Identification of a Mycobacterium tuberculosis gene that enhances mycobacterial survival in macrophages

Intracellular survival plays a central role in the pathogenesis of Mycobacterium tuberculosis. To identify M. tuberculosis genes required for intracellular survival within macrophages, an M. tuberculosis H37Rv plasmid library was constructed by using the shuttle vector pOLYG. This plasmid library was electroporated into Mycobacterium smegmatis 1-2c, and the transformants were used to infect the human macrophage-like cell line U-937. Because M. smegmatis does not readily survive within macrophages, any increased intracellular survival is likely due to cloned M. tuberculosis H37Rv DNA. After six sequential passages of M. smegmatis transformants through U-937 cells, one clone (p69) was enriched more than 70% as determined by both restriction enzyme and PCR analyses. p69 demonstrated significantly enhanced survival compared to that of the vector control, ranging from 2.4- to 5.3-fold at both 24 and 48 h after infection. DNA sequence analysis revealed three open reading frames (ORFs) in the insert of p69. ORF2 (1.2 kb) was the only one which contained a putative promoter region and a ribosome-binding site. Deletion analysis of the p69 insert DNA showed that disruption of ORF2 resulted in complete loss of the enhanced intracellular survival phenotype. This gene was named the enhanced intracellular survival (eis) gene. By using an internal region of eis as a probe for Southern analysis, eis was found in the genomic DNA of various M. tuberculosis strains and of Mycobacterium bovis BCG but not in that of M. smegmatis or 10 other nonpathogenic mycobacterial species. Sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis showed that all M. smegmatis eis-containing constructs expressed a unique protein of 42 kDa, the predicted size of Eis. The expression of this 42-kDa protein directly correlated to the enhanced survival of M. smegmatis p69 in U-937 cells. These results suggest a possible role for eis and its protein product in the intracellular survival of M. tuberculosis.     

Mycobacterium smegmatis laminin-binding glycoprotein shares epitopes with Mycobacterium tuberculosis heparin-binding haemagglutinin

Mycobacterium tuberculosis, the causative agent of tuberculosis, produces a heparin-binding haemagglutinin adhesin (HBHA), which is involved in its epithelial adherence. To ascertain whether HBHA is also present in fast-growing mycobacteria, Mycobacterium smegmatis was studied using anti-HBHA monoclonal antibodies (mAbs). A cross-reactive protein was detected by immunoblotting of M. smegmatis whole-cell lysates. However, the M. tuberculosis HBHA-encoding gene failed to hybridize with M. smegmatis chromosomal DNA in Southern blot analyses. The M. smegmatis protein recognized by the anti-HBHA mAbs was purified by heparin-Sepharose chromatography, and its amino-terminal sequence was found to be identical to that of the previously described histone-like protein, indicating that M. smegmatis does not produce HBHA. Biochemical analysis of the M. smegmatis histone-like protein shows that it is glycosylated like HBHA. Immunoelectron microscopy demonstrated that the M. smegmatis protein is present on the mycobacterial surface, a cellular localization inconsistent with a histone-like function, but compatible with an adhesin activity. In vitro protein interaction assays showed that this glycoprotein binds to laminin, a major component of basement membranes. Therefore, the protein was called M. smegmatis laminin-binding protein (MS-LBP). MS-LBP does not appear to be involved in adherence in the absence of laminin but is responsible for the laminin-mediated mycobacterial adherence to human pneumocytes and macrophages. Homologous laminin-binding adhesins are also produced by virulent mycobacteria such as M. tuberculosis and Mycobacterium leprae, suggesting that this adherence mechanism may contribute to the pathogenesis of mycobacterial diseases.     

Decreased capacity of recombinant 45/47-kDa molecules (Apa) of Mycobacterium tuberculosis to stimulate T lymphocyte responses related to changes in their mannosylation pattern.

