A Carbonic Anhydrase Pseudogene Sensitizes Select Brucella Lineages to Low CO2 Tension

Brucella spp. are intracellular pathogens that cause a disease known as brucellosis. Though the genus is highly monomorphic at the genetic level, species have animal host preferences and some defining physiologic characteristics. Of note is the requirement for CO2 supplementation to cultivate particular species, which confounded early efforts to isolate B. abortus from diseased cattle. Differences in the capacity of Brucella species to assimilate CO2 are determined by mutations in the carbonic anhydrase gene, bcaA Ancestral single-nucleotide insertions in bcaA have resulted in frameshifted pseudogenes in B. abortus and B. ovis lineages, which underlie their inability to grow under the low CO2 tension of a standard atmosphere. Incubation of wild-type B. ovis in air selects for mutations that "rescue" a functional bcaA reading frame, which enables growth under low CO2 and enhances the growth rate under high CO2 Accordingly, we show that heterologous expression of functional Escherichia coli carbonic anhydrases enables B. ovis growth in air. Growth of B. ovis is acutely sensitive to a reduction in CO2 tension, while frame-rescued B. ovis mutants are insensitive to CO2 shifts. B. ovis initiates a gene expression program upon CO2 downshift that resembles the stringent response and results in transcriptional activation of its type IV secretion system. Our study provides evidence that loss-of-function insertion mutations in bcaA sensitize the response of B. ovis and B. abortus to reduced CO2 tension relative to that of other Brucella lineages. CO2-dependent starvation and virulence gene expression programs in these species may influence persistence or transmission in natural hosts.IMPORTANCEBrucella spp. are highly related, but they exhibit differences in animal host preference that must be determined by genome sequence differences. B. ovis and the majority of B. abortus strains require high CO2 tension to be cultivated in vitro and harbor conserved insertional mutations in the carbonic anhydrase gene, bcaA, which underlie this trait. Mutants that grow in a standard atmosphere, first reported nearly a century ago, are easily selected in the laboratory. These mutants harbor varied indel polymorphisms in bcaA that restore its consensus reading frame and rescue its function. Loss of bcaA function has evolved independently in the B. ovis and B. abortus lineages and results in a dramatically increased sensitivity to CO2 limitation.

Induction of Th2-related immune responses and production of systemic IgA in mice intranasally immunized with Brucella abortus malate dehydrogenase loaded chitosan nanoparticles

The aim of this study was to investigate the induction of mucosal immune responses by an important Brucella abortus antigen, malate dehydrogenase (Mdh), loaded in mucoadhesive chitosan nanoparticles (CNs) and immunized intranasally in a BALB/c mouse model. The production of cytokines was investigated in human leukemic monocyte cells (THP-1 cells) after stimulation with the nanoparticles. Mdh-loaded CNs (CNs-Mdh) induced higher interleukin (IL)-6 production than unloaded antigens and TF loaded CNs (CNs-TF). Using ELISpot to quantify cytokines and antibody-secreting cells in the intranasally immunized mice, IL-4 and IgG-secreting cells were found to be significantly increased at 4 weeks and 6 weeks post-immunization in the CNs-Mdh immunized group, respectively. Increases in Mdh-specific IgG, IgG1, and IgG2a antibodies were confirmed at 6 weeks after immunization, indicating a predominant IgG1 response. Analysis of the mucosal immune response in the intranasally immunized mice revealed, Mdh-specific IgA and total IgA in the nasal washes, genital secretions, fecal extracts and sera that were remarkably increased in the CNs-Mdh-immunized group compared to the CNs-TF-immunized group except total IgA of nasal wash. Therefore, the results indicated that the intranasal immunization of CNs-loaded B. abortus Mdh antigen effectively induced antigen-specific mucosal immune responses through the elicitation of Th2-related immune responses.

Exploring the Molecular Basis for Binding of Inhibitors by Threonyl-tRNA Synthetase from Brucella abortus: A Virtual Screening Study

Targeting threonyl-tRNA synthetase (ThrRS) of Brucella abortus is a promising approach to developing small-molecule drugs against bovine brucellosis. Using the BLASTp algorithm, we identified ThrRS from Escherichia coli (EThrRS, PDB ID 1QF6), which is 51% identical to ThrRS from Brucella abortus (BaThrRS) at the amino acid sequence level. EThrRS was used as the template to construct a BaThrRS homology model which was optimized using molecular dynamics simulations. To determine the residues important for substrate ATP binding, we identified the ATP-binding regions of BaThrRS, docked ATP to the protein, and identified the residues whose side chains surrounded bound ATP. We then used the binding site of ATP to virtually screen for BaThrRS inhibitors and got seven leads. We further characterized the BaThrRS-binding site of the compound with the highest predicted inhibitory activity. Our results should facilitate future experimental effects to find novel drugs for use against bovine brucellosis.

LOV Histidine Kinase Modulates the General Stress Response System and Affects the virB Operon Expression in Brucella abortus

Brucella is the causative agent of the zoonotic disease brucellosis, and its success as an intracellular pathogen relies on its ability to adapt to the harsh environmental conditions that it encounters inside the host. The Brucella genome encodes a sensor histidine kinase containing a LOV domain upstream from the kinase, LOVHK, which plays an important role in light-regulated Brucella virulence. In this report we study the intracellular signaling pathway initiated by the light sensor LOVHK using an integrated biochemical and genetic approach. From results of bacterial two-hybrid assays and phosphotransfer experiments we demonstrate that LOVHK functionally interacts with two response regulators: PhyR and LovR, constituting a functional two-component signal-transduction system. LOVHK contributes to the activation of the General Stress Response (GSR) system in Brucella via PhyR, while LovR is proposed to be a phosphate-sink for LOVHK, decreasing its phosphorylation state. We also show that in the absence of LOVHK the expression of the virB operon is down-regulated. In conclusion, our results suggest that LOVHK positively regulates the GSR system in vivo, and has an effect on the expression of the virB operon. The proposed regulatory network suggests a similar role for LOVHK in other microorganisms.

Crystallographic and kinetic study of riboflavin synthase from Brucella abortus, a chemotherapeutic target with an enhanced intrinsic flexibility

Riboflavin synthase (RS) catalyzes the last step of riboflavin biosynthesis in microorganisms and plants, which corresponds to the dismutation of two molecules of 6,7-dimethyl-8-ribityllumazine to yield one molecule of riboflavin and one molecule of 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione. Owing to the absence of this enzyme in animals and the fact that most pathogenic bacteria show a strict dependence on riboflavin biosynthesis, RS has been proposed as a potential target for antimicrobial drug development. Eubacterial, fungal and plant RSs assemble as homotrimers lacking C3 symmetry. Each monomer can bind two substrate molecules, yet there is only one active site for the whole enzyme, which is located at the interface between two neighbouring chains. This work reports the crystallographic structure of RS from the pathogenic bacterium Brucella abortus (the aetiological agent of the disease brucellosis) in its apo form, in complex with riboflavin and in complex with two different product analogues, being the first time that the structure of an intact RS trimer with bound ligands has been solved. These crystal models support the hypothesis of enhanced flexibility in the particle and also highlight the role of the ligands in assembling the unique active site. Kinetic and binding studies were also performed to complement these findings. The structural and biochemical information generated may be useful for the rational design of novel RS inhibitors with antimicrobial activity.

