Type IV secretion systems (T4SS) are multiprotein structures that direct the translocation of specific molecules across the bacterial cell envelope. As in other bacteria, pathogenicity of the genus Brucella essentially depends on the integrity of the T4SS-encoding virB operon, whose expression is regulated by multiple transcription factors belonging to different families. Previously, we identified IHF and HutC, two direct regulators of the virB genes that were isolated from total protein extracts of Brucella. Here, we report the identification of MdrA, a third regulatory element that was isolated using the same screening procedure. This transcription factor, which belongs to the MarR-family of transcriptional regulators, binds at two different sites of the virB promoter and regulates expression in a growth phase-dependent manner. Like other members of the MarR family, specific ligands were able to dissociate MdrA from DNA in vitro. Determination of the MdrA-binding sites by DNase I footprinting and analyses of protein-DNA complexes by electrophoresis mobility shift assays (EMSAs) showed that MdrA competes with IHF and HutC for the binding to the promoter because their target DNA sequences overlap. Unlike IHF, both MdrA and HutC bound to the promoter without inducing bending of DNA. Moreover, the two latter transcription factors activated virB expression to similar extents, and in doing so, they are functionally redundant. Taken together, our results show that MdrA is a regulatory element that directly modulates the activity of the virB promoter and is probably involved in coordinating gene expression in response to specific environmental signals.
Ochrobactrum anthropi is a common soil alphaproteobacterium that colonizes a wide spectrum of organisms and is being increasingly recognized as an opportunistic human pathogen. Potentially life-threatening infections, such as endocarditis, are included in the list of reported O. anthropi infections. These reports, together with the scant number of studies and the organism's phylogenetic proximity to the highly pathogenic brucellae, make O. anthropi an attractive model of bacterial pathogenicity. Here we report the genome sequence of the type strain O. anthropi ATCC 49188, which revealed the presence of two chromosomes and four plasmids.
VjbR is a LuxR-type quorum-sensing (QS) regulator that plays an essential role in the virulence of the intracellular facultative pathogen Brucella, the causative agent of brucellosis. It was previously described that VjbR regulates a diverse group of genes, including the virB operon. The latter codes for a type IV secretion system (T4SS) that is central for the pathogenesis of Brucella. Although the regulatory role of VjbR on the virB promoter (PvirB) was extensively studied by different groups, the VjbR-binding site had not been identified so far. Here, we identified the target DNA sequence of VjbR in PvirB by DNase I footprinting analyses. Surprisingly, we observed that VjbR specifically recognizes a sequence that is identical to a half-binding site of the QS-related regulator MrtR of Mesorhizobium tianshanense. As shown by DNase I footprinting and electrophoretic mobility shift assays, generation of a palindromic MrtR-like-binding site in PvirB increased both the affinity and the stability of the VjbR-DNA complex, which confirmed that the QS regulator of Brucella is highly related to that of M. tianshanense. The addition of N-dodecanoyl homoserine lactone dissociated VjbR from the promoter, which confirmed previous reports that indicated a negative effect of this signal on the VjbR-mediated activation of PvirB. Our results provide new molecular evidence for the structure of the virB promoter and reveal unusual features of the QS target DNA sequence of the main regulator of virulence in Brucella.
Type IV secretion systems (T4SS) are multicomponent machineries involved in the translocation of effector molecules across the bacterial cell envelope. The virB operon of Brucella abortus codes for a T4SS that is essential for virulence and intracellular multiplication of the bacterium in the host. Previous studies showed that the virB operon of B. abortus is tightly regulated within the host cells. In order to identify factors implicated in the control of virB expression, we searched for proteins of Brucella that directly bind to the virB promoter (PvirB). Using different procedures, we isolated a 27-kDa protein that binds specifically to PvirB. This protein was identified as HutC, the transcriptional repressor of the histidine utilization (hut) genes. Analyses of virB and hut promoter activity revealed that HutC exerts two different roles: it acts as a coactivator of transcription of the virB operon, whereas it represses the hut genes. Such activities were observed both intracellularly and in bacteria incubated under conditions that resemble the intracellular environment. Electrophoresis mobility shift assays (EMSA) and DNase I footprinting experiments revealed the structure, affinity, and localization of the HutC-binding sites and supported the regulatory role of HutC in both hut and virB promoters. Taken together, these results indicate that Brucella coopted the function of HutC to coordinate the Hut pathway with transcriptional regulation of the virB genes, probably as a way to sense its own metabolic state and develop adaptive responses to overcome intracellular host defenses.