The Apa molecules secreted by Mycobacterium tuberculosis, Mycobacterium bovis, or BCG have been identified as major immunodominant antigens. Mass spectrometry analysis indicated similar mannosylation, a complete pattern from 1 up to 9 hexose residues/mole of protein, of the native species from the 3 reference strains. The recombinant antigen expressed in M. smegmatis revealed a different mannosylation pattern: species containing 7 to 9 sugar residues/mole of protein were in the highest proportion, whereas species bearing a low number of sugar residues were almost absent. The 45/47-kDa recombinant antigen expressed in E. coli was devoid of sugar residues. The proteins purified from M. tuberculosis, M. bovis, or BCG have a high capacity to elicit in vivo potent delayed-type hypersensitivity (DTH) reactions and to stimulate in vitro sensitized T lymphocytes of guinea pigs immunized with living BCG. The recombinant Apa expressed in Mycobacterium smegmatis was 4-fold less potent in vivo in the DTH assay and 10-fold less active in vitro to stimulate sensitized T lymphocytes than the native proteins. The recombinant protein expressed in Escherichia coli was nearly unable to elicit DTH reactions in vivo or to stimulate T lymphocytes in vitro. Thus the observed biological effects were related to the extent of glycosylation of the antigen.     

Identification of mycobacterium tuberculosis DNA sequences encoding exported proteins by using phoA gene fusions.

The activity of bacterial alkaline phosphatase (PhoA) is dependent on it being exported across the plasma membrane. A plasmid vector (pJEM11) allowing fusions between phoA and genes encoding exported proteins was constructed to study protein export in mycobacteria. Introduction of the Mycobacterium fortuitum beta-lactamase gene (blaF*) into this vector led to the production in M. smegmatis of protein fusions with PhoA activity. A genomic library from M. tuberculosis was constructed in pJEM11 and screened in M. smegmatis for clones with PhoA activity. Sequences of the M. tuberculosis inserts directing the production of protein fusions in these PhoA-positive clones were determined. They include part of the already-known exported 19-kDa lipoprotein, a sequence with similarities to the exported 28-kDa antigen from M. leprae, a sequence encoding a protein sharing conserved amino acid motifs with stearoyl-acyl-carrier-protein desaturases, and unknown sequences. This approach thus appears to identify sequences directing protein export, and we expect that more extensive screening of such libraries will lead to a better understanding of protein export in M. tuberculosis.     

Mycobacterium tuberculosis hemoglobin HbO associates with membranes and stimulates cellular respiration of recombinant Escherichia coli.

The truncated hemoglobins HbN and HbO of Mycobacterium tuberculosis H37Rv share little sequence similarity and display structural differences in their EF-loop regions, suggesting distinct function(s) for these hemoglobins. HbO of M. tuberculosis was expressed in Escherichia coli and Mycobacterium smegmatis as a 14.5-kDa homodimeric heme protein exhibiting nearly 50-fold (P(50) approximately 0.51) lower oxygen affinity than HbN. 40-50% of HbO remained associated with the cell membranes and significantly enhanced its respiration in comparison with the membrane fractions of control cells or cells overproducing HbN. Oxygen uptake of HbO-associated membranes was decreased by washing and restored by adding HbO. Additionally, membrane vesicles prepared from terminal oxidase-deficient (cyo(-), cyd(-)) mutants of E. coli did not exhibit significant enhancement in oxygen uptake in the presence of HbO, suggesting its interaction(s) with the electron transport chain. Expression of HbO in Mycobacterium bovis bacillus Calmette-Guérin, an experimental model of M. tuberculosis, was observed (0.2-0.5% of total cellular proteins) throughout its aerobic growth. These results provided evidence for the involvement of HbO with the component of aerobic electron transport chain, suggesting that its function may be related to the facilitation of oxygen transfer during aerobic metabolism of M. tuberculosis. Membrane association properties of HbO may thus play a crucial role in sequestering oxygen and facilitating its availability to internalized M. tuberculosis (an obligate aerobe) under the hypoxic conditions of its intracellular habitat.     

