Hyperglycosylation of glycopeptidolipid of Mycobacterium smegmatis under nutrient starvation: structural studies

Expression, Purification, and In Silico Characterization of Mycobacterium smegmatis Alternative Sigma Factor SigB

Sigma factor B (SigB), an alternative sigma factor (ASF), is very similar to primary sigma factor SigA (σ ⁷⁰) but dispensable for growth in both Mycobacterium smegmatis (Msmeg) and Mycobacterium tuberculosis (Mtb). It is involved in general stress responses including heat, oxidative, surface, starvation stress, and macrophage infections. Despite having an extremely short half-life, SigB tends to operate downstream of at least three stress-responsive extra cytoplasmic function (ECF) sigma factors (SigH, SigE, SigL) and SigF involved in multiple signaling pathways. There is very little information available regarding the regulation of SigB sigma factor and its interacting protein partners. Hence, we cloned the SigB gene into pET28a vector and optimized its expression in three different strains of E. coli, viz., (BL21 (DE3), C41 (DE3), and CodonPlus (DE3)). We also optimized several other parameters for the expression of recombinant SigB including IPTG concentration, temperature, and time duration. We achieved the maximum expression of SigB at 25°C in the soluble fraction of the cell which was purified by affinity chromatography using Ni-NTA and further confirmed by Western blotting. Further, structural characterization demonstrates the instability of SigB in comparison to SigA that is carried out using homology modeling and structure function relationship. We have done protein-protein docking of RNA polymerase (RNAP) of Msmeg and SigB. This effort provides a platform for pulldown assay, structural, and other studies with the recombinant protein to deduce the SigB interacting proteins, which might pave the way to study its signaling networks along with its regulation.

Glycopeptidolipid glycosylation controls surface properties and pathogenicity in Mycobacterium abscessus

Mycobacterium abscessus is an emerging and difficult-to-manage mycobacterial species that exhibits smooth (S) or rough (R) morphotypes. Disruption of glycopeptidolipid (GPL) production results in transition from S to R and severe lung disease. A structure-activity relationship study was undertaken to decipher the role of GPL glycosylation in morphotype transition and pathogenesis. Deletion of gtf3 uncovered the prominent role of the extra rhamnose in enhancing mannose receptor-mediated internalization of M. abscessus by macrophages. In contrast, the absence of the 6-deoxy-talose and the first rhamnose in mutants lacking gtf1 and gtf2, respectively, affected M abscessus phagocytosis but also resulted in the S-to-R transition. Strikingly, gtf1 and gtf2 mutants displayed a strong propensity to form cords and abscesses in zebrafish, leading to robust and lethal infection. Together, these results underscore the importance and differential contribution of GPL monosaccharides in promoting virulence and infection outcomes.

Mutation on lysX from Mycobacterium avium hominissuis impacts the host-pathogen interaction and virulence phenotype

The lysX gene from Mycobacterium avium hominissuis (MAH) is not only involved in cationic antimicrobial resistance but also regulates metabolic activity. An MAH lysX deficient mutant was shown to exhibit a metabolic shift at the extracellular state preadapting the bacteria to the conditions inside host-cells. It further showed stronger growth in human monocytes. In the present study, the LysX activity on host-pathogen interactions were analyzed. The lysX mutant from MAH proved to be more sensitive toward host-mediated stresses such as reactive oxygen species. Further, the lysX mutant exhibited increased inflammatory response in PBMC and multinucleated giant cell (MGC) formation in human macrophages during infection studies. Coincidentally, the lysX mutant strain revealed to be more reproductive in the Galleria mellonella infection model. Together, these data demonstrate that LysX plays a role in regulating the bacillary load in host organisms and the lack of lysX gene facilitates MAH adaptation to intracellular host-habitat, thereby suggesting an essential role of LysX in the modulation of host-pathogen interaction.