Brucellosis is a worldwide zoonosis that affects livestock and humans and is caused by closely related Brucella spp., which are adapted to intracellular life within cells of a large variety of mammals. Brucella can be considered a furtive pathogen that infects professional and non-professional phagocytes. In these cells Brucella survives in a replicative niche, which is characterized for having a very low oxygen tension and being deprived from nutrients such as amino acids and vitamins. Among these vitamins, we have focused on riboflavin (vitamin B2). Flavin metabolism has been barely implicated in bacterial virulence. We have recently described that Brucella and other Rhizobiales bear an atypical riboflavin metabolic pathway. In the present work we analyze the role of the flavin metabolism on Brucella virulence. Mutants on the two lumazine synthases (LS) isoenzymes RibH1 and RibH2 and a double RibH mutant were generated. These mutants and different complemented strains were tested for viability and virulence in cells and in mice. In this fashion we have established that at least one LS must be present for B. abortus survival and that RibH2 and not RibH1 is essential for intracellular survival due to its LS activity in vivo. In summary, we show that riboflavin biosynthesis is essential for Brucella survival inside cells or in mice. These results highlight the potential use of flavin biosynthetic pathway enzymes as targets for the chemotherapy of brucellosis.
Cyclic β-1,2-glucans (CβG) are periplasmic homopolysaccharides that have been shown to play an important role in several symbiotic and pathogenic relationships. Cyclic β-1,2-glucan synthase (Cgs), the enzyme responsible for the synthesis of CβG, is an integral membrane polyfunctional protein that catalyzes the four enzymatic activities (initiation, elongation, phosphorolysis, and cyclization) required for the synthesis of CβG. Recently, we have identified the glycosyltransferase and the β-1,2-glucooligosaccharide phosphorylase domains of Brucella abortus Cgs. In this study, we performed large-scale linker-scanning mutagenesis to gain further insight into the functional domains of Cgs. This analysis allowed us to construct a functional map of the enzyme and led to the identification of the minimal region required for the catalysis of initiation and elongation reactions. In addition, we identified the Cgs region (residues 991 to 1544) as being the protein domain required for cyclization and demonstrated that upon cyclization and releasing of the CβG, one or more glucose residues remain attached to the protein intermediate that serves as a primer for the next round of CβG synthesis. Finally, our results indicate that the overall control of the degree of polymerization of CβG is the result of a balance between elongation, phosphorolysis, and cyclization reactions.
The Brucella cell envelope contains the zwitterionic phospholipids phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Synthesis of PC occurs exclusively via the PC synthase pathway, implying that the pathogen depends on the choline synthesized by the host cell to form PC. Notably, PC is necessary to sustain a chronic infection process, which suggests that the membrane lipid content is relevant for Brucella virulence. In this study we investigated the first step of PE biosynthesis in B. abortus, which is catalyzed by phosphatidylserine synthase (PssA). Disruption of pssA abrogated the synthesis of PE without affecting the growth in rich complex medium. In minimal medium, however, the mutant required choline supplementation for growth, suggesting that at least PE or PC is necessary for Brucella viability. The absence of PE altered cell surface properties, but most importantly, it impaired several virulence traits of B. abortus, such as intracellular survival in both macrophages and HeLa cells, the maturation of the replicative Brucella-containing vacuole, and mouse colonization. These results suggest that membrane phospholipid composition is critical for the interaction of B. abortus with the host cell.
