Proteins with bacterial immunoglobulin-like (Big) domains, such as the Yersinia pseudotuberculosis invasin and Escherichia coli intimin, are surface-expressed proteins that mediate host mammalian cell invasion or attachment. Here, we report the identification and characterization of a new family of Big domain proteins, referred to as Lig (leptospiral Ig-like) proteins, in pathogenic Leptospira. Screening of L. interrogans and L. kirschneri expression libraries with sera from leptospirosis patients identified 13 lambda phage clones that encode tandem repeats of the 90 amino acid Big domain. Two lig genes, designated ligA and ligB, and one pseudo-gene, ligC, were identified. The ligA and ligB genes encode amino-terminal lipoprotein signal peptides followed by 10 or 11 Big domain repeats and, in the case of ligB, a unique carboxy-terminal non-repeat domain. The organization of ligC is similar to that of ligB but contains mutations that disrupt the reading frame. The lig sequences are present in pathogenic but not saprophytic Leptospira species. LigA and LigB are expressed by a variety of virulent leptospiral strains. Loss of Lig protein and RNA transcript expression is correlated with the observed loss of virulence during culture attenuation of pathogenic strains. High-pressure freeze substitution followed by immunocytochemical electron microscopy confirmed that the Lig proteins were localized to the bacterial surface. Immunoblot studies with patient sera found that the Lig proteins are a major antigen recognized during the acute host infection. These observations demonstrate that the Lig proteins are a newly identified surface protein of pathogenic Leptospira, which by analogy to other bacterial immunoglobulin superfamily virulence factors, may play a role in host cell attachment and invasion during leptospiral pathogenesis.
Transmission of leptospirosis occurs through contact of mucous membranes and abraded skin with freshwater contaminated by pathogenic Leptospira spp. Exposure to physiological osmolarity induces leptospires to express high levels of the Lig surface proteins containing imperfect immunoglobulin-like repeats that are shared or differ between LigA and LigB. We report that osmotic induction of Lig is accompanied by 1.6- to 2.5-fold increases in leptospiral adhesion to immobilized extracellular matrix and plasma proteins, including collagens I and IV, laminin, and especially fibronectin and fibrinogen. Recombinant LigA-unique and LigB-unique repeat proteins bind to these same host ligands. We found that the avidity of LigB in binding fibronectin is comparable to that of the Staphylococcus aureus FnBPA D repeats. Both LigA- and LigB-unique repeats interact with the amino-terminal fibrin- and gelatin-binding domains of fibronectin, which are also recognized by fibronectin-binding proteins mediating the adhesion of other microbial pathogens. In contrast, repeats common to both LigA and LigB do not bind these host proteins, and nonrepeat sequences in the carboxy-terminal domain of LigB show only weak interaction with fibronectin and fibrinogen. A functional role for the binding activity of LigA and LigB is suggested by the ability of the recombinants to inhibit leptospiral adhesion to fibronectin by 28% and 21%, respectively. The binding of LigA and LigB to multiple ligands present in different tissues suggests that these adhesins may be involved in the initial colonization and dissemination stages of leptospirosis. The characterization of the Lig adhesin function should aid the design of Lig-based vaccines and serodiagnostic tests.
The leptospiral immunoglobulin-like (Lig) proteins LigA and LigB possess immunoglobulin-like domains with 90-amino-acid repeats and are adhesion molecules involved in pathogenicity. They are conserved in pathogenic Leptospira spp. and thus are of interest for use as serodiagnostic antigens and in recombinant vaccine formulations. The N-terminal amino acid sequences of the LigA and LigB proteins are identical, but the C-terminal sequences vary. In this study, we evaluated the protective potential of five truncated forms of LigA and LigB proteins from Leptospira interrogans serovar Canicola as DNA vaccines using the pTARGET mammalian expression vector. Hamsters immunized with the DNA vaccines were subjected to a heterologous challenge with L. interrogans serovar Copenhageni strain Spool via the intraperitoneal route. Immunization with a DNA vaccine encoding LigBrep resulted in the survival of 5/8 (62.5%) hamsters against lethal infection (P < 0.05). None of the control hamsters or animals immunized with the other vaccine preparations survived. The vaccine induced an IgG antibody response and, additionally, conferred sterilizing immunity in 80% of the surviving animals. Our results indicate that the LigBrep DNA vaccine is a promising candidate for inclusion in a protective leptospiral vaccine.