Aggregation of Nontuberculous Mycobacteria Is Regulated by Carbon-Nitrogen Balance

Free-living bacteria can assemble into multicellular structures called biofilms. Biofilms help bacteria tolerate multiple stresses, including antibiotics and the host immune system. Nontuberculous mycobacteria are a group of emerging opportunistic pathogens that utilize biofilms to adhere to household plumbing and showerheads and to avoid phagocytosis by host immune cells. Typically, bacteria regulate biofilm formation by controlling expression of adhesive structures to attach to surfaces and other bacterial cells. Mycobacteria harbor a unique cell wall built chiefly of long-chain mycolic acids that confers hydrophobicity and has been thought to cause constitutive aggregation in liquid media. Here we show that aggregation is instead a regulated process dictated by the balance of available carbon and nitrogen. Understanding that mycobacteria utilize metabolic cues to regulate the transition between planktonic and aggregated cells reveals an inroad to controlling biofilm formation through targeted therapeutics.

Nontuberculous mycobacteria (NTM) are emerging opportunistic pathogens that colonize household water systems and cause chronic lung infections in susceptible patients. The ability of NTM to form surface-attached biofilms in the nonhost environment and corded aggregates in vivo is important to their ability to persist in both contexts. Underlying the development of these multicellular structures is the capacity of mycobacterial cells to adhere to one another. Unlike most other bacteria, NTM spontaneously and constitutively aggregate in vitro, hindering our ability to understand the transition between planktonic and aggregated cells. While culturing a model NTM, Mycobacterium smegmatis, in rich medium, we fortuitously discovered that planktonic cells accumulate after ∼3 days of growth. By providing selective pressure for bacteria that disperse earlier, we isolated a strain with two mutations in the oligopeptide permease operon (opp). A mutant lacking the opp operon (Δopp) disperses earlier than wild type (WT) due to a defect in nutrient uptake. Experiments with WT M. smegmatis revealed that growth as aggregates is favored when carbon is replete, but under conditions of low available carbon relative to available nitrogen, M. smegmatis grows as planktonic cells. By adjusting carbon and nitrogen sources in defined medium, we tuned the cellular C/N ratio such that M. smegmatis grows either as aggregates or as planktonic cells. C/N-mediated aggregation regulation is widespread among NTM with the possible exception of rough-colony Mycobacterium abscessus isolates. Altogether, we show that NTM aggregation is a controlled process that is governed by the relative availability of carbon and nitrogen for metabolism.

Synthetic Arabinomannan Heptasaccharide Glycolipids Inhibit Biofilm Growth and Augment Isoniazid Effects in Mycobacterium smegmatis

Biofilm formation, involving attachment to an adherent surface, is a critical survival strategy of mycobacterial colonies in hostile environmental conditions. Here we report the synthesis of heptasaccharide glycolipids based on mannopyranoside units anchored on to a branched arabinofuranoside core. Two types of glycolipids-2,3-branched and 2,5-branched-were synthesized and evaluated for their efficacies in inhibiting biofilm growth by the non-pathogenic mycobacterium variant Mycobacterium smegmatis. Biofilm formation was inhibited at a minimum biofilm growth inhibition concentration (MBIC) of 100 μg mL^-1 in the case of the 2,5-branched heptasaccharide glycolipid. Further, we were able to ascertain that a combination of the drug isoniazid with the branched heptasaccharide glycolipid (50 μg mL^-1 ) potentiates the drug, making it three times more effective, with an improved MBIC of 30 μg mL^-1 . These studies establish that synthetic glycolipids not only act as inhibitors of biofilm growth, but also provide a synergistic effect when combined with significantly lowered concentrations of isoniazid to disrupt the biofilm structures of the mycobacteria.

