UDP-N-acetylglucosamine:N-acetylmuramoyl-(pentapeptide) pyrophosphoryl undecaprenol N-acetylglucosamine transferase from Escherichia coli: overproduction, solubilization, and purification

Discovery of an alternative pathway of peptidoglycan biosynthesis: A new target for pathway specific inhibitors

Peptidoglycan in bacterial cell walls is a biopolymer consisting of sugars and amino acids and plays important role in maintaining cell integrity from the environment. Its biosynthesis is a major target for antibiotics and the genes and enzymes involved in the biosynthetic pathway have been well studied. However, we recently identified an alternative pathway in the early stage of peptidoglycan biosynthesis in Xanthomonas oryzae, a plant pathogen causing bacterial blight disease of rice. The distribution of the alternative pathway is limited to relatively few bacterial genera that contain many pathogenic species, including Xylella and Stenotrophomonas, besides Xanthomonas. Thus, the alternative pathway is an attractive target for the development of narrow spectrum antibiotics specific to pathogens. In this minireview, we summarize the discovery of the alternative pathway and identification of its specific inhibitors.

The Membrane Steps of Bacterial Cell Wall Synthesis as Antibiotic Targets

Peptidoglycan is the major component of the cell envelope of virtually all bacteria. It has structural roles and acts as a selective sieve for molecules from the outer environment. Peptidoglycan synthesis is therefore one of the most important biogenesis pathways in bacteria and has been studied extensively over the last twenty years. The pathway starts in the cytoplasm, continues in the cytoplasmic membrane and finishes in the periplasmic space, where the precursor is polymerized into the peptidoglycan layer. A number of proteins involved in this pathway, such as the Mur enzymes and the penicillin binding proteins (PBPs), have been studied and regarded as good targets for antibiotics. The present review focuses on the membrane steps of peptidoglycan synthesis that involve two enzymes, MraY and MurG, the inhibitors of these enzymes and the inhibition mechanisms. We also discuss the challenges of targeting these two cytoplasmic membrane (associated) proteins in bacterial cells and the perspectives on how to overcome the issues.

Structural Insights into Protein-Protein Interactions Involved in Bacterial Cell Wall Biogenesis

The bacterial cell wall is essential for survival, and proteins that participate in its biosynthesis have been the targets of antibiotic development efforts for decades. The biosynthesis of its main component, the peptidoglycan, involves the coordinated action of proteins that are involved in multi-member complexes which are essential for cell division (the “divisome”) and/or cell wall elongation (the “elongasome”), in the case of rod-shaped cells. Our knowledge regarding these interactions has greatly benefitted from the visualization of different aspects of the bacterial cell wall and its cytoskeleton by cryoelectron microscopy and tomography, as well as genetic and biochemical screens that have complemented information from high resolution crystal structures of protein complexes involved in divisome or elongasome formation. This review summarizes structural and functional aspects of protein complexes involved in the cytoplasmic and membrane-related steps of peptidoglycan biosynthesis, with a particular focus on protein-protein interactions whereby disruption could lead to the development of novel antibacterial strategies.

Colicin M hydrolyses branched lipids II from Gram-positive bacteria

Lipids II found in some Gram-positive bacteria were prepared in radioactive form from l-lysine-containing UDP-MurNAc-pentapeptide. The specific lateral chains of Enterococcus faecalis, Enterococcus faecium and Staphylococcus aureus (di-L-alanine, D-isoasparagine, and pentaglycine, respectively) were introduced by chemical peptide synthesis using the Fmoc chemistry. The branched nucleotides obtained were converted into the corresponding lipids II by enzymatic synthesis using the MraY and MurG enzymes. All of the lipids were hydrolysed by Escherichia coli colicin M at approximately the same rate as the meso-diaminopimelate-containing lipid II found in Gram-negative bacteria, thereby opening the way to the use of this enzyme as a broad spectrum antibacterial agent.

Synthesis and biological evaluation of potential new inhibitors of the bacterial transferase MraY with a β-ketophosphonate structure

Stable analogs of bacterial transferase MraY substrate or product with a pyrophosphate surrogate in their structure are described. β-ketophosphonates were designed as pyrophosphate bioisosteres and were investigated as UDP-GlcNAc mimics. The developed strategy allows introduction of structural diversity at a late stage of the synthesis. The biological activity of the synthesized compounds was evaluated on the MraY enzyme.