Vaccination with LiveEscherichia coliExpressingBrucella abortusCu/Zn Superoxide-Dismutase

Previously we reported that immunization with Escherichia coli DH5alpha-expressing Brucella abortus Cu/Zn superoxide dismutase [E. coli (pBSSOD)] induces a protective immune response in BALB/c mice. Here we studied the type of immune defense that the recombinant E. coli induces in mice using as our experimental model Brucella superoxide dismutase Cu/Zn presented by J744.A1 to sensitized lymphocytes as the target of specific lysis or as cytokine inductors. The results indicate that E. coli carrying the Cu/Zn gene was able to induce specific cytotoxic T cells, mainly from CD8(+) subpopulation and IFN-gamma-producing cells belonging in their vast majority to the CD4(+) subpopulation.

Brucella abortus choloylglycine hydrolase affects cell envelope composition and host cell internalization

Choloylglycine hydrolase (CGH, E.C. 3.5.1.24) is a conjugated bile salt hydrolase that catalyses the hydrolysis of the amide bond in conjugated bile acids. Bile salt hydrolases are expressed by gastrointestinal bacteria, and they presumably decrease the toxicity of host's conjugated bile salts. Brucella species are the causative agents of brucellosis, a disease affecting livestock and humans. CGH confers Brucella the ability to deconjugate and resist the antimicrobial action of bile salts, contributing to the establishment of a successful infection through the oral route in mice. Additionally, cgh-deletion mutant was also attenuated in intraperitoneally inoculated mice, which suggests that CGH may play a role during systemic infection other than hydrolyzing conjugated bile acids. To understand the role CGH plays in B. abortus virulence, we infected phagocytic and epithelial cells with a cgh-deletion mutant (Δcgh) and found that it is defective in the internalization process. This defect along with the increased resistance of Δcgh to the antimicrobial action of polymyxin B, prompted an analysis of the cell envelope of this mutant. Two-dimensional electrophoretic profiles of Δcgh cell envelope-associated proteins showed an altered expression of Omp2b and different members of the Omp25/31 family. These results were confirmed by Western blot analysis with monoclonal antibodies. Altogether, the results indicate that Brucella CGH not only participates in deconjugation of bile salts but also affects overall membrane composition and host cell internalization.

Characterization of recombinant B. abortus strain RB51SOD toward understanding the uncorrelated innate and adaptive immune responses induced by RB51SOD compared to its parent vaccine strain RB51

Brucella abortus is a Gram-negative, facultative intracellular pathogen for several mammals, including humans. Live attenuated B. abortus strain RB51 is currently the official vaccine used against bovine brucellosis in the United States and several other countries. Overexpression of protective B. abortus antigen Cu/Zn superoxide dismutase (SOD) in a recombinant strain of RB51 (strain RB51SOD) significantly increases its vaccine efficacy against virulent B. abortus challenge in a mouse model. An attempt has been made to better understand the mechanism of the enhanced protective immunity of RB51SOD compared to its parent strain RB51. We previously reported that RB51SOD stimulated enhanced Th1 immune response. In this study, we further found that T effector cells derived from RB51SOD-immunized mice exhibited significantly higher cytotoxic T lymphocyte activity than T effector cells derived from RB51-immunized mice against virulent B. abortus-infected target cells. Meanwhile, the macrophage responses to these two strains were also studied. Compared to RB51, RB51SOD cells had a lower survival rate in macrophages and induced lower levels of macrophage apoptosis and necrosis. The decreased survival of RB51SOD cells correlates with the higher sensitivity of RB51SOD, compared to RB51, to the bactericidal action of either Polymyxin B or sodium dodecyl sulfate (SDS). Furthermore, a physical damage to the outer membrane of RB51SOD was observed by electron microscopy. Possibly due to the physical damage, overexpressed Cu/Zn SOD in RB51SOD was found to be released into the bacterial cell culture medium. Therefore, the stronger adaptive immunity induced by RB51SOD did not correlate with the low level of innate immunity induced by RB51SOD compared to RB51. This unique and apparently contradictory profile is likely associated with the differences in outer membrane integrity and Cu/Zn SOD release.

Enzymatic and biological characteristics of enolase in Brucella abortus A19

Brucella abortus is the etiological agent of brucellosis, a disease causing human public health problems as well as major economic losses in domestic animal industries. In this study, the enolase gene of B. abortus A19 was cloned, sequenced and expressed in Escherichia coli BL21. Bacterial-expressed enolase protein (His-eno) was purified and its ability to catalyze the conversion of 2-phosphoglycerate (2-PGE) to phosphoenolpyruvate (PEP) (hereon referred to as enolase activity) was analyzed. Michaelis constant (K(m)) and maximum reaction velocity (V(max)) of the reaction was determined to be 2.0 × 10(-3) M and 178 μM l(-1)min(-1), respectively. Factors influencing the enolase activity of His-eno, such as pH, the presence of metal ions and temperature were investigated in vitro. The results showed that His-eno exhibited maximal enolase activity in pH 8.5 reaction buffer containing 10 mM MgSO(4) at 37 °C. In addition to studying the enzyme activity, binding assays were performed to provide insights into the function of His-eno on pathogenesis and immunity. His-eno exhibits fibronectin-binding ability in immunoblotting assay, suggesting that enolase may play a role in B. abortus colonization, persistence, and invasion of host tissue. Furthermore, Western blot demonstrated His-eno's binding ability to 34 bovine B. abortus positive sera, suggesting that future studies may find enolase a useful as a diagnostic marker or a vaccine candidate for brucellosis.

Identification of a single-nucleotide insertion in the promoter region affecting the sodC promoter activity in Brucella neotomae

Brucella neotomae is not known to be associated with clinical disease in any host species. Previous research suggested that B. neotomae might not express detectable levels of Cu/Zn superoxide dismutase (SOD), a periplasmic enzyme known to be involved in protecting Brucella from oxidative bactericidal effects of host phagocytes. This study was undertaken to investigate the genetic basis for the disparity in SOD expression in B. neotomae. Our Western blot and SOD enzyme assay analyses indicated that B. neotomae does express SOD, but at a substantially reduced level. Nucleotide sequence analysis of region upstream to the sodC gene identified a single-nucleotide insertion in the potential promoter region. The same single-nucleotide insertion was also detected in the sodC promoter of B. suis strain Thomsen, belonging to biovar 2 in which SOD expression was undetectable previously. Examination of the sodC promoter activities using translational fusion constructs with E. coli β-galactosidase demonstrated that the B. neotomae and B. suis biovar 2 promoters were very weak in driving gene expression. Site-directed mutation studies indicated that the insertion of A in the B. neotomae sodC promoter reduced the promoter activity. Increasing the level of SOD expression in B. neotomae through complementation with B. abortus sodC gene did not alter the bacterial survival in J774A.1 macrophage-like cells and in tissues of BALB/c and C57BL/6 mice. These results for the first time demonstrate the occurrence of a single-nucleotide polymorphism affecting promoter function and gene expression in Brucella.

Blue-light-activated histidine kinases: two-component sensors in bacteria

Histidine kinases, used for environmental sensing by bacterial two-component systems, are involved in regulation of bacterial gene expression, chemotaxis, phototaxis, and virulence. Flavin-containing domains function as light-sensory modules in plant and algal phototropins and in fungal blue-light receptors. We have discovered that the prokaryotes Brucella melitensis, Brucella abortus, Erythrobacter litoralis, and Pseudomonas syringae contain light-activated histidine kinases that bind a flavin chromophore and undergo photochemistry indicative of cysteinyl-flavin adduct formation. Infection of macrophages by B. abortus was stimulated by light in the wild type but was limited in photochemically inactive and null mutants, indicating that the flavin-containing histidine kinase functions as a photoreceptor regulating B. abortus virulence.