Sigma Factors: Key Molecules in Mycobacterium tuberculosis Physiology and Virulence

Rapid adaptation to changing environments is one of the keys to the success of microorganisms. Since infection is a dynamic process, it is possible to predict that Mycobacterium tuberculosis adaptation involves continuous modulation of its global transcriptional profile in response to the changing environment found in the human body. In the last 18 years several studies have stressed the role of sigma (σ) factors in this process. These are small interchangeable subunits of the RNA polymerase holoenzyme that are required for transcriptional initiation and that determine promoter specificity. The M. tuberculosis genome encodes 13 of these proteins, one of which--the principal σ factor σA--is essential. Of the other 12 σ factors, at least 6 are required for virulence. In this article we review our current knowledge of mycobacterial σ factors, their regulons, the complex mechanisms determining their regulation, and their roles in M. tuberculosis physiology and virulence.

A GMC oxidoreductase homologue is required for acetylation of glycopeptidolipid in Mycobacterium smegmatis

The Mycobacterium tuberculosis Rv3409c gene is required for modulation of the Toll-like receptor 2 (TLR-2) signaling response in infected macrophages. Although each is annotated as encoding a cholesterol oxidase, neither Rv3409c nor its ortholog MSMEG1604 is required for the metabolism of cholesterol in mycobacteria. Here we report that a unique lipid, L1334, accumulates in a MSMEG1604 transposon mutant in the Mycobacterium smegmatis cell envelope. L1334 is a polar glycopeptidolipid that is hyperrhamnosylated and in which the 6-deoxytalose moiety is not acetylated. The alteration of L1334 acetylation is consistent with a reduced level of interference with TLR-2 signaling in mutant infected macrophages.

Rough colony morphology of Mycobacterium massiliense Type II genotype is due to the deletion of glycopeptidolipid locus within its genome

Background

Recently, we introduced the complete genome sequence of Mycobacterium massiliense clinical isolates, Asan 50594 belonging to Type II genotype with rough colony morphology. Here, to address the issue of whether the rough colony morphotype of M. massiliense Type II genotype is genetically determined or not, we compared polymorphisms of the glycopeptidolipid (GPL) gene locus between M. massiliense Type II Asan 50594 and other rapidly growing mycobacteria (RGM) strains via analysis of genome databases.

Results

We found deletions of 10 genes (24.8 kb), in the GPL biosynthesis related gene cluster of Asan 50594 genome, but no deletions in those of other smooth RGMs. To check the presence of deletions of GPL biosynthesis related genes in Mycobacterium abscessus − complex strains, PCRs targeting 12 different GPL genes (10 genes deleted in Asan 50594 genome as well as 2 conserved genes) were applied into 76 clinical strains of the M. abscessus complex strains [54 strains (Type I: 33, and Type II: 21) of M. massiliense and 22 strains (rough morphoype: 11 and smooth morphotype: 11) of M. abscessus]. No strains of the Type II genotype produced PCR amplicons in a total of 10 deleted GPL genes, suggesting loss of GPL biosynthesis genes in the genome of M. massiliense type II genotype strains.

Conclusions

Our data suggested that the rough colony morphotype of the M. massiliense Type II genotype may be acquired via deletion events at the GPL gene locus for evolutionary adaptation between the host and pathogen.

Structure and function of mycobacterium glycopeptidolipids from comparative genomics perspective

Glycopeptidolipids (GPLs) attached to the outer surface of the greasy cell envelope, are a class of important glycolipids synthesized by several non-tuberculosis mycobacteria. The deletion or structure change of GPLs confers several phenotypical changes including colony morphology, hydrophobicity, aggregation, sliding motility, and biofilm formation. In addition, GPLs, particular serovar specific GPLs, are important immunomodulators. This review aims to summarize the advance on the structure, function and biosynthesis of mycobacterium GPLs.