Cloning and expression of the trehalose-phosphate phosphatase of Mycobacterium tuberculosis: comparison to the enzyme from Mycobacterium smegmatis

Two open reading frames in the Mycobacterium tuberculosis genome, Rv3372 and Rv2006, have about 25% sequence identity at the amino acid level to the trehalose-phosphate phosphatase (TPP) purified from Mycobacterium smegmatis. However, the protein produced from the cloned Rv3372 gene has a molecular weight of about 45kDa whereas the trehalose-P phosphatase purified from M. smegmatis has a molecular weight of about 27kDa. We expressed the Rv3372 protein in Escherichia coli and show here that it is a trehalose-P phosphatase with very similar properties to the M. smegmatis TPP, i.e., complete specificity for trehalose-phosphate as the substrate, an almost absolute requirement for Mg(2+), and a pH optimum of 7-7.5. On the other hand, in contrast to the M. smegmatis enzyme, the Rv3372 protein was much less stable to heat and much less sensitive to inhibition by diumycin and moenomycin. In fact, both of these antibiotics stimulate enzyme activity at low concentrations and only inhibit the activity at higher antibiotic concentrations. Antibody prepared against the 27kDa TPP does not cross react with the 45kDa TPP nor does antibody against the 45kDa TPP cross react with the 27kDa TPP. Nevertheless, studies of secondary structure by circular dichroism indicate that the two enzymes are quite similar in structure. The product of the other gene, Rv2006, is a 159kDa protein with no detectable phosphatase activity. Thus, its function is currently unknown.     

Functional complementation of the essential gene fabG1 of Mycobacterium tuberculosis by Mycobacterium smegmatis fabG but not Escherichia coli fabG

Mycolic acids are a key component of the mycobacterial cell wall, providing structure and forming a major permeability barrier. In Mycobacterium tuberculosis mycolic acids are synthesized by type I and type II fatty acid synthases. One of the enzymes of the type II system is encoded by fabG1. We demonstrate here that this gene can be deleted from the M. tuberculosis chromosome only when another functional copy is provided elsewhere, showing that under normal culture conditions fabG1 is essential. FabG1 activity can be replaced by the corresponding enzyme from the closely related species Mycobacterium smegmatis but not by the enzyme from Escherichia coli. M. tuberculosis carrying FabG from M. smegmatis showed no phenotypic changes, and both the mycolic acids and cell wall permeability were unchanged. Thus, M. tuberculosis and M. smegmatis enzymes are interchangeable and do not control the lengths and types of mycolic acids synthesized.     

Studies on transfer RNA from mycobacteria.

Active preparations of tRNA and aminoacyl-tRNA synthetases have been isolated from exponentially growing cells of Mycobacterium smegmatis and Mycobacterium tuberculosis H37Rv. Though the aminoacyl-tRNA synthetases of older cells retain their activity, the tRNAs seem to undergo modification and show poorer activity. The mycobacterial enzyme preparations catalyse homologous and heterologous aminoacylation between tRNA from the two species (M. smegmatis and M. tuberculosis H37Rv) or from Escherichia coli, with equal efficiency; tRNA samples from eukaryotic cells (yeast and rat liver) do not serve as substrates for the mycobacterial synthetases. The analytical separation of the different amino acid specific tRNAs from M. smegmatis resembles the pattern found in other bacteria. Purification of valine- (three species) and methionine-specific tRNA (two species) to 70-80% purity has been accomplished by using column-chromatographic techniques. Of the two species of tRNAMet, one can be formylated in the presence of formyl tetrahydrofolate and the transformylase from mycobacteria.     

Oxidative stress response and its role in sensitivity to isoniazid in mycobacteria: characterization and inducibility of ahpC by peroxides in Mycobacterium smegmatis and lack of expression in M. aurum and M. tuberculosis.