Campylobacter fetus subspecies venerealis is the causative agent of bovine genital campylobacteriosis, asymptomatic in bulls the disease is spread to female cattle causing extensive reproductive loss. The microbiological and molecular differentiation of C. fetus subsp. venerealis from C. fetus subsp. fetus is extremely difficult. This study describes the analysis of the available C. fetus subsp. venerealis AZUL-94 strain genome (~75–80%) to identify elements exclusively found in C. fetus subsp. venerealis strains as potential diagnostic targets and the characterisation of subspecies virulence genes.
Eighty Kb of genomic sequence (22 contigs) was identified as unique to C. fetus subsp. venerealis AZUL-94 and consisted of type IV secretory pathway components, putative plasmid genes and hypothetical proteins. Of the 9 PCR assays developed to target C. fetus subsp. venerealis type IV secretion system genes, 4 of these were specific for C. fetus subsp. venerealis biovar venerealis and did not detect C. fetus subsp. venerealis biovar intermedius. Two assays were specific for C. fetus subsp. venerealis AZUL-94 strain, with a further single assay specific for the AZUL-94 strain and C. fetus subsp. venerealis biovar intermedius (and not the remaining C. fetus subsp. venerealis biovar venerealis strains tested). C. fetus subsp. fetus and C. fetus subsp. venerealis were found to share most common Campylobacter virulence factors such as SAP, chemotaxis, flagellar biosynthesis, 2-component systems and cytolethal distending toxin subunits (A, B, C). We did not however, identify in C. fetus the full complement of bacterial adherence candidates commonly found in other Campylobacter spp.
The comparison of the available C. fetus subsp. venerealis genome sequence with the C. fetus subsp. fetus genome identified 80 kb of unique C. fetus subsp. venerealis AZUL94 sequence, with subsequent PCR confirmation demonstrating inconsistent amplification of these targets in all other C. fetus subsp. venerealis strains and biovars tested. The assays developed here highlight the complexity of targeting strain specific virulence genes for field studies for the molecular identification and epidemiology of C. fetus.
Brucella is an intracellular pathogen able to persist for long periods of time within the host and establish a chronic disease. We show that soon after Brucella inoculation in intestinal loops, dendritic cells from ileal Peyer's patches become infected and constitute a cell target for this pathogen. In vitro, we found that Brucella replicates within dendritic cells and hinders their functional activation. In addition, we identified a new Brucella protein Btp1, which down-modulates maturation of infected dendritic cells by interfering with the TLR2 signaling pathway. These results show that intracellular Brucella is able to control dendritic cell function, which may have important consequences in the development of chronic brucellosis.
A key determinant for intracellular pathogenic bacteria to induce infectious diseases is their ability to avoid recognition by the host immune system. Although most microorganisms internalized by host cells are efficiently cleared, Brucella behave as a Trojan horse causing a zoonosis called brucellosis that affects both humans and animals. Here we show that pathogenic Brucella are able to target host cell defense mechanisms by controlling the function of the sentinels of the immune system, the dendritic cells. In particular, the Brucella TIR-containing protein (Btp1) targets the Toll-like receptor 2 activation pathway, which is a major host response system involved in bacterial recognition. Btp1 is involved in the inhibition of dendritic cell maturation. The direct consequence is a control of inflammatory cytokine secretion and antigen presentation to T lymphocytes. These bacterial proteins are not specific for Brucella and have been identified in other pathogens and may be part of a general virulence mechanism used by several intracellular pathogens to induce disease.