The high-molecular-weight leptospiral immunoglobulin-like repeat (Lig) proteins are expressed only by virulent low-passage forms of pathogenic Leptospira species. We examined the effects of growth phase and environmental signals on the expression, surface exposure, and extracellular release of LigA and LigB. LigA was lost from stationary-phase cells, while LigB expression was maintained. The loss of cell-associated LigA correlated with selective release of a lower-molecular-weight form of LigA into the culture supernatant, while LigB and the outer membrane lipoprotein LipL41 remained associated with cells. Addition of tissue culture medium to leptospiral culture medium induced LigA and LigB expression and caused a substantial increase in released LigA. The sodium chloride component of tissue culture medium was primarily responsible for the enhanced release of LigA. Addition of sodium chloride, potassium chloride, or sodium sulfate to leptospiral medium to physiological osmolarity caused the induction of both cell-associated LigA and LigB, indicating that osmolarity regulates the expression of Lig proteins. Osmotic induction of Lig expression also resulted in enhanced release of LigA and increased surface exposure of LigB, as determined by surface immunofluorescence. Osmolarity appears to be a key environmental signal that controls the expression of LigA and LigB.
The pathogenic mechanisms of Leptospira interrogans, the causal agent of leptospirosis, remain largely unknown. This is mainly due to the lack of tools for genetically manipulating pathogenic Leptospira species. Thus, homologous recombination between introduced DNA and the corresponding chromosomal locus has never been demonstrated for this pathogen. Leptospiral immunoglobulin-like repeat (Lig) proteins were previously identified as putative Leptospira virulence factors. In this study, a ligB mutant was constructed by allelic exchange in L. interrogans; in this mutant a spectinomycin resistance (Spcr) gene replaced a portion of the ligB coding sequence. Gene disruption was confirmed by PCR, immunoblot analysis, and immunofluorescence studies. The ligB mutant did not show decrease virulence compared to the wild-type strain in the hamster model of leptospirosis. In addition, inoculation of rats with the ligB mutant induced persistent colonization of the kidneys. Finally, LigB was not required to mediate bacterial adherence to cultured cells. Taken together, our data provide the first evidence of site-directed homologous recombination in pathogenic Leptospira species. Furthermore, our data suggest that LigB does not play a major role in dissemination of the pathogen in the host and in the development of acute disease manifestations or persistent renal colonization.
Leptospira spp. are pathogenic spirochetes that cause the zoonotic disease leptospirosis. Leptospiral immunoglobulin (Ig)-like protein B (LigB) contributes to the binding of Leptospira to extracellular matrix proteins such as fibronectin, fibrinogen, laminin, elastin, tropoelastin and collagen. A high-affinity Fn-binding region of LigB has been localized to LigBCen2, which contains the partial 11th and full 12th Ig-like repeats (LigBCen2R) and 47 amino acids of the non-repeat region (LigBCen2NR) of LigB. In this study, the gelatin binding domain of fibronectin was shown to interact with LigBCen2R (KD = 1.91±0.40 µM). Not only LigBCen2R but also other Ig-like domains of Lig proteins including LigAVar7'-8, LigAVar10, LigAVar11, LigAVar12, LigAVar13, LigBCen7'-8, and LigBCen9 bind to GBD. Interestingly, a large gain in affinity was achieved through an avidity effect, with the terminal domains, 13th (LigA) or 12th (LigB) Ig-like repeat of Lig protein (LigAVar7'-13 and LigBCen7'-12) enhancing binding affinity approximately 51 and 28 fold, respectively, compared to recombinant proteins without this terminal repeat. In addition, the inhibited effect on MDCKs cells can also be promoted by Lig proteins with terminal domains, but these two domains are not required for gelatin binding domain binding and cell adhesion. Interestingly, Lig proteins with the terminal domains could form compact structures with a round shape mediated by multidomain interaction. This is the first report about the interaction of gelatin binding domain of Fn and Lig proteins and provides an example of Lig-gelatin binding domain binding mediating bacterial-host interaction.
We previously reported the cloning and characterization of leptospiral immunoglobulin-like proteins LigA and LigB of Leptospira interrogans. LigA and LigB are conserved at the amino-terminal region but are variable at the carboxyl-terminal region. Here, we evaluate the potential of recombinant LigA (rLigA) as a vaccine candidate against infection by L. interrogans serovar Pomona in a hamster model. rLigA was truncated into conserved (rLigAcon) and variable (rLigAvar) regions and expressed in Escherichia coli as a fusion protein with glutathione-S-transferase (rLigA). Golden Syrian hamsters were immunized at 3 and 6 weeks of age with rLigA (rLigAcon and rLigAvar) with aluminum hydroxide as an adjuvant. Hamsters given recombinant glutathione-S-transferase (rGST)-adjuvant and phosphate-buffered saline-adjuvant served as nonvaccinated controls. Three weeks after the last vaccination, all animals were challenged intraperitoneally with 108 L. interrogans serovar Pomona bacteria (NVSL 1427-35-093002). All hamsters immunized with recombinant LigA survived after challenge and had no significant histopathological changes. In contrast, nonimmunized and rGST-immunized hamsters were subjected to lethal doses, and the hamsters that survived showed severe tubulointerstitial nephritis. All vaccinated animals showed a rise in antibody titers against rLigA. Results from this study indicate that rLigA is a potential vaccine candidate against L. interrogans serovar Pomona infection.