Peptidoglycan biosynthesis machinery: a rich source of drug targets

The range of antibiotic therapy for the control of bacterial infections is becoming increasingly limited because of the rapid rise in multidrug resistance in clinical bacterial isolates. A few diseases, such as tuberculosis, which were once thought to be under control, have re-emerged as serious health threats. These problems have resulted in intensified research to look for new inhibitors for bacterial pathogens. Of late, the peptidoglycan (PG) layer, the most important component of the bacterial cell wall has been the subject of drug targeting because, first, it is essential for the survivability of eubacteria and secondly, it is absent in humans. The last decade has seen tremendous inputs in deciphering the 3-D structures of the PG biosynthetic enzymes. Many inhibitors against these enzymes have been developed using virtual and high throughput screening techniques. This review discusses the mechanistic and structural properties of the PG biosynthetic enzymes and inhibitors developed in the last decade.

Inhibition of Escherichia coli glycosyltransferase MurG and Mycobacterium tuberculosis Gal transferase by uridine-linked transition state mimics

Glycosyltransferase MurG catalyses the transfer of N-acetyl-d-glucosamine to lipid intermediate I on the bacterial peptidoglycan biosynthesis pathway, and is a target for development of new antibacterial agents. A transition state mimic was designed for MurG, containing a functionalised proline, linked through the alpha-carboxylic acid, via a spacer, to a uridine nucleoside. A set of 15 functionalised prolines were synthesised, using a convergent dipolar cycloaddition reaction, which were coupled via either a glycine, proline, sarcosine, or diester linkage to the 5'-position of uridine. The library of 18 final compounds were tested as inhibitors of Escherichia coli glycosyltransferase MurG. Ten compounds showed inhibition of MurG at 1mM concentration, the most active with IC(50) 400microM. The library was also tested against Mycobacterium tuberculosis galactosyltransferase GlfT2, and one compound showed effective inhibition at 1mM concentration.

Human- and plant-pathogenic Pseudomonas species produce bacteriocins exhibiting colicin M-like hydrolase activity towards peptidoglycan precursors

Genes encoding proteins that exhibit similarity to the C-terminal domain of Escherichia coli colicin M were identified in the genomes of some Pseudomonas species, namely, P. aeruginosa, P. syringae, and P. fluorescens. These genes were detected only in a restricted number of strains. In P. aeruginosa, for instance, the colicin M homologue gene was located within the ExoU-containing genomic island A, a large horizontally acquired genetic element and virulence determinant. Here we report the cloning of these genes from the three Pseudomonas species and the purification and biochemical characterization of the different colicin M homologues. All of them were shown to exhibit Mg(2+)-dependent diphosphoric diester hydrolase activity toward the two undecaprenyl phosphate-linked peptidoglycan precursors (lipids I and II) in vitro. In all cases, the site of cleavage was localized between the undecaprenyl and pyrophospho-MurNAc moieties of these precursors. These enzymes were not active on the cytoplasmic precursor UDP-MurNAc-pentapeptide or (or only very poorly) on undecaprenyl pyrophosphate. These colicin M homologues have a narrow range of antibacterial activity. The P. aeruginosa protein at low concentrations was shown to inhibit growth of sensitive P. aeruginosa strains. These proteins thus represent a new class of bacteriocins (pyocins), the first ones reported thus far in the genus Pseudomonas that target peptidoglycan metabolism.

Purification and characterization of the bacterial UDP-GlcNAc:undecaprenyl-phosphate GlcNAc-1-phosphate transferase WecA

To date, the structural and functional characterization of proteins belonging to the polyprenyl-phosphate N-acetylhexosamine-1-phosphate transferase superfamily has been relentlessly held back by problems encountered with their overexpression and purification. In the present work and for the first time, the integral membrane protein WecA that catalyzes the transfer of the GlcNAc-1-phosphate moiety from UDP-GlcNAc onto the carrier lipid undecaprenyl phosphate, yielding undecaprenyl-pyrophosphoryl-GlcNAc, the lipid intermediate involved in the synthesis of various bacterial cell envelope components, was overproduced and purified to near homogeneity in milligram quantities. An enzymatic assay was developed, and the kinetic parameters of WecA as well as the effects of pH, salts, cations, detergents, and temperature on the enzyme activity were determined. A minimal length of 35 carbons was required for the lipid substrate, and tunicamycin was shown to inhibit the enzyme at submicromolar concentrations.