Brucella periplasmic cyclic β-1,2-glucan plays an important role during bacterium-host interaction. Nuclear magnetic resonance spectrometry analysis, thin-layer chromatography, and DEAE-Sephadex chromatography were used to characterize Brucella abortus cyclic glucan. In the present study, we report that a fraction of B. abortus cyclic β-1,2-glucan is substituted with succinyl residues, which confer anionic character on the cyclic β-1,2-glucan. The oligosaccharide backbone is substituted at C-6 positions with an average of two succinyl residues per glucan molecule. This O-ester-linked succinyl residue is the only substituent of Brucella cyclic glucan. A B. abortus open reading frame (BAB1_1718) homologous to Rhodobacter sphaeroides glucan succinyltransferase (OpgC) was identified as the gene encoding the enzyme responsible for cyclic glucan modification. This gene was named cgm for cyclic glucan modifier and is highly conserved in Brucella melitensis and Brucella suis. Nucleotide sequencing revealed that B. abortus cgm consists of a 1,182-bp open reading frame coding for a predicted membrane protein of 393 amino acid residues (42.7 kDa) 39% identical to Rhodobacter sphaeroides succinyltransferase. cgm null mutants in B. abortus strains 2308 and S19 produced neutral glucans without succinyl residues, confirming the identity of this protein as the cyclic-glucan succinyltransferase enzyme. In this study, we demonstrate that succinyl substituents of cyclic β-1,2-glucan of B. abortus are necessary for hypo-osmotic adaptation. On the other hand, intracellular multiplication and mouse spleen colonization are not affected in cgm mutants, indicating that cyclic-β-1,2-glucan succinylation is not required for virulence and suggesting that no low-osmotic stress conditions must be overcome during infection.
The Brucella cell envelope is characterized by the presence of phosphatidylcholine (PC), a common phospholipid in eukaryotes that is rare in prokaryotes. Studies on the composition of Brucella abortus 2308 phospholipids revealed that the synthesis of PC depends on the presence of choline in the culture medium, suggesting that the methylation biosynthetic pathway is not functional. Phospholipid composition of pmtA and pcs mutants indicated that in Brucella, PC synthesis occurs exclusively via the phosphatidylcholine synthase pathway. Transformation of Escherichia coli with an expression vector containing the B. abortus pcs homologue was sufficient for PC synthesis upon induction with IPTG (isopropyl-β-d-thiogalactopyranoside), while no PC formation was detected when bacteria were transformed with a vector containing pmtA. These findings imply that Brucella depends on choline provided by the host cell to form PC. We could not detect any obvious associated phenotype in the PC-deficient strain under vegetative or intracellular growth conditions in macrophages. However, the pcs mutant strain displays a reproducible virulence defect in mice, which suggests that PC is necessary to sustain a chronic infection process.
Despite their high DNA identity and a proposal to group classical Brucella species as biovars of Brucella melitensis, the commonly recognized Brucella species can be distinguished by distinct biochemical and fatty acid characters, as well as by a marked host range (e.g., Brucella suis for swine, B. melitensis for sheep and goats, and Brucella abortus for cattle). Here we present the genome of B. abortus 2308, the virulent prototype biovar 1 strain, and its comparison to the two other human pathogenic Brucella species and to B. abortus field isolate 9-941. The global distribution of pseudogenes, deletions, and insertions supports previous indications that B. abortus and B. melitensis share a common ancestor that diverged from B. suis. With the exception of a dozen genes, the genetic complements of both B. abortus strains are identical, whereas the three species differ in gene content and pseudogenes. The pattern of species-specific gene inactivations affecting transcriptional regulators and outer membrane proteins suggests that these inactivations may play an important role in the establishment of host specificity and may have been a primary driver of speciation in the genus Brucella. Despite being nonmotile, the brucellae contain flagellum gene clusters and display species-specific flagellar gene inactivations, which lead to the putative generation of different versions of flagellum-derived structures and may contribute to differences in host specificity and virulence. Metabolic changes such as the lack of complete metabolic pathways for the synthesis of numerous compounds (e.g., glycogen, biotin, NAD, and choline) are consistent with adaptation of brucellae to an intracellular life-style.
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 β-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 β-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.