A Combined DNA Vaccine Encoding BCSP31, SOD, and L7/L12 Confers High Protection Against Brucella abortus 2308 by Inducing Specific CTL Responses

We constructed a combined DNA vaccine comprising genes encoding the antigens BCSP31, superoxide dismutase (SOD), and L7/L12 and evaluated its immunogenicity and protective efficacy. Immunization of mice with the combined DNA vaccine offered high protection against Brucella abortus (B. abortus) infection. The vaccine induced a vigorous specific immunoglobulin G (IgG) response, with higher IgG2a than IgG1 titers. Cytokine profiling performed at the same time showed a biased Th1-type immune response with significantly increased interferon-gamma and tumor necrosis factor-alpha stimulation. CD8(+), but not CD4(+), T cells accumulated at significantly higher levels after administration of the vaccine. Granzyme B-producing CD8(+) T cells were significantly higher in number in samples prepared from combined DNA-vaccinated mice compared with S19-vaccinated mice, demonstrating that the cytotoxicity lysis pathway is involved in the response to Brucella infection. The success of our combined DNA vaccine in a mouse model suggests its potential efficacy against brucellosis infection in large animals.

Structural properties of periplasmic SodCI that correlate with virulence in Salmonella enterica serovar Typhimurium

Salmonella enterica strains survive and propagate in macrophages by both circumventing and resisting the antibacterial effectors normally delivered to the phagosome. An important aspect of Salmonella resistance is the production of periplasmic superoxide dismutase to combat phagocytic superoxide. S. enterica serovar Typhimurium strain 14028 produces two periplasmic superoxide dismutases: SodCI and SodCII. Both enzymes are produced during infection, but only SodCI contributes to virulence in the animal. Although 60% identical to SodCII at the amino acid level with very similar enzymatic properties, SodCI is dimeric, protease resistant, and tethered within the periplasm via a noncovalent interaction. In contrast, SodCII is monomeric and protease sensitive and is released from the periplasm normally by osmotic shock. We have constructed an enzymatically active monomeric SodCI enzyme by site-directed mutagenesis. The resulting protein was released by osmotic shock and sensitive to protease and could not complement the loss of wild-type dimeric SodCI during infection. To distinguish which property is most critical during infection, we cloned and characterized related SodC proteins from a variety of bacteria. Brucella abortus SodC was monomeric and released by osmotic shock but was protease resistant and could complement SodCI in the animal. These data suggest that protease resistance is a critical property that allows SodCI to function in the harsh environment of the phagosome to combat phagocytic superoxide. We propose a model to account for the various properties of SodCI and how they contribute to bacterial survival in the phagosome.

The asymmetric distribution of the essential histidine kinase PdhS indicates a differentiation event in Brucella abortus

Many organisms use polar localization of signalling proteins to control developmental events in response to completion of asymmetric cell division. Asymmetric division was recently reported for Brucella abortus, a class III facultative intracellular pathogen generating two sibling cells of slightly different size. Here we characterize PdhS, a cytoplasmic histidine kinase essential for B. abortus viability and homologous to the asymmetrically distributed PleC and DivJ histidine kinases from Caulobacter crescentus. PdhS is localized at the old pole of the large cell, and after division and growth, the small cell acquires PdhS at its old pole. PdhS may therefore be considered as a differentiation marker as it labels the old pole of the large cell. Moreover, PdhS colocalizes with its paired response regulator DivK. Finally, PdhS is able to localize at one pole in other alpha-proteobacteria, suggesting that a polar structure associating PdhS with one pole is conserved in these bacteria. We propose that a differentiation event takes place after the completion of cytokinesis in asymmetrically dividing alpha-proteobacteria. Altogether, these data suggest that prokaryotic differentiation may be much more widespread than expected.

Characterization of the urease operon of Brucella abortus and assessment of its role in virulence of the bacterium

Most members of the genus Brucella show strong urease activity. However, the role of this enzyme in the pathogenesis of Brucella infections is poorly understood. We isolated several Tn5 insertion mutants deficient in urease activity from Brucella abortus strain 2308. The mutations of most of these mutants mapped to a 5.7-kbp DNA region essential for urease activity. Sequencing of this region, designated ure1, revealed the presence of seven open reading frames corresponding to the urease structural proteins (UreA, UreB, and UreC) and the accessory proteins (UreD, UreE, UreF, and UreG). In addition to the urease genes, another gene (cobT) was identified, and inactivation of this gene affected urease activity in Brucella. Subsequent analysis of the previously described sequences of the genomes of Brucella spp. revealed the presence of a second urease cluster, ure2, in all them. The ure2 locus was apparently inactive in B. abortus 2308. Urease-deficient mutants were used to evaluate the role of urease in Brucella pathogenesis. The urease-producing strains were found to be resistant in vitro to strong acid conditions in the presence of urea, while urease-negative mutants were susceptible to acid treatment. Similarly, the urease-negative mutants were killed more efficiently than the urease-producing strains during transit through the stomach. These results suggested that urease protects brucellae during their passage through the stomach when the bacteria are acquired by the oral route, which is the major route of infection in human brucellosis.

A bile salt hydrolase of Brucella abortus contributes to the establishment of a successful infection through the oral route in mice

Choloylglycine hydrolase (CGH), a bile salt hydrolase, has been annotated in all the available genomes of Brucella species. We obtained the Brucella CGH in recombinant form and demonstrated in vitro its capacity to cleave glycocholate into glycine and cholate. Brucella abortus 2308 (wild type) and its isogenic Deltacgh deletion mutant exhibited similar growth rates in tryptic soy broth in the absence of bile. In contrast, the growth of the Deltacgh mutant was notably impaired by both 5% and 10% bile. The bile resistance of the complemented mutant was similar to that of the wild-type strain. In mice infected through the intragastric or the intraperitoneal route, splenic infection was significantly lower at 10 and 20 days postinfection in animals infected with the Deltacgh mutant than in those infected with the wild-type strain. For both routes, no differences in spleen CFU were found between animals infected with the wild-type strain and those infected with the complemented mutant. Mice immunized intragastrically with recombinant CGH mixed with cholera toxin (CGH+CT) developed a specific mucosal humoral (immunoglobulin G [IgG] and IgA) and cellular (interleukin-2) immune responses. Fifteen days after challenge by the same route with live B. abortus 2308 cells, splenic CFU counts were 10-fold lower in mice immunized with CGH+CT than in mice immunized with CT or phosphate-buffered saline. This study shows that CGH confers on Brucella the ability to resist the antimicrobial action of bile salts. The results also suggest that CGH may contribute to the ability of Brucella to infect the host through the oral route.

Evolution of vitamin B2 biosynthesis: 6,7-dimethyl-8-ribityllumazine synthases of Brucella

The penultimate step in the biosynthesis of riboflavin (vitamin B2) involves the condensation of 3,4-dihydroxy-2-butanone 4-phosphate with 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione, which is catalyzed by 6,7-dimethyl-8-ribityllumazine synthase (lumazine synthase). Pathogenic Brucella species adapted to an intracellular lifestyle have two genes involved in riboflavin synthesis, ribH1 and ribH2, which are located on different chromosomes. The ribH2 gene was shown previously to specify a lumazine synthase (type II lumazine synthase) with an unusual decameric structure and a very high Km for 3,4-dihydroxy-2-butanone 4-phosphate. Moreover, the protein was found to be an immunodominant Brucella antigen and was able to generate strong humoral as well as cellular immunity against Brucella abortus in mice. We have now cloned and expressed the ribH1 gene, which is located inside a small riboflavin operon, together with two other putative riboflavin biosynthesis genes and the nusB gene, specifying an antitermination factor. The RibH1 protein (type I lumazine synthase) is a homopentamer catalyzing the formation of 6,7-dimethyl-8-ribityllumazine at a rate of 18 nmol mg(-1) min(-1). Sequence comparison of lumazine synthases from archaea, bacteria, plants, and fungi suggests a family of proteins comprising archaeal lumazine and riboflavin synthases, type I lumazine synthases, and the eubacterial type II lumazine synthases.

