hemin-binding protein with a molecular mass of approximately 44 kDa, whose expression was independent of iron levels. This protein was seen in several serovars, including nonpathogenic L. biflexa. Sequence analysis and immunoreactivity with specific antibodies showed this protein to be LipL41.