Subunit vaccines are a potential intervention strategy against leptospirosis, which is a major public health problem in developing countries and a veterinary disease in livestock and companion animals worldwide. Leptospiral immunoglobulin-like (Lig) proteins are a family of surface-exposed determinants that have Ig-like repeat domains found in virulence factors such as intimin and invasin. We expressed fragments of the repeat domain regions of LigA and LigB from Leptospira interrogans serovar Copenhageni. Immunization of Golden Syrian hamsters with Lig fragments in Freund’s adjuvant induced robust antibody responses against recombinant protein and native protein, as detected by ELISA and immunoblot, respectively. A single fragment, LigANI, which corresponds to the six carboxy-terminal Ig-like repeat domains of the LigA molecule, conferred immunoprotection against mortality (67-100%, P <0.05) in hamsters which received a lethal inoculum of L. interrogans serovar Copenhageni. However, immunization with this fragment did not confer sterilizing immunity. These findings indicate that the carboxy-terminal portion of LigA is an immunoprotective domain and may serve as a vaccine candidate for human and veterinary leptospirosis.
Leptospirosis; subunit vaccine; Leptospiral immunoglobulin-like protein; recombinant protein; immunity; antibodies; hamsters
Protocatechuate (PCA) is the key intermediate metabolite in the lignin degradation pathway of Sphingomonas paucimobilis SYK-6 and is metabolized to pyruvate and oxaloacetate via the PCA 4,5-cleavage pathway. We characterized the 4-carboxy-2-hydroxymuconate-6-semialdehyde (CHMS) dehydrogenase gene (ligC). CHMS is the 4,5-cleavage product of PCA and is converted into 2-pyrone-4,6-dicarboxylate (PDC) by LigC. We found that ligC was located 295 bp downstream of ligB, which encodes the large subunit of the PCA 4,5-dioxygenase. The ligC gene consists of a 945-bp open reading frame encoding a polypeptide with a molecular mass of 34,590 Da. The deduced amino acid sequence of ligC showed 19 to 20% identity with 3-chlorobenzoate cis-dihydrodiol dehydrogenase of Alcaligenes sp. strain BR60 and phthalate cis-dihydrodiol dehydrogenases of Pseudomonas putida NMH102-2 and Burkholderia cepacia DBO1, which are unrelated to group I, II, and III microbial alcohol dehydrogenases (M. F. Reid and C. A. Fewson, Crit. Rev. Microbiol. 20:13–56, 1994). The ligC gene was expressed in Escherichia coli and LigC was purified to near homogeneity. Production of PDC from CHMS catalyzed by LigC was confirmed in the presence of NADP+ by electrospray ionization-mass spectrometry and gas chromatography-mass spectrometry. LigC is a homodimer. The isoelectric point, optimum pH, and optimum temperature were estimated to be 5.3, 8.0, and 25°C, respectively. The Km for NADP+ was estimated to be 24.6 ± 1.5 μM, which was approximately 10 times lower than that for NAD+ (252 ± 3.9 μM). The Kms for CHMS in the presence of NADP+ and NAD+ are 26.0 ± 0.5 and 20.6 ± 1.0 μM, respectively. Disruption of ligC in S. paucimobilis SYK-6 prevented growth with vanillate. Only PCA was accumulated during the incubation of vanillate with the whole cells of the ligC insertion mutant (DLC), indicating a lack of PCA 4,5-dioxygenase activity in DLC. However, the introduction of ligC into DLC restored its ability to grow on vanillate. PDC was suggested to be an inducer for ligAB gene expression.