Mycobacterium tuberculosis is a natural mutant with inactivated oxidative stress regulatory gene oxyR. This characteristic has been linked to the exquisite sensitivity of M. tuberculosis to isonicotinic acid hydrazide (INH). In the majority of mycobacteria tested, including M. tuberculosis, oxyR is divergently transcribed from ahpC, a gene encoding a homolog of the subunit of alkyl hydroperoxide reductase that carries out substrate peroxide reduction. Here we compared ahpC expression in Mycobacterium smegmatis, a mycobacterium less sensitive to INH, with that in two highly INH sensitive species, M. tuberculosis and Mycobacterium aurum. The ahpC gene of M. smegmatis was cloned and characterized, and the 5' ends of ahpC mRNA were mapped by S1 nuclease protection analysis. M. smegmatis AhpC and eight other polypeptides were inducible by exposure to H2O2 or organic peroxides, as determined by metabolic labeling and Western blot (immunoblot) analysis. In contrast, M. aurum displayed differential induction of only one 18-kDa polypeptide when exposed to organic peroxides. AhpC could not be detected in this organism by immunological means. AhpC was also below detection levels in M. tuberculosis H37Rv. These observations are consistent with the interpretation that ahpC expression and INH sensitivity are inversely correlated in the mycobacterial species tested. In further support of this conclusion, the presence of plasmid-borne ahpC reduced M. smegmatis susceptibility to INH. Interestingly, mutations in the intergenic region between oxyR and ahpC were identified and increased ahpC expression observed in deltakatG M. tuberculosis and Mycobacterium bovis INH(r) strains. We propose that mutations activating ahpC expression may contribute to the emergence of INH(r) strains.     

Identification of a novel peptidoglycan hydrolase CwlM in Mycobacterium tuberculosis

Mycobacterium tuberculosis is a major global pathogen whose threat has increased with the emergence of multidrug-resistant strains. The cell wall of M. tuberculosis is thick, rigid, and hydrophobic, which serves to protect the organism from the environment and makes it highly impermeable to conventional antimicrobial agents. There is little known about cell wall autolysins (also referred to as peptidoglycan hydrolases) of mycobacteria. We identified an open reading frame (Rv3915) in the M. tuberculosis genome designated cwlM that appeared consistent with a peptidoglycan hydrolase. The 1218-bp gene was amplified by PCR, cloned and expressed in E. coli strain HMS174(DE-3), and its gene product, a 47-kDa recombinant protein, was purified and partially characterized. Purified CwlM was able to lyse whole mycobacteria, release peptidoglycan from the cell wall of Micrococcus luteus and Mycobacterium smegmatis, and cleave N-acetylmuramoyl-L-alanyl-D-isoglutamine, releasing free N-acetylmuramic acid. These results indicate that CwlM is a novel autolysin and identify cwlM as the first, to our knowledge, autolysin gene identified and cloned from M. tuberculosis. CwlM offers a new target for a unique class of drugs that could alter the permeability of the mycobacterial cell wall and enhance the effectiveness of treatments for tuberculosis.     

Functional analyses of mycobacterial lipoprotein diacylglyceryl transferase and comparative secretome analysis of a mycobacterial lgt mutant

Preprolipopoprotein diacylglyceryl transferase (Lgt) is the gating enzyme of lipoprotein biosynthesis, and it attaches a lipid structure to the N-terminal part of preprolipoproteins. Using Lgt from Escherichia coli in a BLASTp search, we identified the corresponding Lgt homologue in Mycobacterium tuberculosis and two homologous (MSMEG_3222 and MSMEG_5408) Lgt in Mycobacterium smegmatis. M. tuberculosis lgt was shown to be essential, but an M. smegmatis ΔMSMEG_3222 mutant could be generated. Using Triton X-114 phase separation and [(14)C]palmitic acid incorporation, we demonstrate that MSMEG_3222 is the major Lgt in M. smegmatis. Recombinant M. tuberculosis lipoproteins Mpt83 and LppX are shown to be localized in the cell envelope of parental M. smegmatis but were absent from the cell membrane and cell wall in the M. smegmatis ΔMSMEG_3222 strain. In a proteomic study, 106 proteins were identified and quantified in the secretome of wild-type M. smegmatis, including 20 lipoproteins. All lipoproteins were secreted at higher levels in the ΔMSMEG_3222 mutant. We identify the major Lgt in M. smegmatis, show that lipoproteins lacking the lipid anchor are secreted into the culture filtrate, and demonstrate that M. tuberculosis lgt is essential and thus a validated drug target.     