Brucella abortusd-alanyl-d-alanine carboxypeptidase contributes to its intracellular replication and resistance against nitric oxide

Brucella spp. are facultative intracellular pathogens that have the ability to survive and multiply in professional and nonprofessional phagocytes, and cause abortion in domestic animals and undulant fever in humans. However, the mechanism and factors of virulence are not fully understood. In the present study, a D-alanyl-D-alanine carboxypeptidase (DAP) mutant of Brucella abortus failed to replicate in mouse macrophages and HeLa cells, and showed less virulence than the wild type in mice. Under nitric oxide (NO) stress, the growth of the DAP mutant in vitro decreased and it also had less capability to reduce NO than the wild type. Intracellular replication of the DAP mutant was partially restored by pretreatment of macrophages with the NO synthase inhibitor, 1-phenyl-imidazole, and the level of expression of the NO reductase gene, norB, in the DAP mutant was lower than that in the wild type. These results suggest that DAP contributes to resistance against NO and that it is required for the intracellular growth of the bacterium.

Identification of active site residues of the inverting glycosyltransferase Cgs required for the synthesis of cyclic beta-1,2-glucan, a Brucella abortus virulence factor

Brucella abortus cyclic glucan synthase (Cgs) is a 320-kDa (2868-amino acid) polytopic integral inner membrane protein responsible for the synthesis of the virulence factor cyclic beta-1,2-glucan by a novel mechanism in which the enzyme itself acts as a protein intermediate. Cgs functions as an inverting processive beta-1,2-autoglucosyltransferase and has the three enzymatic activities required for the synthesis of the cyclic glucan: initiation, elongation, and cyclization. To gain further insight into the protein domains that are essential for the enzymatic activity, we have compared the Cgs sequence with other glycosyltransferases (GTs). This procedure allowed us to identify in the Cgs region (475-818) the widely spaced D, DxD, E/D, (Q/R)xxRW motif that is highly conserved in the active site of numerous GTs. By site-directed mutagenesis and in vitro and in vivo activity assays, we have demonstrated that most of the amino acid residues of this motif are essential for Cgs activity. These sequence and site-directed mutagenesis analyses also indicate that Cgs should be considered a bi-functional modular GT, with an N-terminal GT domain belonging to a new GT family related to GT-2 (GT-84) followed by a GH-94 glycoside hydrolase C-terminal domain. Furthermore, over-expression of inactive mutants results in wild-type (WT) production of cyclic glucan when bacteria co-express the mutant and the WT form, indicating that Cgs may function in the membrane as a monomeric enzyme. Together, these results are compatible with a single addition model by which Cgs acts in the membrane as a monomer and uses the identified motif to form a single center for substrate binding and glycosyl-transfer reaction.

The Brucella abortus xthA-1 gene product participates in base excision repair and resistance to oxidative killing but is not required for wild-type virulence in the mouse model

Exonuclease III, encoded by the xthA gene, plays a central role in the base excision pathway of DNA repair in bacteria. Studies with Escherichia coli xthA mutants have also shown that exonuclease III participates in the repair of oxidative damage to DNA. An isogenic xthA-1 mutant (designated CAM220) derived from virulent Brucella abortus 2308 exhibited increased sensitivity to the alkylating agent methyl methanesulfonate (MMS) compared to the parent strain. In contrast, 2308 and the isogenic xthA-1 mutant displayed similar levels of resistance to the DNA cross-linker mitomycin C. These phenotypic properties are those that would be predicted for a strain defective in base excision repair. The B. abortus xthA-1 mutant also displayed reduced resistance to killing by H2O2 and the ONOO(-)-generating compound 3-morpholinosydnonimine (SIN-1) compared to strain 2308, indicating that the xthA-1 gene product participates in protecting B. abortus 2308 from oxidative damage. Introducing a plasmid-borne copy of the parental xthA-1 gene into CAM220 restored wild-type resistance of this mutant to MMS, H2O2, and SIN-1. Although the B. abortus xthA-1 mutant exhibited increased sensitivity to oxidative killing compared to the parental strain in laboratory assays, CAM220 and 2308 displayed equivalent spleen colonization profiles in C57BL/6 [corrected] mice through 8 weeks postinfection and equivalent intracellular survival and replication profiles in cultured murine macrophages. Thus, although the xthA-1 gene product participates in base excision repair and resistance to oxidative killing in B. abortus 2308, XthA-1 is not required for wild-type virulence of this strain in the mouse model.

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

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

Membrane topology analysis of cyclic glucan synthase, a virulence determinant of Brucella abortus

Brucella abortus cyclic glucan synthase (Cgs) is a 316-kDa (2,831-amino-acid) integral inner membrane protein that is responsible for the synthesis of cyclic beta-1,2-glucan by a novel mechanism in which the enzyme itself acts as a protein intermediate. B. abortus Cgs uses UDP-glucose as a sugar donor and has the three enzymatic activities necessary for synthesis of the cyclic polysaccharide (i.e., initiation, elongation, and cyclization). Cyclic glucan is required in B. abortus for effective host interaction and complete expression of virulence. To gain further insight into the structure and mechanism of action of B. abortus Cgs, we studied the membrane topology of the protein using a combination of in silico predictions, a genetic approach involving the construction of fusions between the cgs gene and the genes encoding alkaline phosphatase (phoA) and beta-galactosidase (lacZ), and site-directed chemical labeling of lysine residues. We found that B. abortus Cgs is a polytopic membrane protein with the amino and carboxyl termini located in the cytoplasm and with six transmembrane segments, transmembrane segments I (residues 419 to 441), II (residues 452 to 474), III (residues 819 to 841), IV (residues 847 to 869), V (residues 939 to 961), and VI (residues 968 to 990). The six transmembrane segments determine four large cytoplasmic domains and three very small periplasmic regions.

N-terminal-capturing screening system for the isolation of Brucella abortus genes encoding surface exposed and secreted proteins

Secreted as well as surface exposed proteins are assumed to play major roles in bacterial virulence. In this report we describe the construction of an N-terminal protein-capturing system and its use for the isolation of Brucella abortus S2308 genes coding for putative surface exposed or secreted proteins. For this purpose, a cloning vector that generates gene fusions to a ribosome binding site and start codon deficient Chloramphenicol Acetyl Transferase (CAT) reporter gene was constructed and the resulting library introduced into B. abortus S2308 and virB mutant strains. Secreted translational fusions were identified by determining CAT activity in culture supernatants. Secretion was confirmed by Western Blot using a polyclonal anti-CAT antibody. A total of 864 clones were screened and 10 genes encoding putative secreted/surface exposed proteins were identified. Seven are Brucella proteins with an assigned function, whereas three are hypothetical proteins. The number of amino acid residues that promotes CAT secretion varies from 5 to 386 and no conserved motifs were detected. Secretion in a virB mutant background of some of the isolated fusion proteins was also determined. Interestingly, some hybrid proteins seemed to require a full VirB system for their secretion.