Microbial pathogens acquire the immediate imperative to avoid or counteract the formidable defense of innate immunity as soon as they overcome the initial physical barriers of the host. Many have adopted the strategy of directly disrupting the complement system through the capture of its components, using proteins on the pathogen's surface. In leptospirosis, pathogenic Leptospira spp. are resistant to complement-mediated killing, in contrast to the highly vulnerable non-pathogenic strains. Pathogenic L. interrogans uses LenA/LfhA and LcpA to respectively sequester and commandeer the function of two regulators, factor H and C4BP, which in turn bind C3b or C4b to interrupt the alternative or classical pathways of complement activation. LigB, another surface-proximal protein originally characterized as an adhesin binding multiple host proteins, has other activities suggesting its importance early in infection, including binding extracellular matrix, plasma, and cutaneous repair proteins and inhibiting hemostasis. In this study, we used a recent model of ectopic expression of LigB in the saprophyte, L. biflexa, to test the hypothesis that LigB also interacts with complement proteins C3b and C4b to promote the virulence of L. interrogans. The surface expression of LigB partially rescued the non-pathogen from killing by 5% normal human serum, showing 1.3- to 48-fold greater survival 4 to 6 d following exposure to complement than cultures of the non-expressing parental strain. Recombinant LigB7′-12 comprising the LigB-specific immunoglobulin repeats binds directly to human complement proteins, C3b and C4b, with respective Kds of 43±26 nM and 69±18 nM. Repeats 9 to 11, previously shown to contain the binding domain for fibronectin and fibrinogen, are also important in LigB-complement interactions, which interfere with the alternative and classical pathways measured by complement-mediated hemolysis of erythrocytes. Thus, LigB is an adaptable interface for L. interrogans to efficiently counteract the multiple homeostatic processes of the host.
The spirochete Leptospira interrogans causes a systemic infection that provokes a febrile illness. The putative lipoproteins LigA and LigB promote adhesion of Leptospira to host proteins, interfere with coagulation, and capture complement regulators. In this study, we demonstrate that the expression level of the LigA and LigB proteins was substantially higher when L. interrogans proliferated at 37°C instead of the standard culture temperature of 30°C. The RNA comprising the 175-nucleotide 5′ untranslated region (UTR) and first six lig codons, whose sequence is identical in ligA and ligB, is predicted to fold into two distinct stem-loop structures separated by a single-stranded region. The ribosome-binding site is partially sequestered in double-stranded RNA within the second structure. Toeprint analysis revealed that in vitro formation of a 30S-tRNAfMet-mRNA ternary complex was inhibited unless a 5′ deletion mutation disrupted the second stem-loop structure. To determine whether the lig sequence could mediate temperature-regulated gene expression in vivo, the 5′ UTR and the first six codons were inserted between the Escherichia coli
l-arabinose promoter and bgaB (β-galactosidase from Bacillus stearothermophilus) to create a translational fusion. The lig fragment successfully conferred thermoregulation upon the β-galactosidase reporter in E. coli. The second stem-loop structure was sufficient to confer thermoregulation on the reporter, while sequences further upstream in the 5′ UTR slightly diminished expression at each temperature tested. Finally, the expression level of β-galactosidase was significantly higher when point mutations predicted to disrupt base pairs in the second structure were introduced into the stem. Compensatory mutations that maintained base pairing of the stem without restoring the wild-type sequence reinstated the inhibitory effect of the 5′ UTR on expression. These results indicate that ligA and ligB expression is limited by double-stranded RNA that occludes the ribosome-binding site. At elevated temperatures, the ribosome-binding site is exposed to promote translation initiation.
There is an urgent need for improved diagnosis of leptospirosis, an emerging infectious disease which imparts a large disease burden in developing countries. We evaluated the use of Leptospira immunoglobulin (Ig)-like (Lig) proteins as a serodiagnostic marker for leptospirosis. Lig proteins have bacterial immunoglobulin-like (Big) tandem repeat domains, a moiety found in virulence factors in other pathogens. Sera from patients identified during urban outbreaks in Brazil reacted strongly with immunoblots of a recombinant fragment comprised of the second to sixth Big domains of LigB from L. interrogans serovar Copenhageni, the principal agent for transmission in this setting. Furthermore, the sera recognized an analogous LigB fragment derived from L. kirschneri serovar Grippotyphosa, a pathogenic serovar which is not endemic to the study area. The immunoblot assay detected anti-LigB IgM antibodies in sera from 92% (95% confidence interval, 85 to 96%) of patients during acute-phase leptospirosis. The assay had a sensitivity of 81% for sera from patients with less than 7 days of illness. Anti-LigB antibodies were found in sera from 57% of the patients who did not have detectable anti-whole-Leptospira responses as detected by IgM enzyme-linked immunosorbent assay and microagglutination test. The specificities of the assay were 93 to 100% and 90 to 97% among sera from healthy individuals and patients with diseases that have clinical presentations that overlap with those of leptospirosis, respectively. These findings indicate that the antibody response to this putative virulence determinant is a sensitive and specific marker for acute infection. The use of this marker may aid the prompt and timely diagnosis required to reduce the high mortality associated with severe forms of the disease.