The structure of arylamine N-acetyltransferase from Mycobacterium smegmatis--an enzyme which inactivates the anti-tubercular drug,isoniazid

Arylamine N-acetyltransferases which acetylate and inactivate isoniazid, an anti-tubercular drug, are found in mycobacteria including Mycobacterium smegmatis and Mycobacterium tuberculosis. We have solved the structure of arylamine N-acetyltransferase from M. smegmatis at a resolution of 1.7 A as a model for the highly homologous NAT from M. tuberculosis. The fold closely resembles that of NAT from Salmonella typhimurium, with a common catalytic triad and domain structure that is similar to certain cysteine proteases. The detailed geometry of the catalytic triad is typical of enzymes which use primary alcohols or thiols as activated nucleophiles. Thermal mobility and structural variations identify parts of NAT which might undergo conformational changes during catalysis. Sequence conservation among eubacterial NATs is restricted to structural residues of the protein core, as well as the active site and a hinge that connects the first two domains of the NAT structure. The structure of M. smegmatis NAT provides a template for modelling the structure of the M. tuberculosis enzyme and for structure-based ligand design as an approach to designing anti-TB drugs.     

Protein inactivation in mycobacteria by controlled proteolysis and its application to deplete the beta subunit of RNA polymerase

Using a component of the Escherichia coli protein degradation machinery, we have established a system to regulate protein stability in mycobacteria. A protein tag derived from the E. coli SsrA degradation signal did not affect several reporter proteins in wild-type Mycobacterium smegmatis or Mycobacterium tuberculosis. Expression of the adaptor protein SspB, which recognizes this modified tag and helps deliver tagged proteins to the protease ClpXP, strongly decreased the activities and protein levels of different reporters. This inactivation did not occur when the function of ClpX was inhibited. Using this system, we constructed a conditional M. smegmatis knockdown mutant in which addition of anhydrotetracycline (atc) caused depletion of the beta subunit of RNA polymerase, RpoB. The impact of atc on this mutant was dose-dependent. Very low amounts of atc did not prevent growth but increased sensitivity to an antibiotic that inactivates RpoB. Intermediate amounts of RpoB knockdown resulted in bacteriostasis and a more substantial depletion led to a decrease in viability by up to 99%. These studies identify SspB-mediated proteolysis as an efficient approach to conditionally inactivate essential proteins in mycobacteria. They further demonstrate that depletion of RpoB by ～ 93% is sufficient to cause death of M. smegmatis.     

Investigating the function of the putative mycolic acid methyltransferase UmaA: divergence between the Mycobacterium smegmatis and Mycobacterium tuberculosis proteins

Mycolic acids are major and specific lipid components of the cell envelope of mycobacteria that include the causative agents of tuberculosis and leprosy, Mycobacterium tuberculosis and Mycobacterium leprae, respectively. Subtle structural variations that are known to be crucial for both their virulence and the permeability of their cell envelope occur in mycolic acids. Among these are the introduction of cyclopropyl groups and methyl branches by mycolic acid S-adenosylmethionine-dependent methyltransferases (MA-MTs). While the functions of seven of the M. tuberculosis MA-MTs have been either established or strongly presumed nothing is known of the roles of the remaining umaA gene product and those of M. smegmatis MA-MTs. Mutants of the M. tuberculosis umaA gene and its putative M. smegmatis orthologue, MSMEG0913, were created. The lipid extracts of the resulting mutants were analyzed in detail using a combination of analytical techniques such as matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and proton nuclear magnetic resonance spectroscopy, and chemical degradation methods. The M. smegmatis mutants no longer synthesized subtypes of mycolates containing a methyl branch adjacent to either trans cyclopropyl group or trans double bond at the "proximal" position of both alpha- and epoxy-mycolates. Complementation with MSMEG0913, but not with umaA, fully restored the wild-type phenotype in M. smegmatis. Consistently, no modification was observed in the structures of mycolic acids produced by the M. tuberculosis umaA mutant. These data proved that despite their synteny and high similarity umaA and MSMEG0913 are not functionally orthologous.     