Lipid intermediates in the biosynthesis of bacterial peptidoglycan

This review is an attempt to bring together and critically evaluate the now-abundant but dispersed data concerning the lipid intermediates of the biosynthesis of bacterial peptidoglycan. Lipid I, lipid II, and their modified forms play a key role not only as the specific link between the intracellular synthesis of the peptidoglycan monomer unit and the extracytoplasmic polymerization reactions but also in the attachment of proteins to the bacterial cell wall and in the mechanisms of action of antibiotics with which they form specific complexes. The survey deals first with their detection, purification, structure, and preparation by chemical and enzymatic methods. The recent important advances in the study of transferases MraY and MurG, responsible for the formation of lipids I and II, are reported. Various modifications undergone by lipids I and II are described, especially those occurring in gram-positive organisms. The following section concerns the cellular location of the lipid intermediates and the translocation of lipid II across the cytoplasmic membrane. The great efforts made since 2000 in the study of the glycosyltransferases catalyzing the glycan chain formation with lipid II or analogues are analyzed in detail. Finally, examples of antibiotics forming complexes with the lipid intermediates are presented.

The biosynthesis of peptidoglycan lipid-linked intermediates

The biosynthesis of bacterial cell wall peptidoglycan is a complex process involving many different steps taking place in the cytoplasm (synthesis of the nucleotide precursors) and on the inner and outer sides of the cytoplasmic membrane (assembly and polymerization of the disaccharide-peptide monomer unit, respectively). This review summarizes the current knowledge on the membrane steps leading to the formation of the lipid II intermediate, i.e. the substrate of the polymerization reactions. It makes the point on past and recent data that have significantly contributed to the understanding of the biosynthesis of undecaprenyl phosphate, the carrier lipid required for the anchoring of the peptidoglycan hydrophilic units in the membrane, and to the characterization of the MraY and MurG enzymes which catalyze the successive transfers of the N-acetylmuramoyl-peptide and N-acetylglucosamine moieties onto the carrier lipid, respectively. Enzyme inhibitors and antibacterial compounds interfering with these essential metabolic steps and interesting targets are presented.

Role of the amino acid invariants in the active site of MurG as evaluated by site-directed mutagenesis

To evaluate their role in the active site of the MurG enzyme from Escherichia coli, 13 residues conserved in the sequences of 73 MurG orthologues were submitted to site-directed mutagenesis. All these residues lay within, or close to, the active site of MurG as defined by its tridimensional structure [Ha et al., Prot. Sci. 9 (2000) 1045-1052, and Hu et al., Proc. Natl. Acad. Sci. USA 100 (2003) 845-849]. Thirteen mutants proteins, in which residues T15, H18, Y105, H124, E125, N127, N134, S191, N198, R260, E268, Q288 or N291 have been replaced by alanine, were obtained as the C-terminal His-tagged forms. The effects of the mutations on the activity were checked: (i) by functional complementation of an E. coli murG mutant strain by the mutated genes; and (ii) by the determination of the steady-state kinetic parameters of the purified proteins. Most mutations resulted in an important loss of activity and, in the case of N134A, in the production of a highly unstable protein. The results correlated with the assigned or putative functions of the residues based on the tridimensional structure.

The essential peptidoglycan glycosyltransferase MurG forms a complex with proteins involved in lateral envelope growth as well as with proteins involved in cell division in Escherichia coli

In Escherichia coli many enzymes including MurG are directly involved in the synthesis and assembly of peptidoglycan. MurG is an essential glycosyltransferase catalysing the last intracellular step of peptidoglycan synthesis. To elucidate its role during elongation and division events, localization of MurG using immunofluorescence microscopy was performed. MurG exhibited a random distribution in the cell envelope with a relatively higher intensity at the division site. This mid-cell localization was dependent on the presence of a mature divisome. Its localization in the lateral cell wall appeared to require the presence of MreCD. This could be indicative of a potential interaction between MurG and other proteins. Investigating this by immunoprecipitation revealed the association of MurG with MreB and MraY in the same protein complex. In view of this, the loss of rod shape of ΔmreBCD strain could be ascribed to the loss of MurG membrane localization. Consequently, this could prevent the localized supply of the lipid II precursor to the peptidoglycan synthesizing machinery involved in cell elongation. It is postulated that the involvement of MurG in the peptidoglycan synthesis concurs with two complexes, one implicated in cell elongation and the other in division. A model representing the first complex is proposed.

Colicin M Exerts Its Bacteriolytic Effect via Enzymatic Degradation of Undecaprenyl Phosphate-linked Peptidoglycan Precursors