Adhesion through microbial surface components that recognize adhesive matrix molecules is an essential step in infection for most pathogenic bacteria. In this study, we report that LigB interacts with fibronectin (Fn) through its variable region. A possible role for LigB in bacterial attachment to host cells during the course of infection is supported by the following observations: (i) binding of the variable region of LigB to Madin-Darby canine kidney (MDCK) cells in a dose-dependent manner reduces the adhesion of Leptospira, (ii) inhibition of leptospiral attachment to Fn by the variable region of LigB, and (iii) decrease in binding of the variable region of LigB to the MDCK cells in the presence of Fn. Furthermore, we found a significant reduction in binding of the variable region of LigB to Fn using small interfering RNA (siRNA). Finally, the isothermal titration calorimetric results confirmed the interaction between the variable region of LigB and Fn. This is the first report to demonstrate that LigB binds to MDCK cells. In addition, the reduction of Fn expression in the MDCK cells, by siRNA, reduced the binding of LigB. Taken together, the data from the present study showed that LigB is a Fn-binding protein of pathogenic Leptospira spp. and may play a pivotal role in Leptospira-host interaction during the initial stage of infection.
adhesion; Fn; Leptospira; LigB; MDCK cell; siRNA
A clone expressing a novel immunoreactive leptospiral immunoglobulin-like protein A of 130 kDa (LigA) from Leptospira interrogans serovar pomona type kennewicki was isolated by screening a genomic DNA library with serum from a mare that had recently aborted due to leptospiral infection. LigA is encoded by an open reading frame of 3,675 bp, and the deduced amino acid sequence consists of a series of 90-amino-acid tandem repeats. A search of the NCBI database found that homology of the LigA repeat region was limited to an immunoglobulin-like domain of the bacterial intimin binding protein of Escherichia coli, the cell adhesion domain of Clostridium acetobutylicum, and the invasin of Yersinia pestis. Secondary structure prediction analysis indicates that LigA consists mostly of beta sheets with a few alpha-helical regions. No LigA was detectable by immunoblot analysis of lysates of the leptospires grown in vitro at 30°C or when cultures were shifted to 37°C. Strikingly, immunohistochemistry on kidney from leptospira-infected hamsters demonstrated LigA expression. These findings suggest that LigA is specifically induced only in vivo. Sera from horses, which aborted as a result of natural Leptospira infection, strongly recognize LigA. LigA is the first leptospiral protein described to have 12 tandem repeats and is also the first to be expressed only during infection. Thus, LigA may have value in serodiagnosis or as a protective immunogen in novel vaccines.
Leptospiral immunoglobulin-like (Lig) proteins are of great interest due to their ability to act as mediators of pathogenesis, serodiagnostic antigens, and immunogens. Purified recombinant LigA protein is the most promising subunit vaccine candidate against leptospirosis reported to date, however, as purified proteins are weak immunogens the use of a potent adjuvant is essential for the success of LigA as a subunit vaccine. In the present study, we compared xanthan pv. pruni (strain 106), aluminium hydroxide (alhydrogel), and CpG ODN as adjuvants in a LigA subunit vaccine preparation. Xanthan gum is a high molecular weight extracellular polysaccharide produced by fermentation of Xanthomonas spp., a plant-pathogenic bacterium genus. Preparations containing xanthan induced a strong antibody response comparable to that observed when alhydrogel was used. Upon challenge with a virulent strain of L. interrogans serovar Copenhageni, significant protection (Fisher test, P < 0.05) was observed in 100%, 100%, and 67% of hamsters immunized with rLigANI-xanthan, LigA-CpG-xanthan, and rLigANI-alhydrogel, respectively. Furthermore, xanthan did not cause cytotoxicity in Chinese hamster ovary (CHO) cells in vitro. The use of xanthan as an adjuvant is a novel alternative for enhancing the immunogenicity of vaccines against leptospirosis and possibly against other pathogens.