Purification,cloning, expression,and properties of mycobacterial trehalose-phosphate phosphatase

The trehalose-phosphate phosphatase (TPP) was purified from the cytosol of Mycobacterium smegmatis to near homogeneity using a variety of conventional steps to achieve a purification of about 1600-fold with a yield of active enzyme of about 1%. Based on gel filtration, the active enzyme had a molecular weight of about 27,000, and the most purified fraction also gave a major band on SDS-PAGE corresponding to a molecular weight of about 27,000. A number of peptides from the 27-kDa protein were sequenced and these sequences showed considerable homology to the trehalose-P phosphatase (otsB) of Escherichia coli. Based on these peptides, the M. smegmatis gene for TPP was cloned and expressed in E. coli. The recombinant protein was synthesized with a (His)(6) tag at the amino terminus. Most of the TPP activity in the crude E. coli sonicate was initially found in the membrane fraction, but it became solubilized in the presence of 0.2% Sarkosyl. The solubilized protein was purified to apparent homogeneity on a metal ion column and this fraction had high phosphatase activity that was completely specific for trehalose-P. The purified enzyme, either isolated from M. smegmatis, or expressed in E. coli, rapidly dephosphorylated trehalose-6-P, but had essentially no activity on any other sugar phosphates, or on p-nitrophenyl phosphate. The K(m) for trehalose-6-P was about 1.6 mm, and the pH optimum was about 7.5. The native enzyme showed an almost absolute requirement for Mg(2+) and was not very active with Mn(2+), whereas both of these cations were equally effective with the recombinant TPP. The enzyme activity was inhibited by the antibiotics, diumycin and moenomycin, but not by a number of other antibiotics or trehalose analogs. TPP activity was strongly inhibited by the detergents, Sarkosyl and deoxycholate, even at 0.025%, but it was not inhibited by Nonidet P-40, Triton X-100, or octyl glucoside, even at concentrations up to 0.3%. The purified enzyme was stable to heating at 60 degrees C for up to 6 min, but was slowly inactivated at 70 degrees C. Circular dichroism studies on recombinant TPP indicate that the secondary structure of this protein has considerable beta-pleated sheet and is very compact. TPP may play a key role in the biosynthesis of trehalose compounds, such as trehalose mycolates, and therefore may represent an excellent target site for chemotherapy against tuberculosis and other mycobacterial diseases.     

Identification of the mycothiol synthase gene (mshD) encoding the acetyltransferase producing mycothiol in actinomycetes

Mycothiol is the predominant thiol in most actinomycetes, including Mycobacterium tuberculosis, and appears to play a role analogous to glutathione, which is not found in these bacteria. The enzymes involved in mycothiol biosynthesis are of interest as potential targets for new drugs directed against tuberculosis. In this work we describe the isolation and characterization of a Tn 5 transposon mutant of Mycobacterium smegmatis that is blocked in the production of mycothiol and accumulates its precursor, 1 D-myo-inosityl 2- L-cysteinylamido-2-deoxy-alpha-D-glucopyranoside (Cys-GlcN-Ins). Cys-GlcN-Ins isolated from this mutant was used to assay for acetyl-CoA:Cys-GlcN-Ins acetyltransferase (mycothiol synthase, MshD) activity, which was found in wild-type cells, but not in the mutant. Sequencing outward of the DNA of the mutant strain from the site of insertion permitted identification of the mshD gene in the M. smegmatis genome, as well as the orthologous gene Rv0819 in the M. tuberculosis genome. Cloning and expression of mshD from M. tuberculosis (Rv0819) in Escherichia coli gave a transformant with MshD activity, demonstrating that Rv0819 is the mshD mycothiol biosynthesis gene.     