Retrobiosynthetic approach delineates the biosynthetic pathway and the structure of the acyl chain of mycobacterial glycopeptidolipids

Glycopeptidolipids (GPLs) are dominant cell surface molecules present in several non-tuberculous and opportunistic mycobacterial species. GPLs from Mycobacterium smegmatis are composed of a lipopeptide core unit consisting of a modified C(26)-C(34) fatty acyl chain that is linked to a tetrapeptide (Phe-Thr-Ala-alaninol). The hydroxyl groups of threonine and terminal alaninol are further modified by glycosylations. Although chemical structures have been reported for 16 GPLs from diverse mycobacteria, there is still ambiguity in identifying the exact position of the hydroxyl group on the fatty acyl chain. Moreover, the enzymes involved in the biosynthesis of the fatty acyl component are unknown. In this study we show that a bimodular polyketide synthase in conjunction with a fatty acyl-AMP ligase dictates the synthesis of fatty acyl chain of GPL. Based on genetic, biochemical, and structural investigations, we determine that the hydroxyl group is present at the C-5 position of the fatty acyl component. Our retrobiosynthetic approach has provided a means to understand the biosynthesis of GPLs and also resolve the long-standing debate on the accurate structure of mycobacterial GPLs.

Glycopeptidolipids: immuno-modulators in greasy mycobacterial cell envelope

Species of opportunistic mycobacteria are the major causative agent for disseminating pulmonary infections in immuno-compromised individuals. These naturally resistant strains recruit a unique type of glycolipid known as glycopeptidolipids (GPLs), noncovalently attached to the outer surface of their thick lipid rich cell envelope. Species specific GPLs constitute the chemical determinants of most nontuberculous mycobacterial serotypes, and their absence from the cell surface confers altered colony morphology, hydrophobicity, and inability to grow as biofilms. The objective of this review is to present a comprehensive account and highlight the renewed interest on this much neglected group of pleiotropic molecules with respect to their structural diversity and biosynthesis. In addition, the role of GPLs in mycobacterial survival, both intracellular and in the environment is also discussed. It also explores the possibility of identifying new targets for intervening Mycobacterium avium complex-related infections. These antigenic molecules have been considered to play a pivotal role in immune suppression and can also induce various cytokine mediated innate immune responses, the molecular mechanism of which remains obscure.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for the period 2005-2006

This review is the fourth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2006. The review covers fundamental studies, fragmentation of carbohydrate ions, method developments, and applications of the technique to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, glycated proteins, glycolipids from bacteria, glycosides, and various other natural products. There is a short section on the use of MALDI-TOF mass spectrometry for the study of enzymes involved in glycan processing, a section on industrial processes, particularly the development of biopharmaceuticals and a section on the use of MALDI-MS to monitor products of chemical synthesis of carbohydrates. Large carbohydrate-protein complexes and glycodendrimers are highlighted in this final section.

Mycobacterium avium glycopeptidolipids require specific acetylation and methylation patterns for signaling through toll-like receptor 2

Toll-like receptors (TLRs) recognize pathogen-associated molecules and play a vital role in promoting an immune response against invading microbes. TLR2, one of the key members of the TLR family, recognizes a wide variety of microbial products, including lipoproteins and lipopeptides, from a number of pathogens. Recent studies from our laboratory indicate that glycopeptidolipids (GPLs), a major surface component of Mycobacterium avium and other non-tuberculosis mycobacteria, are ligands for TLR2. However, the molecular requirements necessary for the GPL-TLR2 interaction were not defined in this report. In the present study we isolated different GPL species from M. avium, and using mass spectrometry and NMR analyses, characterized the molecular requirements of the GPL-TLR2 interaction. Interestingly, the extent of the respective acetylation and methylation of the 6-deoxytalose and rhamnose contained within the core GPL structure dictated whether the GPL signaled through TLR2. These experiments illustrate how subtle changes in a complex TLR2 ligand can alter its affinity for this important receptor, and suggest that M. avium could potentially modify its GPL structure to limit its interaction with TLR2.