The animal pathogen Brucella abortus contains a gene cgt, which complemented Sinorhizobium meliloti nodule development (ndvA) and Agrobacterium tumefaciens chromosomal virulence (chvA) mutants. Complemented strains recovered the presence of anionic cyclic β-1,2-glucan, motility, tumor induction in A. tumefaciens, and nodule occupancy in S. meliloti, all traits strictly associated with the presence of cyclic β-1,2-glucan in the periplasm. Nucleotide sequencing revealed that B. abortus cgt contains a 1,797-bp open reading frame coding for a predicted membrane protein of 599 amino acids (65.9 kDa) that is 58.5 and 59.9% identical to S. meliloti NdvA and A. tumefaciens ChvA, respectively. Additionally, B. abortus cgt, like S. meliloti ndvA and A. tumefaciens chvA possesses ATP-binding motifs and the ABC signature domain features of a typical ABC transporter. Characterization of Cgt was carried out by the construction of null mutants in B. abortus 2308 and S19 backgrounds. Both mutants do not transport cyclic β-1,2-glucan to the periplasm, as shown by the absence of anionic cyclic glucan, and they display reduced virulence in mice and defective intracellular multiplication in HeLa cells. These results suggest that cyclic β-1,2-glucan must be transported into the periplasmatic space to exert its action as a virulence factor.
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 Δpgm 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 Δpgm strain is unable to assemble the O side chain in the complete LPS. Vaccination with the Δpgm 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 Δpgm mutant could be used as a vaccination strain is discussed.
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.
Null cyclic β-1,2-glucan synthetase mutants (cgs mutants) were obtained from Brucella abortus virulent strain 2308 and from B. abortus attenuated vaccinal strain S19. Both mutants show greater sensitivity to surfactants like deoxycholic acid, sodium dodecyl sulfate, and Zwittergent than the parental strains, suggesting cell surface alterations. Although not to the same extent, both mutants display reduced virulence in mice and defective intracellular multiplication in HeLa cells. The B. abortus S19 cgs mutant was completely cleared from the spleens of mice after 4 weeks, while the 2308 mutant showed a 1.5-log reduction of the number of brucellae isolated from the spleens after 12 weeks. These results suggest that cyclic β-1,2-glucan plays an important role in the residual virulence of the attenuated B. abortus S19 strain. Although the cgs mutant was cleared from the spleens earlier than the wild-type parental strain (B. abortus S19) and produced less inflammatory response, its ability to confer protection against the virulent strain B. abortus 2308 was fully retained. Equivalent levels of induction of spleen gamma interferon mRNA and anti-lipopolysaccharide (LPS) of immunoglobulin G2a (IgG2a) subtype antibodies were observed in mice injected with B. abortus S19 or the cgs mutant. However, the titer of anti-LPS antibodies of the IgG1 subtype induced by the cgs mutant was lower than that observed with the parental S19 strain, thus suggesting that the cgs mutant induces a relatively exclusive Th1 response.
Brucella abortus is the etiological agent of brucellosis, a disease that affects bovines and human. We generated DNA random sequences from the genome of B. abortus strain 2308 in order to characterize molecular targets that might be useful for developing immunological or chemotherapeutic strategies against this pathogen. The partial sequencing of 1,899 clones allowed the identification of 1,199 genomic sequence surveys (GSSs) with high homology (BLAST expect value < 10−5) to sequences deposited in the GenBank databases. Among them, 925 represent putative novel genes for the Brucella genus. Out of 925 nonredundant GSSs, 470 were classified in 15 categories based on cellular function. Seven hundred GSSs showed no significant database matches and remain available for further studies in order to identify their function. A high number of GSSs with homology to Agrobacterium tumefaciens and Rhizobium meliloti proteins were observed, thus confirming their close phylogenetic relationship. Among them, several GSSs showed high similarity with genes related to nodule nitrogen fixation, synthesis of nod factors, nodulation protein symbiotic plasmid, and nodule bacteroid differentiation. We have also identified several B. abortus homologs of virulence and pathogenesis genes from other pathogens, including a homolog to both the Shda gene from Salmonella enterica serovar Typhimurium and the AidA-1 gene from Escherichia coli. Other GSSs displayed significant homologies to genes encoding components of the type III and type IV secretion machineries, suggesting that Brucella might also have an active type III secretion machinery.