5'-adenosinephosphosulfate lies at a metabolic branch point in mycobacteria

Bacterial sulfate assimilation pathways provide for activation of inorganic sulfur for the biosynthesis of cysteine and methionine, through either adenosine 5'-phosphosulfate (APS) or 3'-phosphoadenosine 5'-phosphosulfate (PAPS) as intermediates. PAPS is also the substrate for sulfotransferases that produce sulfolipids, putative virulence factors, in Mycobacterium tuberculosis such as SL-1. In this report, genetic complementation using Escherichia coli mutant strains deficient in APS kinase and PAPS reductase was used to define the M. tuberculosis and Mycobacterium smegmatis CysH enzymes as APS reductases. Consequently, the sulfate assimilation pathway of M. tuberculosis proceeds from sulfate through APS, which is acted on by APS reductase in the first committed step toward cysteine and methionine. Thus, M. tuberculosis most likely produces PAPS for the sole use of this organism's sulfotransferases. Deletion of CysH from M. smegmatis afforded a cysteine and methionine auxotroph consistent with a metabolic branch point centered on APS. In addition, we have redefined the substrate specificity of the B. subtilis CysH, formerly designated a PAPS reductase, as an APS reductase, based on its ability to complement a mutant E. coli strain deficient in APS kinase. Together, these studies show that two conserved sequence motifs, CCXXRKXXPL and SXGCXXCT, found in the C termini of all APS reductases, but not in PAPS reductases, may be used to predict the substrate specificity of these enzymes. A functional domain of the M. tuberculosis CysC protein was cloned and expressed in E. coli, confirming the ability of this organism to make PAPS. The expression of recombinant M. tuberculosis APS kinase provides a means for the discovery of inhibitors of this enzyme and thus of the biosynthesis of SL-1.     

Expression and purification of immunologically reactive DPPD, a recombinant Mycobacterium tuberculosis skin test antigen, using Mycobacterium smegmatis and Escherichia coli host cells

DPPD is a Mycobacterium tuberculosis recombinant antigen that elicits specific delayed type hypersensitivity reactions similar in size and morphological aspects to that elicited by purified protein derivative, in both guinea pigs and humans infected with M. tuberculosis. In addition, earlier clinical studies with DPPD suggested that this molecule could improve the specificity of the tuberculin skin test, which is used as an important aid for the diagnosis of tuberculosis. However, these studies could only be performed with DPPD engineered as a fusion molecule with another Mycobacterium spp. protein because no expression of DPPD could be achieved as a single molecule or as a conventional fusion protein in any commercial system. Although recombinant fusion proteins are in general suitable for several biological studies, they are by definition not ideal for studies involving highly purified and defined polypeptide sequences. Here, we report two alternative approaches for the expression of immunologically reactive recombinant genuine DPPD. The first approach used the rapidly growing, nonpathogenic Mycobacterium smegmatis as host cells transformed with the pSMT3 plasmid vector containing the full-length DPPD gene. The second approach used Escherichia coli transformed with the pET-17b plasmid vector containing the DPPD gene engineered in a three-copy fusion manner in tandem with itself. Though at low levels, expression and purification of immunologically reactive DPPD in M. smegmatis could be achieved. More abundant expression and purification of DPPD as a homo-trimer molecule was achieved in E. coli (> or =2 mg/L of bacterial broth cultures). Interestingly, expression could only be achieved in host cells transformed with the DPPD gene containing its leader peptide. However, the expressed proteins lacked the leader sequence, which indicates that processing of the M. tuberculosis DPPD gene was accurately achieved and necessary in both M. smegmatis and E. coli. More importantly, the delayed type hypersensitivity reactions elicited by purified molecules in guinea pigs infected with M. tuberculosis were indistinguishable from that elicited by purified protein derivative. Because the DPPD gene is present only in the tuberculosis-complex organisms of the Mycobacterium genus, these highly purified molecules should be helpful in identifying individuals sensitized with tubercle bacilli.