Brucella abortus nicotinamidase (PncA) contributes to its intracellular replication and infectivity in mice

Brucella spp. are facultative intracellular pathogens that have the ability to survive and multiply in professional and non-professional phagocytes, and cause abortion in domestic animals and undulant fever in humans. The mechanism and factors of virulence are not fully understood. Nicotinamidase/pyrazinamidase mutant (pncA mutant) of Brucella abortus failed to replicate in HeLa cells, and showed a lower rate of intracellular replication than that of wild-type strain in macrophages. Addition of nicotinic acid, but not nicotinamide, into medium supported intracellular replication of pncA mutant in HeLa cells and macrophages. The pncA mutant was not co-localizing with either late endosomes or lysosomes. The B. abortus virB4 mutant was completely cleared from the spleens of mice after 4 weeks, while the pncA mutant showed a 1.5-log reduction of the number of bacteria isolated from spleens after 10 weeks. Although pncA mutant showed reduced virulence in mice and defective intracellular replication, its ability to confer protection against the virulent B. abortus strain 544 was fully retained. These results suggest that PncA does not contribute to intracellular trafficking of B. abortus, but contributes to utilization of nutrients required for intracellular growth. Our results indicate that detailed characterizations of the pncA mutant may help the improvement of currently available live vaccines.

Intraspleen delivery of a DNA vaccine coding for superoxide dismutase (SOD) of Brucella abortus induces SOD-specific CD4+ and CD8+ T cells

In the development of vaccines capable of providing immunity against brucellosis, Cu-Zn superoxide dismutase (SOD) has been demonstrated to be one of the protective immunogens of Brucella abortus. In an earlier study, we provided strong evidence that intramuscular injection with a plasmid DNA carrying the SOD gene (pcDNA-SOD) was able to induce a protective immune response. The present study was designed to characterize T-cell immune responses after an intraspleen (i.s.) vaccination of BALB/c mice with pcDNA-SOD. Animals vaccinated with pcDNA-SOD did not develop SOD-specific antibodies, at least until week 4 after immunization (the end of the experiment), and in vitro stimulation of their splenocytes with either recombinant Cu-Zn SOD or crude Brucella protein induced the secretion of gamma interferon (IFN-gamma), but not interleukin-4, and elicited the induction of cytotoxic-T-lymphocyte activity. Upon analyzing the SOD-specific T-cell responses, the pcDNA-SOD vaccination was found to be stimulating both CD4(+)- and CD8(+)-T-cell populations. However, only the CD4(+) population was able to produce IFN-gamma and only the CD8(+) population was able to induce cytotoxic activity. Nevertheless, although i.s. route vaccination induces a significant level of protection in BALB/c mice against challenge with the virulent B. abortus strain 2308, vaccination by the intramuscular route with a similar amount of plasmid DNA does not protect. Based on these results, we conclude that i.s. immunization with pcDNA-SOD vaccine efficiently induced a Th1 type of immune response and a protective response that could be related to IFN-gamma production and cytotoxic activity against infected cells by SOD-specific CD4(+) and CD8(+) T cells, respectively.

Evaluation of Brucella abortus phosphoglucomutase (pgm) mutant as a new live rough-phenotype vaccine

Brucella abortus S19 is the vaccine most frequently used against bovine brucellosis. Although it induces good protection levels, it cannot be administered to pregnant cattle, revaccination is not advised due to interference in the discrimination between infected and vaccinated animals during immune-screening procedures, and the vaccine is virulent for humans. Due to these reasons, there is a continuous search for new bovine vaccine candidates that may confer protection levels comparable to those conferred by S19 but without its disadvantages. A previous study characterized the phenotype associated with the phosphoglucomutase (pgm) gene disruption in Brucella abortus S2308, as well as the possible role for the smooth lipopolysaccharide (LPS) in virulence and intracellular multiplication in HeLa cells (J. E. Ugalde, C. Czibener, M. F. Feldman, and R. A. Ugalde, Infect. Immun. 68:5716-5723, 2000). In this report, we analyze the protection, proliferative response, and cytokine production induced in BALB/c mice by a deltapgm deletion strain. We show that this strain synthesizes O antigen with a size of approximately 45 kDa but is rough. This is due to the fact that the deltapgm strain is unable to assemble the O side chain in the complete LPS. Vaccination with the deltapgm strain induced protection levels comparable to those induced by S19 and generated a proliferative splenocyte response and a cytokine profile typical of a Th1 response. On the other hand, we were unable to detect a specific anti-O-antigen antibody response by using the fluorescence polarization assay. In view of these results, the possibility that the deltapgm mutant could be used as a vaccination strain is discussed.

A DNA vaccine encoding Cu,Zn superoxide dismutase of Brucella abortus induces protective immunity in BALB/c mice

This study was conducted to evaluate the immunogenicity and protective efficacy of a DNA vaccine encoding Brucella abortus Cu,Zn superoxide dismutase (SOD). Intramuscular injection of plasmid DNA carrying the SOD gene (pcDNA-SOD) into BALB/c mice elicited both humoral and cellular immune responses. Animals injected with pcDNA-SOD developed SOD-specific antibodies which exhibited a dominance of immunoglobulin G2a (IgG2a) over IgG1. In addition, the DNA vaccine elicited a T-cell-proliferative response and also induced the production of gamma interferon, but not interleukin-10 (IL-10) or IL-4, upon restimulation with either recombinant SOD or crude Brucella protein, suggesting the induction of a typical T-helper-1-dominated immune response in mice. The pcDNA-SOD (but not the control vector) induced a strong, significant level of protection in BALB/c mice against challenge with B. abortus virulent strain 2308; the level of protection was similar to the one induced by B. abortus vaccine strain RB51. Altogether, these data suggest that pcDNA-SOD is a good candidate for use in future studies of vaccination against brucellosis.

Functional expression and characterization of EryA, the erythritol kinase of Brucella abortus, and enzymatic synthesis of L-erythritol-4-phosphate

The eryA gene of the bacterial pathogen Brucella abortus has been functionally expressed in Escherichia coli. The resultant EryA was shown to catalyze the ATP-dependent conversion of erythritol to L-erythritol-4-phosphate (L-E4P). The steady state kinetic parameters of this reaction were determined and the enzyme was used to prepare L-E4P which was shown to be a weak inhibitor of 2-C-methyl-D-erythritol-4-phosphate cytidyltransferase (YgbP).

Molecular and immunological characterisation of recombinant Brucella abortus glyceraldehyde-3-phosphate-dehydrogenase, a T- and B-cell reactive protein that induces partial protection when co-administered with an interleukin-12-expressing plasmid in a DNA vaccine formulation

To identify antigens of Brucella spp. that are potentially involved in stimulating a protective T-cell-mediated immune response, previous studies identified 10 clones from a Brucella abortus 2308 genomic library with primed lymphocytes as probes. One selected positive clone (182) contained an insert of 1.2 kb which was identified, sequenced and characterised. The deduced amino acid sequence of the open reading frame (ORF) revealed 82% and 81% identity to the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) enzymes from Agrobacterium tumefaciens and Xanthobacter flavus, respectively. Southern blot analysis demonstrated that the gap gene is present in only one copy in the Brucella genome. B. abortus GAPDH was then expressed in Escherichia coli as a fusion protein with the maltose-binding protein (MBP). To demonstrate the functional activity of Brucella GAPDH, E. coli gap mutants were transformed with a Brucella pMAL-gap construct. Genetic complementation was achieved and as a result E. coli mutants were able to grow on glucose or other carbon source medium. The humoral and cellular immune responses to the recombinant (r) GAPDH were characterised. In Western blots, sera from naturally infected cattle and sheep showed antibody reactivity against rGAPDH. In response to in-vitro stimulation by rGAPDH, splenocytes from mice vaccinated with rGAPDH or B. abortus S19 were able to produce gamma-interferon and tumour necrosis factor-a but not interleukin (IL)-4. Furthermore, gap associated with murine IL-12 gene in a DNA vaccine formulation partially protected mice against experimental infection.