The protocatechuate (PCA) 4,5-cleavage pathway is the essential metabolic route for degradation of low-molecular-weight products derived from lignin by Sphingomonas paucimobilis SYK-6. In the 10.5-kb EcoRI fragment carrying the genes for PCA 4,5-dioxygenase (ligAB), 2-pyrone-4,6-dicarboxylate hydrolase (ligI), 4-oxalomesaconate hydratase (ligJ), and a part of 4-carboxy-2-hydroxymuconate-6-semialdehyde dehydrogenase (ligC), we found the ligK gene, which encodes 4-carboxy-4-hydroxy-2-oxoadipate (CHA) aldolase. The ligK gene was located 1,183 bp upstream of ligI and transcribed in the same direction as ligI. We also found the ligR gene encoding a LysR-type transcriptional activator, which was located 174 bp upstream of ligK. The ligK gene consists of a 684-bp open reading frame encoding a polypeptide with a molecular mass of 24,131 Da. The deduced amino acid sequence of ligK showed 57 to 88% identity with those of the corresponding genes recently reported in Sphingomonas sp. strain LB126, Comamonas testosteroni BR6020, Arthrobacter keyseri 12B, and Pseudomonas ochraceae NGJ1. The ligK gene was expressed in Escherichia coli, and the gene product (LigK) was purified to near homogeneity. Electrospray-ionization mass spectrometry indicated that LigK catalyzes not only the conversion of CHA to pyruvate and oxaloacetate but also that of oxaloacetate to pyruvate and CO2. LigK is a hexamer, and its isoelectric point is 5.1. The Km for CHA and oxaloacetate are 11.2 and 136 μM, respectively. Inactivation of ligK in S. paucimobilis SYK-6 resulted in the growth deficiency of vanillate and syringate, indicating that ligK encodes the essential CHA aldolase for catabolism of these compounds. Reverse transcription-PCR analysis revealed that the PCA 4,5-cleavage pathway genes of S. paucimobilis SYK-6 consisted of four transcriptional units, including the ligK-orf1-ligI-lsdA cluster, the ligJAB cluster, and the monocistronic ligR and ligC genes.
Transmission of pathogenic Leptospira between mammalian hosts usually involves dissemination via soil or water contaminated by the urine of carrier animals. The ability of Leptospira to adapt to the diverse conditions found inside and outside the host is reflected in its relatively large genome size and high percentage of signal transduction genes. An exception is Leptospira borgpetersenii serovar Hardjo, which is transmitted by direct contact and appears to have lost genes necessary for survival outside the mammalian host. Invasion of host tissues by Leptospira interrogans involves a transition from a low osmolar environment outside the host to a higher physiologic osmolar environment within the host. Expression of the lipoprotein LigA and LigB adhesins is strongly induced by an upshift in osmolarity to the level found in mammalian host tissues. These data suggest that Leptospira utilizes changes in osmolarity to regulate virulence characteristics. To better understand how L. interrogans serovar Copenhageni adapts to osmolar conditions that correspond with invasion of a mammalian host, we quantified alterations in transcript levels using whole-genome microarrays. Overnight exposure in leptospiral culture medium supplemented with sodium chloride to physiologic osmolarity significantly altered the transcript levels of 6% of L. interrogans genes. Repressed genes were significantly more likely to be absent or pseudogenes in L. borgpetersenii, suggesting that osmolarity is relevant in studying the adaptation of L. interrogans to host conditions. Genes induced by physiologic osmolarity encoded a higher than expected number of proteins involved in signal transduction. Further, genes predicted to encode lipoproteins and those coregulated by temperature were overrepresented among both salt-induced and salt-repressed genes. In contrast, leptospiral homologues of hyperosmotic or general stress genes were not induced at physiologic osmolarity. These findings suggest that physiologic osmolarity is an important signal for regulation of gene expression by pathogenic leptospires during transition from ambient conditions to the host tissue environment.
Escherichia coli DNA ligase (LigA) is the prototype of the NAD+-dependent class of DNA ligases found in all bacteria. Here we report the characterization of E.coli LigB, a second NAD+-dependent DNA ligase identified by virtue of its sequence similarity to LigA. LigB differs from LigA in that it lacks the BRCA1 C-terminus domain (BRCT) and two of the four Zn-binding cysteines that are present in LigA and all other bacterial NAD+ ligases. We found that recombinant LigB catalyzed strand joining on a singly-nicked DNA in the presence of a divalent cation and NAD+, and that LigB reacted with NAD+ to form a covalent ligase-adenylate intermediate. Alanine substitution for the motif I lysine (126KxDG) abolished nick joining and ligase-adenylate formation by LigB, thus confirming that the ligase and adenylyltransferase activities are intrinsic to the LigB protein.
Infection by pathogenic strains of Leptospira hinges on the pathogen’s ability to adhere to host cells via extracellular matrix such as fibronectin (Fn). Previously, the immunoglobulin-like domains of Leptospira Lig proteins were recognized as adhesins binding to N-terminal domain (NTD) and gelatin binding domain (GBD) of Fn. In this study, we identified another Fn-binding motif on the C-terminus of the Leptospira adhesin LigB (LigBCtv), residues 1708–1712 containing sequence LIPAD with a β-strand and nascent helical structure. This motif binds to 15th type III modules (15F3) (KD = 10.70 μM), and association (kon = 600 M−1 s−1) and dissociation (koff = 0.0129 s−1) rate constants represents a slow binding kinetics in this interaction. Moreover, pretreatment of MDCK cells with LigB1706–1716 blocked the binding of Leptospira by 39%, demonstrating a significant role of LigB1706–1716 in cellular adhesion. These data indicate that the LIPAD residues (LigB1708–1712) of the Leptospira interrogans LigB protein bind 15F3 of Fn at a novel binding site, and this interaction contributes to adhesion to host cells.