The mycobacterial glycopeptidolipids: structure, function, and their role in pathogenesis

Glycopeptidolipids (GPLs) are a class of glycolipids produced by several nontuberculosis-causing members of the Mycobacterium genus including pathogenic and nonpathogenic species. GPLs are expressed in different forms with production of highly antigenic, typeable serovar-specific GPLs in members of the Mycobacterium avium complex (MAC). M. avium and M. intracellulare, which comprise this complex, are slow-growing mycobacteria noted for producing disseminated infections in AIDS patients and pulmonary infections in non-AIDS patients. Previous studies have defined the gene cluster responsible for GPL biosynthesis and more recent work has characterized the function of the individual genes. Current research has also focused on the GPL's role in colony morphology, sliding motility, biofilm formation, immune modulation and virulence. These topics, along with new information on the enzymes involved in GPL biosynthesis, are the subject of this review.

Proteomics and mass spectrometric studies reveal planktonic growth of Mycobacterium smegmatis in biofilm cultures in the absence of rpoZ

Mycobacterium smegmatis is known to form biofilms and many cell surface molecules like core glycopeptidolipids and short-chain mycolates appear to play important role in the process. However, the involvement of the cell surface molecules in mycobacteria towards complete maturation of biofilms is still not clear. This work demonstrates the importance of the glycopeptidolipid species with hydroxylated alkyl chain and the epoxylated mycolic acids, during the process of biofilm development. In our previous study, we reported the impairment of biofilm formation in rpoZ-deleted M. smegmatis, where rpoZ codes for the omega subunit of RNA polymerase (R. Mathew, R. Mukherjee, R. Balachandar, D. Chatterji, Microbiology 152 (2006) 1741). Here we report the occurrence of planktonic growth in a mc(2)155 strain which is devoid of rpoZ gene. This strain is deficient in selective incorporation of the hydroxylated glycopeptidolipids and the epoxy mycolates to their respective locations in the cell wall. Hence it forms a mutant biofilm defective in maturation, wherein the cells undertake various alternative metabolic pathways to survive in an environment where oxygen, the terminal electron acceptor, is limiting.

Roles of SigB and SigF in the Mycobacterium tuberculosis sigma factor network

To characterize the roles of SigB and SigF in sigma factor regulation in Mycobacterium tuberculosis, we used chemically inducible recombinant strains to conditionally overexpress sigB and sigF. Using whole genomic microarray analysis and quantitative reverse transcription-PCR, we investigated the resulting global transcriptional changes after sigB induction, and we specifically tested the relative expression of other sigma factor genes after knock-in expression of sigB and sigF. Overexpression of sigB resulted in significant upregulation of genes encoding several early culture filtrate antigens (ESAT-6-like proteins), ribosomal proteins, PE-PGRS proteins, the keto-acyl synthase, KasA, and the regulatory proteins WhiB2 and IdeR. Of note, the induction of sigB did not alter the expression of other sigma factor genes, indicating that SigB is likely to serve as an end regulator for at least one branch of the M. tuberculosis sigma factor regulatory cascade. Analysis of the 5'-untranslated region (UTR) of SigB-dependent transcripts revealed a putative consensus sequence of NGTGG-N(14-18)-NNGNNG. This sequence appeared upstream of both sigB (Rv2710) and the gene following it, ideR (Rv2711), and in vitro transcription analysis with recombinant SigB-reconstituted RNA polymerase confirmed SigB-dependent transcription from each of these promoters. Knock-in expression of sigF revealed that only the sigC gene was significantly upregulated 6 and 12 h after sigF induction. The previously identified SigF promoter consensus sequence AGTTTG-N(15)-GGGTTT was identified in the 5' UTR of the sigC gene, and SigF-dependent in vitro transcription of the promoter upstream of sigC was confirmed by using recombinant SigF-reconstituted RNA polymerase. These two knock-in recombinant strains were tested in a macrophage model of infection which showed that overexpression of sigB and sigF resulted in reduced rates of M. tuberculosis intracellular growth. These results define the SigB promoter consensus recognition sequence and members of the SigB regulon. Moreover, the data suggest that, in addition to serving as an end regulator in a sigma factor cascade, SigB may auto-amplify its own expression under certain conditions.