A DNA vaccine encoding lumazine synthase from Brucella abortus induces protective immunity in BALB/c mice

This study was conducted to evaluate the immunogenicity of the Brucella abortus lumazine synthase (BLS) gene cloned into the pcDNA3 plasmid, which is driven by the cytomegalovirus promoter. Injection of plasmid DNA carrying the BLS gene (pcDNA-BLS) into BALB/c mice elicited both humoral and cellular immune responses. Antibodies to the encoded BLS included immunoglobulin G1 (IgG1) IgG2a, IgG2b, IgG3, and IgM isotypes. Animals injected with pcDNA-BLS exhibited a dominance of IgG2a over IgG1. In addition, spleen cells from vaccinated animals produced interleukin-2 and gamma interferon but not IL-10 or IL-4 after in vitro stimulation with recombinant BLS (rBLS), suggesting the induction of a Th1 response. Protection was evaluated by comparing the levels of infection in the spleens of vaccinated mice challenged with B. abortus 544. Immunization with pcDNA-BLS- reduced the bacterial burden relative to those in the control groups. Mice immunized with rBLS produced a significant humoral response but did not show a specific cellular response or any protection from challenge. Altogether, these data suggest that pcDNA-BLS is a good immunogen for the production of humoral and cell-mediated responses in mice and is a candidate for use in future studies of vaccination against brucellosis.

GTPases of the Rho subfamily are required for Brucella abortus internalization in nonprofessional phagocytes: direct activation of Cdc42

Members of the genus Brucella are intracellular alpha-Proteobacteria responsible for brucellosis, a chronic disease of humans and animals. Little is known about Brucella virulence mechanisms, but the abilities of these bacteria to invade and to survive within cells are decisive factors for causing disease. Transmission electron and fluorescence microscopy of infected nonprofessional phagocytic HeLa cells revealed minor membrane changes accompanied by discrete recruitment of F-actin at the site of Brucella abortus entry. Cell uptake of B. abortus was negatively affected to various degrees by actin, actin-myosin, and microtubule chemical inhibitors. Modulators of MAPKs and protein-tyrosine kinases hampered Brucella cell internalization. Inactivation of Rho small GTPases using clostridial toxins TcdB-10463, TcdB-1470, TcsL-1522, and TcdA significantly reduced the uptake of B. abortus by HeLa cells. In contrast, cytotoxic necrotizing factor from Escherichia coli, known to activate Rho, Rac, and Cdc42 small GTPases, increased the internalization of both virulent and non-virulent B. abortus. Expression of dominant-positive Rho, Rac, and Cdc42 forms in HeLa cells promoted the uptake of B. abortus, whereas expression of dominant-negative forms of these GTPases in HeLa cells hampered Brucella uptake. Cdc42 was activated upon cell contact by virulent B. abortus, but not by a noninvasive isogenic strain, as proven by affinity precipitation of active Rho, Rac, and Cdc42. The polyphasic approach used to discern the molecular events leading to Brucella internalization provides new alternatives for exploring the complexity of the signals required by intracellular pathogens for cell invasion.

Ferrochelatase is present in Brucella abortus and is critical for its intracellular survival and virulence

Brucella spp. are pathogenic bacteria that cause brucellosis, an animal disease which can also affect humans. Although understanding the pathogenesis is important for the health of animals and humans, little is known about virulence factors associated with it. In order for chronic disease to be established, Brucella spp. have developed the ability to survive inside phagocytes by evading cell defenses. It hides inside vacuoles, where it then replicates, indicating that it has an active metabolism. The purpose of this work was to obtain better insight into the intracellular metabolism of Brucella abortus. During a B. abortus genomic sequencing project, a clone coding a putative gene homologous to hemH was identified and sequenced. The amino acid sequence revealed high homology to members of the ferrochelatase family. A knockout mutant displayed auxotrophy for hemin, defective intracellular survival inside J774 and HeLa cells, and lack of virulence in BALB/c mice. This phenotype was overcome by complementing the mutant strain with a plasmid harboring wild-type hemH. These data demonstrate that B. abortus synthesizes its own heme and also has the ability to use an external source of heme; however, inside cells, there is not enough available heme to support its intracellular metabolism. It is concluded that ferrochelatase is essential for the multiplication and intracellular survival of B. abortus and thus for the establishment of chronic disease as well.

Identification and characterization of the Brucella abortus phosphoglucomutase gene: role of lipopolysaccharide in virulence and intracellular multiplication

Smooth lipopolysaccharide (LPS) of Brucella abortus has been reported to be an important virulence factor, although its precise role in pathogenesis is not yet clear. While the protective properties of LPS against complement are well accepted, there is still some controversy about the capacity of rough mutants to replicate intracellularly. The B. abortus phosphoglucomutase gene (pgm) was cloned, sequenced, and disrupted. The gene has a high index of identity to Agrobacterium tumefaciens pgm but is not part of the glycogen operon. A B. abortus null mutant lacks LPS O antigen but has an LPS core with an electrophoretic profile undistinguishable from that of the wild-type core, suggesting that glucose, galactose, or a derivative of these sugars may be part of the linkage between the core and the O antigen. This mutant is unable to survive in mice but replicates in HeLa cells, indicating that the complete LPS is not essential either for invasion or for intracellular multiplication. This behavior suggests that the LPS may play a role in extracellular survival in the animal, probably protecting the cell against complement-mediated lysis, but is not involved in intracellular survival.

Residual virulence of Brucella abortus in the absence of the cytochrome bc(1)complex in a murine model in vitro and in vivo

To maintain survival in macrophages, Brucella must overcome a hostile phagosomal environment defined as low pH, limited nutrition and low oxygen tension. The specific mechanisms utilized by Brucella to surmount such unfavorable environmental factors in phagosomes are not well understood. In general, to adapt to a change in environmental oxygen tension, bacteria use different terminal oxidases that have different oxygen affinity. To survive in phagosomes where low oxygen tension exists, Brucella, like other bacteria, may require high oxygen affinity terminal oxidases that can accept electrons through a cytochrome bc(1)complex dependent or independent pathway. Using a Brucella abortus cytochrome bc(1)complex deficient mutant, delta fbcF, the requirement for a high oxygen affinity terminal oxidase governed by the cytochrome bc(1)complex dependent pathway was tested. The number of cfu from RAW 264.7 macrophage cells and spleens of BALB/c mice infected with wild-type or the cytochrome bc(1)complex deficient mutant was similar during the course of infection. These results suggest that B. abortus contains no essential terminal oxidase utilized at low oxygen tension in phagosomes requiring the cytochrome bc(1)complex. Alternatively, other branched cytochrome bc(1)complex independent respiratory mechanisms that contain the high oxygen affinity terminal oxidases likely exist to facilitate Brucella survival in phagosomes. This is the first investigation regarding the Brucella respiratory system at the molecular level and the involvement of a respiratory system in Brucella pathogenesis.