Leptospira interrogans; Fibronectin; Type III modules; LigB
Sphingomonas paucimobilis SYK-6 is able to grow on various dimeric lignin compounds, which are converted to vanillate and syringate by the actions of unique lignin degradation enzymes in this strain. Vanillate and syringate are degraded by the O-demethylase and converted into protocatechuate (PCA) and 3-O-methylgallate (3MGA), respectively. PCA is further degraded via the PCA 4,5-cleavage pathway, while the results suggested that 3MGA is degraded through another pathway in which PCA 4,5-dioxygenase is not involved. In a 10.5-kb EcoRI fragment carrying the genes for PCA 4,5-dioxygenase (ligAB), 2-pyrone-4,6-dicarboxylate hydrolase (ligI), and a portion of 4-carboxy-2-hydroxymuconate-6-semialdehyde dehydrogenase (ligC), we found the ligJ gene encoding 4-oxalomesaconate (OMA) hydratase, which catalyzes the conversion of OMA into 4-carboxy-4-hydroxy-2-oxoadipate. The ligJ gene is transcribed in the same direction as ligABC genes and consists of an 1,023-bp open reading frame encoding a polypeptide with a molecular mass of 38,008 Da, which is located 73-bp upstream from ligA. The ligJ gene product (LigJ), expressed in Escherichia coli, was purified to near homogeneity and was estimated to be a homodimer (69.5 kDa) by gel filtration chromatography. The isoelectric point was determined to be 4.9, and the optimal temperature is 30°C. The Km for OMA and the Vmax were determined to be 138 μM and 440 U/mg, respectively. LigJ activity was inhibited by the addition of thiol reagents, suggesting that some cysteine residue is part of the catalytic site. The ligJ gene disruption in SYK-6 caused the growth defect on and the accumulation of common metabolites from both vanillate and syringate, indicating that the ligJ gene is essential to the degradation of these two compounds. These results indicated that syringate is converted into OMA via 3MGA, and it enters the PCA 4,5-cleavage pathway.
Leptospira immunoglobulin (Ig)-like (Lig) proteins are a novel family of surface-associated proteins in which the N-terminal 630 amino acids are conserved. In this study, we truncated the LigA conserved region into 7 fragments comprising the 1st to 3rd (LigACon1-3), 4th to 7.5th (LigACon4-7.5), 4th (LigACon4), 4.5th to 5.5th (LigACon4.5–5.5), 5.5th to 6.5th (LigACon5.5–6.5), 4th to 5th (LigACon4-5), and 6th to 7.5th (LigACon6-7.5) repeat domains. All 7 recombinant Lig proteins were screened using a slot-shaped dot blot assay for the diagnosis of equine leptospirosis. Our results showed that LigACon4-7.5 is the best candidate diagnostic antigen in a slot-shaped dot blot assay. LigACon4-7.5 was further evaluated as an indirect enzyme-linked immunosorbent assay (ELISA) antigen for the detection of Leptospira antibodies in equine sera. This assay was evaluated with equine sera (n = 60) that were microscopic agglutination test (MAT) negative and sera (n = 220) that were MAT positive to the 5 serovars that most commonly cause equine leptospirosis. The indirect ELISA results showed that at a single serum dilution of 1:250, the sensitivity and specificity of ELISA were 80.0% and 87.2%, respectively, compared to those of MAT. In conclusion, an indirect ELISA was developed utilizing a recombinant LigA fragment comprising the 4th to 7.5th repeat domain (LigACon4-7.5) as a diagnostic antigen for equine leptospirosis. This ELISA was found to be sensitive and specific, and it yielded results that concurred with those of the standard MAT.
Genetic analyses of an Escherichia coli K-12 mutant possessing the amber mutation lig-321 were carried out. This mutant is defective in deoxyribonucleic acid (DNA) ligase and conditionally lethal. We constructed strains harboring an F'lig+ or F'lig-321 plasmid. Genetic complementation analyses were done by using these plasmids and by constructing a lig-4/F'lig-321 merodiploid. It was shown that lig-321 does not complement lig-4, unless the former is suppressed by an amber suppressor. The same was found to be the case between lig-321 and lig-ts7. Transductional mapping of lig-321, by a four-factor cross, revealed that lig-321 is very closely linked to lig-4. The frequency of recombinants between the two alleles was not unreasonable for assuming that they arose by intragenic recombination. The lig-4 and lig-ts7 alleles are known to reside in the structural gene for DNA ligase, in which lig-321 may also be located.