Genomics of glycopeptidolipid biosynthesis in Mycobacterium abscessus and M. chelonae

Background

The outermost layer of the bacterial surface is of crucial importance because it is in constant interaction with the host. Glycopeptidolipids (GPLs) are major surface glycolipids present on various mycobacterial species. In the fast-grower model organism Mycobacterium smegmatis, GPL biosynthesis involves approximately 30 genes all mapping to a single region of 65 kb.

Results

We have recently sequenced the complete genomes of two fast-growers causing human infections, Mycobacterium abscessus (CIP 104536T) and M. chelonae (CIP 104535T). We show here that these two species contain genes corresponding to all those of the M. smegmatis "GPL locus", with extensive conservation of the predicted protein sequences consistent with the production of GPL molecules indistinguishable by biochemical analysis. However, the GPL locus appears to be split into several parts in M. chelonae and M. abscessus. One large cluster (19 genes) comprises all genes involved in the synthesis of the tripeptide-aminoalcohol moiety, the glycosylation of the lipopeptide and methylation/acetylation modifications. We provide evidence that a duplicated acetyltransferase (atf1 and atf2) in M. abscessus and M. chelonae has evolved through specialization, being able to transfer one acetyl at once in a sequential manner. There is a second smaller and distant (M. chelonae, 900 kb; M. abscessus, 3 Mb) cluster of six genes involved in the synthesis of the fatty acyl moiety and its attachment to the tripeptide-aminoalcohol moiety. The other genes are scattered throughout the genome, including two genes encoding putative regulatory proteins.

Conclusion

Although these three species produce identical GPL molecules, the organization of GPL genes differ between them, thus constituting species-specific signatures. An hypothesis is that the compact organization of the GPL locus in M. smegmatis represents the ancestral form and that evolution has scattered various pieces throughout the genome in M. abscessus and M. chelonae.

The sigma factors of Mycobacterium tuberculosis

Mycobacterium tuberculosis is a remarkable pathogen capable of adapting and surviving in various harsh conditions. Correct gene expression regulation is essential for the success of this process. The reversible association of different sigma factors is a common mechanism for reprogramming bacterial RNA polymerase and modulating the transcription of numerous genes. Thirteen putative sigma factors are encoded in the M. tuberculosis genome, several being important for virulence. Here, we analyse the latest information available on mycobacterial sigma factors and discuss their roles in the physiology and virulence of M. tuberculosis.

Deletion of the rpoZ gene, encoding the ω subunit of RNA polymerase, results in pleiotropic surface-related phenotypes in Mycobacterium smegmatis

Theωsubunit, the smallest subunit of bacterial RNA polymerase, is known to be involved in maintaining the conformation of theβ′ subunit and aiding its recruitment to the rest of the core enzyme assembly inEscherichia coli. It has recently been shown inMycobacterium smegmatis, by creating a deletion mutation of therpoZgene encodingω, that the physiological role of theωsubunit also includes providing physical protection toβ′. Interestingly, the mutant had altered colony morphology. This paper demonstrates that the mutant mycobacterium has pleiotropic phenotypes including reduced sliding motility and defective biofilm formation. Analysis of the spatial arrangement of biofilms by electron microscopy suggests that the altered phenotype of the mutant arises from a deficiency in generation of extracellular matrix. Complementation of the mutant strain with a copy of the wild-typerpoZgene integrated in the bacterial chromosome restored both sliding motility and biofilm formation to the wild-type state, unequivocally proving the role ofωin the characteristics observed for the mutant bacterium. Analysis of the cell wall composition demonstrated that the mutant bacterium had an identical glycopeptidolipid profile to the wild-type, but failed to synthesize the short-chain mycolic acids characteristic of biofilm growth inM. smegmatis.