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.
As part of a Brucella abortus 2308 genome project carried out in our laboratory, we identified, cloned, and sequenced a genomic DNA fragment containing a locus (virB) highly homologous to bacterial type IV secretion systems. The B. abortus virB locus is a collinear arrangement of 13 open reading frames (ORFs). Between virB1 and virB2 and downstream of ORF12, two degenerated, palindromic repeat sequences characteristic of Brucella intergenic regions were found. Gene reporter studies demonstrated that the B. abortus virB locus constitutes an operon transcribed from virB1 which is turned on during the stationary phase of growth. A B. abortus polar virB1 mutant failed to replicate in HeLa cells, indicating that the virB operon plays a critical role in intracellular multiplication. Mutants with polar and nonpolar mutations introduced in virB10 showed different behaviors in mice and in the HeLa cell infection assay, suggesting that virB10 per se is necessary for the correct function of this type IV secretion apparatus. Mouse infection assays demonstrated that the virB operon constitutes a major determinant of B. abortus virulence. It is suggested that putative effector molecules secreted by this type IV secretion system determine routing of B. abortus to an endoplasmic reticulum-related replication compartment.
The gene organization and transcription of the Agrobacterium glg operon differ from those in other bacteria. Agrobacterium tumefaciens A348 contains a 9.1-kb gene cluster harboring genes for glycogen metabolism. The nucleotide sequence and gene organization of a region containing ADP-glucose pyrophosphorylase (glgC), glycogen synthetase (glgA), and phosphoglucomutase (pgm) genes have been previously described (A. Uttaro and R. A. Ugalde, Gene 150:117–122, 1994). In this work we report that the glycogen phosphorylase (glgP) and branching enzyme (glgB) genes are located immediately upstream of this region. The complete nucleotide sequences of the glgP and glgB genes were obtained, and mutants were constructed by targeted insertional mutagenesis with a kanamycin cassette. Enzymatic assays and reverse transcription PCR carried out with the wild type and with glgP and glgB mutants, as well as primer extension experiments and β-galactosidase fusions, revealed that this region containing five open reading frames (glgPBCA and pgm) is transcribed unidirectionally as a single operon under the control of a promoter located upstream of the glycogen phosphorylase gene (glgP). An alternative transcript was identified starting 168 bp upstream of an internal ATG start codon of the pgm gene, which is translated as a 71-amino-acid-shorter Pgm protein which complements in vivo a pgm mutant. This alternative transcript has a promoter with the motif TATCAAN5G, identified in octopine Ti plasmid as an autoinducible TraR promoter. This promoter is >200 times more efficient in A. tumefaciens than in Escherichia coli, as judged by the level of enzymatic activity of a lacZ-pgm fusion.
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 β(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 β(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 β(1-2) glucan synthesis. Confirmation of the identity of this protein as B. abortus cyclic β(1-2) glucan synthetase was done by the construction of a B. abortus Tn3-HoHo1 insertion mutant that does not form cyclic β(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 β(1-2) glucan may be a virulence factor in Brucella infection.
A vector for the expression of foreign antigens in the vaccine strain Brucella abortus S19 was developed by using a DNA fragment containing the regulatory sequences and the signal peptide of the Brucella bcsp31 gene. This fragment was cloned in broad-host-range plasmid pBBR4MCS, resulting in plasmid pBEV. As a reporter protein, a repetitive antigen of Trypanosoma cruzi was used. The recombinant fusion protein is stably expressed and secreted into the Brucella periplasmic space, inducing a good antibody response against the T. cruzi antigen. The expression of the repetitive antigen in Brucella neither altered its growth pattern nor generated a toxic or lethal effect during experimental infection. The application of this strategy for the generation of live recombinant vaccines and the tagging of B. abortus S19 vaccine is discussed. This is the first time that a recombinant protein has been expressed in the periplasm of brucellae.