Mycobacteria contain genes for several DNA ligases, including ligA, which encodes a NAD+-dependent enzyme that has been postulated to be a target for novel antibacterial compounds. Using a homologous recombination system, direct evidence is presented that wild-type ligA cannot be deleted from the chromosome of Mycobacterium smegmatis. Deletions of native ligA in M. smegmatis could be obtained only after the integration of an extra copy of M. smegmatis or Mycobacterium tuberculosis ligA into the attB site of the chromosome, with expression controlled by chemically inducible promoters. The four ATP-dependent DNA ligases encoded by the M. smegmatis chromosome were unable to replace the function of LigA. Interestingly, the LigA protein from M. smegmatis could be substituted with the NAD+-dependent DNA ligase of Escherichia coli or the ATP-dependent ligase of bacteriophage T4. The conditional mutant strains allowed the analysis of the effect of LigA depletion on the growth of M. smegmatis. The protein level of the conditional mutants was estimated by Western blot analysis using antibodies raised against LigA of M. tuberculosis. This revealed that a strong overproduction or depletion of LigA did not affect the growth or survival of mycobacteria under standard laboratory conditions. In conclusion, although NAD+-dependent DNA ligase is essential for mycobacterial viability, only low levels of protein are required for growth. These findings suggest that very efficient inhibition of enzyme activity would be required if NAD+-dependent DNA ligase is to be useful as an antibiotic target in mycobacteria. The strains developed here will provide useful tools for the evaluation of the efficacy of any appropriate compounds in mycobacteria.
Canine leptospirosis is underdiagnosed due to its wide spectrum of clinical presentations and the lack of a rapid and sensitive test for the accurate diagnosis of acute and chronic infections. In this study, we developed a highly sensitive and specific fluorescence resonance energy transfer (FRET)-PCR to detect common pathogenic leptospires in dogs, including Leptospira interrogans serovars Autumnalis, Canicola, Copenhageni (Icterohaemorrhagiae serogroup) and Pomona, and Leptospira kirschneri serovar Grippotyphosa. This PCR targets the lig genes, exclusively found in the pathogenic Leptospira species but not in saprophytic species (L. biflexa). A robust, high-stringency step-down real-time platform was coupled to the highly specific detection of leptospiral DNA by fluorescently labeled FRET probes. This enabled the detection of a single copy of the lig gene in a PCR containing DNA from up to 50 µL canine blood or 400 µL urine. Sensitivity determination by use of limiting serial dilutions of extracted leptospiral DNA indicated that the lig FRET-PCR we established was almost 100-fold more sensitive than the widely accepted lipL32 SYBR assay and 10-fold more sensitive than a 16S rRNA TaqMan assay. Application of this method to 207 dogs with potential leptospiral infection enabled us to diagnose three cases of canine leptospirosis characterized by low amounts of leptospiral DNA in body fluids. Detection of canine leptospirosis with the lig FRET-PCR was more sensitive with the lig FRET-PCR than with the 16S rRNA TaqMan PCR, which detected only 2 of the 3 cases, and the lipL32 SYBR PCR, which detected none of the 3 dogs with leptospirosis.
DNA ligases are required for DNA strand joining in all forms of cellular life. NAD+-dependent DNA ligases are found primarily in eubacteria but also in some eukaryotic viruses, bacteriophage and archaea. Among the archaeal NAD+-dependent DNA ligases is the LigN enzyme of the halophilic euryarchaeon Haloferax volcanii, the gene for which was apparently acquired by Hfx.volcanii through lateral gene transfer (LGT) from a halophilic eubacterium. Genetic studies show that the LGT-acquired LigN enzyme shares an essential function with the native Hfx.volcanii ATP-dependent DNA ligase protein LigA.
To characterise the enzymatic properties of the LigN protein, wild-type and three mutant forms of the LigN protein were separately expressed in recombinant form in E.coli and purified to apparent homogeneity by immobilised metal ion affinity chromatography (IMAC). Non-isotopic DNA ligase activity assays using λ DNA restriction fragments with 12 bp cos cohesive ends were used to show that LigN activity was dependent on addition of divalent cations and salt. No activity was detected in the absence of KCl, whereas maximum activity could be detected at 3.2 M KCl, close to the intracellular KCl concentration of Hfx.volcanii cells.
LigN is unique amongst characterised DNA ligase enzymes in displaying maximal DNA strand joining activity at high (> 3 M) salt levels. As such the LigN enzyme has potential both as a novel tool for biotechnology and as a model enzyme for studying the adaptation of proteins to high intracellular salt levels.