Identification and characterization of the genes involved in glycosylation pathways of mycobacterial glycopeptidolipid biosynthesis

Glycopeptidolipids (GPLs) are major components present on the outer layers of the cell walls of several nontuberculous mycobacteria. GPLs are antigenic molecules and have variant oligosaccharides in mycobacteria such as Mycobacterium avium. In this study, we identified four genes (gtf1, gtf2, gtf3, and gtf4) in the genome of Mycobacterium smegmatis. These genes were independently inactivated by homologous recombination in M. smegmatis, and the structures of GPLs from each gene disruptant were analyzed. Thin-layer chromatography, gas chromatography-mass spectrometry, and matrix-assisted laser desorption ionization-time-of-flight mass spectrometry analyses revealed that the mutants Deltagtf1 and Deltagtf2 accumulated the fatty acyl-tetrapeptide core having O-methyl-rhamnose and 6-deoxy-talose as sugar residues, respectively. The mutant Deltagtf4 possessed the same GPLs as the wild type, whereas the mutant Deltagtf3 lacked two minor GPLs, consisting of 3-O-methyl-rhamnose attached to O-methyl-rhamnose of the fatty acyl-tetrapeptide core. These results indicate that the gtf1 and gtf2 genes are responsible for the early glycosylation steps of GPL biosynthesis and the gtf3 gene is involved in transferring a rhamnose residue not to 6-deoxy-talose but to an O-methyl-rhamnose residue. Moreover, a complementation experiment showed that M. avium gtfA and gtfB, which are deduced glycosyltransferase genes of GPL biosynthesis, restore complete GPL production in the mutants Deltagtf1 and Deltagtf2, respectively. Our findings propose that both M. smegmatis and M. avium have the common glycosylation pathway in the early steps of GPL biosynthesis but differ at the later stages.

A glycosyltransferase involved in biosynthesis of triglycosylated glycopeptidolipids in Mycobacterium smegmatis: impact on surface properties

The cell envelope of mycobacteria is a complex structure that plays an important role in the interactions of the cell with its environment and in the protection against the antimicrobial activity of the immune system. Glycopeptidolipids (GPLs) are species- or type species-specific glycolipids that are present at the surface of a number of mycobacteria and that are characterized by a high variability in glycosylation patterns. These GPLs possess various biological activities that depend mostly on the sugars capping the core molecule. In Mycobacterium smegmatis, the GPL core can be substituted by either two or three deoxyhexoses. In this study, we show that Gtf3 is a glycosyltransferase responsible for the synthesis of the triglycosylated GPLs. Biochemical analysis of these molecules, with a combination of mass spectrometry and chemical degradation methods, has shown that they contain three deoxyhexose moieties. The presence of the triglycosylated GPLs is associated with cell surface modifications that lead to a decrease in sliding motility as well as a modification in cellular aggregation and colony appearance on Congo red. Phylogenetic analysis indicated that Gtf3 is a member of a yet-uncharacterized glycosyltransferase family conserved among the mycobacteria.

Evaluation of the role of sigma B in Mycobacterium smegmatis

The alternate sigma factor, sigB, is known to play a crucial role in maintaining the stationary phase in mycobacteria. In this communication, we have studied the proteomics of Mycobacterium smegmatis mc(2)155 and its two derivatives, one of which has a disrupted sigB gene and the other, PMVSigB, which contains a multicopy plasmid containing sigB. We have identified by two-dimensional gel analyses, several proteins that are over-expressed in PMVSigB compared to mc(2)155. These proteins are either stress proteins or participate actively in different metabolic pathways of the organisms. On the other hand, when sigB deleted mycobacteria were grown until the stationary phase and its two-dimensional protein profile was compared to that of mc(2)155, few DNA binding proteins were found to be up-regulated. We have shown recently that upon over-expressing sigB, the cell surface glycopeptidolipids of M. smegmatis are hyperglycosylated, a situation similar to what was observed for nutritionally starved bacteria. Gene expression profile through quantitative PCR presented here identified a Rhamnosyltransferase responsible for this hyperglycosylation.