Regulation of Brucella abortus catalase

All aerobic organisms have mechanisms that protect against oxidative compounds. Catalase, peroxidase, superoxide dismutase, glutathione, and thioredoxin are widely distributed in many taxa and constitute elements of a nearly ubiquitous antioxidant metabolic strategy. Interestingly, the regulatory mechanisms that control these elements are rather different depending on the nature of the oxidative stress and the organism. Catalase is well documented to play an important role in protecting cells from oxidative stress. In particular, pathogenic bacteria seem to use this enzyme as a defensive tool against attack by the host. To investigate the significance of catalase in hostile environments, we made catalase deletion mutations in two different B. abortus strains and used two-dimensional gel analysis, survival tests, and adaptation experiments to explore the behavior and role of catalase under several oxidative stress conditions. These studies show that B. abortus strains that do not express catalase activity exhibit increased sensitivity to hydrogen peroxide. We also demonstrate that catalase expression is regulated in this species, and that preexposure to a sublethal concentration of hydrogen peroxide allows B. abortus to adapt so as to survive subsequent exposure to higher concentrations of hydrogen peroxide.

Structural, functional and immunological studies on a polymeric bacterial protein

The characterization of proteins from Brucella spp, the causative agent of brucellosis, has been the subject of intensive research. We have described an 18-kDa cytoplasmic protein of Brucella abortus and shown the potential usefulness of this protein as an antigen for the serologic diagnosis of brucellosis. The amino acid sequence of the protein showed a low but significant homology with that of lumazine synthases. Lumazine is an intermediate product in bacterial riboflavin biosynthesis. The recombinant form of the 18-kDa protein (expressed in E. coli) folds like the native Brucella protein and has lumazine-synthase enzymatic activity. Three-dimensional analysis by X-ray crystallography of the homolog Bacillus subtilis lumazine synthase has revealed that the enzyme forms an icosahedral capsid. Recombinant lumazine synthase from B. abortus was crystallized, diffracted X rays to 2.7-A resolution at room temperature, and the structure successfully solved by molecular replacement procedures. The macromolecular assembly of the enzyme differs from that of the enzyme from B. subtilis. The Brucella enzyme remains pentameric (90 kDa) in its crystallographic form. Nonetheless, the active sites of the two enzymes are virtually identical at the structural level, indicating that inhibitors of these enzymes could be viable pharmaceuticals across a broad species range. We describe the structural reasons for the differences in their quaternary arrangement and also discuss the potential use of this protein as a target for the development of acellular vaccines.

The Brucella abortus CcrM DNA methyltransferase is essential for viability, and its overexpression attenuates intracellular replication in murine macrophages

The CcrM DNA methyltransferase of the alpha-proteobacteria catalyzes the methylation of the adenine in the sequence GAnTC. Like Dam in the enterobacteria, CcrM plays a regulatory role in Caulobacter crescentus and Rhizobium meliloti. CcrM is essential for viability in both of these organisms, and we show here that it is also essential in Brucella abortus. Further, increased copy number of the ccrM gene results in striking changes in B. abortus morphology, DNA replication, and growth in murine macrophages. We generated strains that carry ccrM either on a low-copy-number plasmid (strain GR131) or on a moderate-copy-number plasmid (strain GR132). Strain GR131 has wild-type morphology and chromosome number, as assessed by flow cytometry. In contrast, strain GR132 has abnormal branched morphology, suggesting aberrant cell division, and increased chromosome number. Although these strains exhibit different morphologies and DNA content, the replication of both strains in macrophages is attenuated. These data imply that the reduction in survival in host cells is not due solely to a cell division defect but is due to additional functions of CcrM. Because CcrM is essential in B. abortus and increased ccrM copy number attenuates survival in host cells, we propose that CcrM is an appropriate target for new antibiotics.

Divergence in macromolecular assembly: X-ray crystallographic structure analysis of lumazine synthase from Brucella abortus

We have determined the three-dimensional structure of 6, 7-dimethyl-8-ribityllumazine synthase (lumazine synthase) from Brucella abortus, the infectious organism of the disease brucellosis in animals. This enzyme catalyses the formation of 6, 7-dimethyl-8-ribityllumazine, the penultimate product in the synthesis of riboflavin. The three-dimensional X-ray crystal structure of the enzyme from B. abortus has been solved and refined at 2.7 A resolution to a final R-value of 0.18 (R(free)=0.23). The macromolecular assembly of the enzyme differs from that of the enzyme from Bacillus subtilis, the only other lumazine synthase structure known. While the protein from B. subtilis assembles into a 60 subunit icosahedral capsid built from 12 pentameric units, the enzyme from B. abortus is pentameric in its crystalline form. Nonetheless, the active sites of the two enzymes are virtually identical indicating inhibitors to theses enzymes could be effective pharmaceuticals across a broad species range. Furthermore, we compare the structures of the enzyme from B. subtilis and B. abortus and describe the C teminus structure which accounts for the differences in quaternary structure.

The Brucella abortus Lon functions as a generalized stress response protease and is required for wild-type virulence in BALB/c mice

The gene encoding a Lon protease homologue has been cloned from Brucella abortus. The putative Brucella abortus Lon shares > 60% amino acid identity with its Escherichia coli counterpart and the recombinant form of this protein restores the capacity of an Escherichia coli lon mutant to resist killing by ultraviolet irradiation and regulate the expression of a cpsB:lacZ fusion to wild-type levels. A sigma32 type promoter was identified upstream of the predicted lon coding region and Northern analysis revealed that transcription of the native Brucella abortus lon increases in response to heat shock and other environmental stresses. ATP-dependent proteolytic activity was also demonstrated for purified recombinant Lon. To evaluate the capacity of the Brucella abortus Lon homologue to function as a stress response protease, the majority of the lon coding region was removed from virulent strain Brucella abortus 2308 via allelic exchange. In contrast to the parent strain, the Brucella abortus lon mutant, designated GR106, was impaired in its capacity to form isolated colonies on solid medium at 41 degrees C and displayed an increased sensitivity to killing by puromycin and H2O2. GR106 also displayed reduced survival in cultured murine macrophages and significant attenuation in BALB/c mice at 1 week post infection compared with the virulent parental strain. Beginning at 2 weeks and continuing for 6 weeks post infection, however, GR106 and 2308 displayed equivalent spleen and liver colonization levels in mice. These findings suggest that the Brucella abortus Lon homologue functions as a stress response protease that is required for wild-type virulence during the initial stages of infection in the mouse model, but is not essential for the establishment and maintenance of chronic infection in this host.

Identification of an IS711 element interrupting the wboA gene of Brucella abortus vaccine strain RB51 and a PCR assay to distinguish strain RB51 from other Brucella species and strains

Brucella abortus vaccine strain RB51 is a natural stable attenuated rough mutant derived from the virulent strain 2308. The genetic mutations that are responsible for the roughness and the attenuation of strain RB51 have not been identified until now. Also, except for an assay based on pulsed-field gel electrophoresis, no other simple method to differentiate strain RB51 from its parent strain 2308 is available. In the present study, we demonstrate that the wboA gene encoding a glycosyltransferase, an enzyme essential for the synthesis of O antigen, is disrupted by an IS711 element in B. abortus vaccine strain RB51. Exploiting this feature, we developed a PCR assay that distinguishes strain RB51 from all other Brucella species and strains tested.

Vaccination with live Escherichia coli expressing Brucella abortus Cu/Zn superoxide dismutase protects mice against virulent B. abortus

Vaccination of mice with Escherichia coli expressing Brucella Cu/Zn superoxide dismutase (SOD) [E. coli(pBSSOD)] induced a significant level of protection against virulent Brucella abortus challenge, although this level was not as high as the one reached with B. abortus vaccine strain RB51. In addition, vaccination with E. coli(pBSSOD) induced antibodies to Cu/Zn SOD and a strong proliferative response of splenocytes when stimulated in vitro with a thioredoxin-Cu/Zn SOD fusion protein.

Crystallization and preliminary x-ray diffraction analysis of the lumazine synthase from Brucella abortus

Lumazine synthase from Brucella abortus was overexpressed in Escherichia coli, refolded, and purified to apparent homogeneity. Crystals of lumazine synthase were grown by the hanging drop vapor diffusion method using polyethylene glycol 8000 or ammonium sulfate as precipitants. They belong to the trigonal space group P321 with cell parameters a = b = 132.00A, c = 167.25 A. A complete diffraction data set to 3.7 A resolution has been collected using synchrotron radiation. Preliminary analysis of the quaternary structure of this protein by means of a self-rotation function calculated with the diffraction data clearly indicates 532 symmetry compatible with the presence of an icosahedral lumazine synthase particle.

Molecular cloning and characterization of cgs, the Brucella abortus cyclic beta(1-2) glucan synthetase gene: genetic complementation of Rhizobium meliloti ndvB and Agrobacterium tumefaciens chvB mutants

The animal pathogen Brucella abortus contains a gene, cgs, that complemented a Rhizobium meliloti nodule development (ndvB) mutant and an Agrobacterium tumefaciens chromosomal virulence (chvB) mutant. The complemented strains recovered the synthesis of cyclic beta(1-2) glucan, motility, virulence in A. tumefaciens, and nitrogen fixation in R. meliloti; all traits were strictly associated with the presence of an active cyclic beta(1-2) glucan synthetase protein in the membranes. Nucleotide sequencing revealed the presence in B. abortus of an 8.49-kb open reading frame coding for a predicted membrane protein of 2,831 amino acids (316.2 kDa) and with 51% identity to R. meliloti NdvB. Four regions of the B. abortus protein spanning amino acids 520 to 800, 1025 to 1124, 1284 to 1526, and 2400 to 2660 displayed similarities of higher than 80% with R. meliloti NdvB. Tn3-HoHo1 mutagenesis showed that the C-terminal 825 amino acids of the Brucella protein, although highly conserved in Rhizobium, are not necessary for cyclic beta(1-2) glucan synthesis. Confirmation of the identity of this protein as B. abortus cyclic beta(1-2) glucan synthetase was done by the construction of a B. abortus Tn3-HoHo1 insertion mutant that does not form cyclic beta(1-2) glucan and lacks the 316.2-kDa membrane protein. The recovery of this mutant from the spleens of inoculated mice was decreased by 3 orders of magnitude compared with that of the parental strain; this result suggests that cyclic beta(1-2) glucan may be a virulence factor in Brucella infection.

Isolation and characterization of the UDP-glucose 4'-epimerase-encoding gene, galE, from Brucella abortus 2308

The UDP-glucose 4'-epimerase-encoding gene, galE or exoB, was isolated from Brucella abortus 2308 by complementation of an exoB mutant of Sinorhizobium meliloti. Confirmation of the identity was done by constructing an in-frame deletion of 660 bp with galE of the B. abortus genome by marker exchange. The resulting galE mutant lacked UDP-glucose 4'-epimerase activity. This activity was restored by in trans complementation with the intact gene. The B. abortus gal E mutant is not altered in colony morphology compared to wt 2308. The lack of UDP-glucose 4'-epimerase activity in the mutant and PCR analysis strongly suggest that only one copy of galE exists in B. abortus 2308. The galE sequence of B. abortus 2308 is more similar to galE from other animal-inhabiting bacteria than it is to exoB from the Sinorhizobium legume symbionts. We propose that galE in B. abortus evolved by lateral transfer from other animal-inhabiting bacteria rather than from a common ancestor of Brucella and Sinorhizobium.

Brucella abortus arginase and ornithine cyclodeaminase genes are similar to Ti plasmid arginase and ornithine cyclodeaminase

Brucella abortus arginase and ornithine cyclodeaminase genes have been cloned and sequenced. These gene sequences are located in the same operon and occur in the same order as the homologous genes in Agrobacterium tumefaciens Ti C58 plasmid. The nucleotide sequences of the two genes have 72% and 65% identity to the respective Ti plasmid genes. Both genes are present in a single copy, and expression of arginase is regulated in response to arginine.

The HtrA stress response protease contributes to resistance of Brucella abortus to killing by murine phagocytes

Compared with virulent Brucella abortus 2308, the isogenic htrA mutant PHE1 shows decreased resistance to killing by cultured murine neutrophils and macrophages and significant attenuation during the early stages of infection in the BALB/c mouse model. These findings further define the contributions of the htrA gene product to the pathogenesis of B. abortus infections.

Cloning and characterization of the glucokinase gene of Brucella abortus 19 and identification of three other genes

A clone from Brucella abortus 19 complemented an Escherichia coli strain deficient in phosphorylation of glucose. Open reading frames similar to E. coli mepA, glk, and genes encoding ATP-coupled exporters were found in the sequence. A fourth affected growth on minimal media of the ptsI glk strain with various carbon sources.

Structural characterization of the active site of Brucella abortus Cu-Zn superoxide dismutase: a 15N and 1H NMR investigation

Prokaryotic Cu-Zn superoxide dismutases (SODs) are rare and poorly characterized compared to their eukaryotic counterparts. To better characterize the structure of the prokaryotic enzyme, an NMR investigation of Brucella abortus Cu-Zn SOD in the reduced form was undertaken. The enzyme studied was a recombinant form, expressed in Escherichia coli. The enzyme initially lacked a full complement of Cu and Zn ion. After demetallation and remetallation with a stoichiometric amount of Cu and Zn ion, the specific activity of the recombinant B. abortus Cu-Zn SOD was comparable to the specific activity of the bovine enzyme. The 15N and 1H resonances of seven active site histidine imidazole rings were assigned using two-dimensional NMR methods. A self-consistent set of nuclear Overhauser effects between imidazole ring protons was observed, which was in agreement with the predictions of a model based on the X-ray crystallographic structure of the oxidized bovine enzyme (Tainer, J.A., Getzoff, E. D., Beem, K. M., Richardson, J.S., & Richardson, D.C. (1982) J. Mol. Biol. 160, 181-217). These observations strongly suggest that the structure of the active site of the prokaryotic enzyme is similar to that of the eukaryotic enzyme. Differences in the observed and predicted nuclear Overhauser effects could be ascribed to differences in the oxidation state of the Cu ion (Cu(I) in the reduced B. abortus enzyme and Cu(II) in the oxidized bovine enzyme), as much as they could to the different origins of the enzymes. The NMR data were also compared to a similar 1H NMR study of the human enzyme (Bertini, I., Capozzi, F., Luchinat, C., Piccioli, M., & Viezzoli, M. S. (1991) Eur. J. Biochem. 197, 691-697). The pattern of nuclear Overhauser effects and the chemical shifts of corresponding resonances were very similar in 1H NMR spectra of the human and B. abortus enzymes. Significant differences in the chemical shifts or exchange behavior of a few resonances indicated differences in the environments of several histidines in the active sites of reduced B. abortus and human Cu-Zn SODs. This is consistent with the presence of a number of insertions and deletions in the loop regions that make up the active site as indicated by amino acid sequence alignment studies. The tautomeric and protonation states of the active site histidines were also determined in this study, and the results were in agreement with previous studies. The resonances of nitrogen atoms coordinated to metal ions were found to fall between those of protonated and unprotonated nitrogens on histidine imidazoles.

Brucella abortus catalase is a periplasmic protein lacking a standard signal sequence

A periplasmic catalase has been purified and cloned from Brucella abortus. The functional enzyme is a tetramer with a subunit molecular weight of 55,000. All evidence indicates that a typical N-terminal signal sequence is not associated with the export of this protein to the periplasm.