Colicin M was earlier demonstrated to provoke Escherichia coli cell lysis via inhibition of cell wall peptidoglycan (murein) biosynthesis. As the formation of the O-antigen moiety of lipopolysaccharides was concomitantly blocked, it was hypothesized that the metabolism of undecaprenyl phosphate, an essential carrier lipid shared by these two pathways, should be the target of this colicin. However, the exact target and mechanism of action of colicin M was unknown. Colicin M was now purified to near homogeneity, and its effects on cell wall peptidoglycan metabolism reinvestigated. It is demonstrated that colicin M exhibits both in vitro and in vivo enzymatic properties of degradation of lipid I and lipid II peptidoglycan intermediates. Free undecaprenol and either 1-pyrophospho-MurNAc-pentapeptide or 1-pyrophospho-MurNAc-(pentapeptide)-Glc-NAc were identified as the lipid I and lipid II degradation products, respectively, showing that the cleavage occurred between the lipid moiety and the pyrophosphoryl group. This is the first time such an activity is described. Neither undecaprenyl pyrophosphate nor the peptidoglycan nucleotide precursors were substrates of colicin M, indicating that both undecaprenyl and sugar moieties were essential for activity. The bacteriolytic effect of colicin M therefore appears to be the consequence of an arrest of peptidoglycan polymerization steps provoked by enzymatic degradation of the undecaprenyl phosphate-linked peptidoglycan precursors.

Molecular modeling and site-directed mutagenesis of plant chloroplast monogalactosyldiacylglycerol synthase reveal critical residues for activity

Monogalactosyldiacylglycerol (MGDG), the major lipid of plant and algal plastids, is synthesized by MGD (or MGDG synthase), a dimeric and membrane-bound glycosyltransferase of the plastid envelope that catalyzes the transfer of a galactosyl group from a UDP-galactose donor onto a diacylglycerol acceptor. Although this enzyme is essential for biogenesis, and therefore an interesting target for herbicide design, no structural information is available. MGD monomers share sequence similarity with MURG, a bacterial glycosyltransferase catalyzing the transfer of N-acetyl-glucosamine on Lipid 1. Using the x-ray structure of Escherichia coli MURG as a template, we computed a model for the fold of Spinacia oleracea MGD. This structural prediction was supported by site-directed mutagenesis analyses. The predicted monomer architecture is a double Rossmann fold. The binding site for UDP-galactose was predicted in the cleft separating the two Rossmann folds. Two short segments of MGD (beta2-alpha2 and beta6-beta7 loops) have no counterparts in MURG, and their structure could not be determined. Combining the obtained model with phylogenetic and biochemical information, we collected evidence supporting the beta2-alpha2 loop in the N-domain as likely to be involved in diacylglycerol binding. Additionally, the monotopic insertion of MGD in one membrane leaflet of the plastid envelope occurs very likely at the level of hydrophobic amino acids of the N-terminal domain.

Scintillation proximity assay for inhibitors of Escherichia coli MurG and, optionally, MraY

MurG and MraY, essential enzymes involved in the synthesis of bacterial peptidoglycan, are difficult to assay because the substrates are lipidic and hard to prepare in large quantities. Based on the use of Escherichia coli membranes lacking PBP1b, we report a high-throughput method to measure the activity of MurG and, optionally, MraY as well. In these membranes, incubation with the two peptidoglycan sugar precursors results in accumulation of lipid II rather than the peptidoglycan produced by wild-type membranes. MurG was assayed by addition of UDP-[3H]N-acetylglucosamine to membranes in which lipid I was preformed by incubation with UDP-N-acetyl-muramylpentapeptide, and the product was captured by wheat germ agglutinin scintillation proximity assay beads. In a modification of the assay, the activity of MraY was coupled to that of MurG by addition of both sugar precursors together in a single step. This allows simultaneous detection of inhibitors of either enzyme. Both assays could be performed using wild-type membranes by addition of the transglycosylase inhibitor moenomycin. Nisin and vancomycin inhibited the MurG reaction; the MraY-MurG assay was inhibited by tunicamycin as well. Inhibitors of other enzymes of peptidoglycan synthesis--penicillin G, moenomycin, and bacitracin--had no effect. Surprisingly, however, the beta-lactam cephalosporin C inhibited both the MurG and MraY-MurG assays, indicating a secondary mechanism by which this drug inhibits bacterial growth. In addition, it inhibited NADH dehydrogenase in membranes, a hitherto-unreported activity. These assays can be used to screen for novel antibacterial agents.

Two proteins homologous to the N- and C-terminal domains of the bacterial glycosyltransferase Murg are required for the second step of dolichyl-linked oligosaccharide synthesis in Saccharomyces cerevisiae

Two highly conserved eukaryotic gene products of unknown function showing homology to glycosyltransferases involved in the second steps of bacterial peptidoglycan (Murg) and capsular polysaccharide (Cps14f/Cps14g) biosynthesis have been identified in silico. The amino acid sequence of the eukaryotic protein that is homologous to the lipid acceptor- and membrane-associating N-terminal domain of Murg and the Cps14f beta4-galactosyltransferase enhancer protein is predicted to possess a cleavable signal peptide and transmembrane helices. The other eukaryotic protein is predicted to possess neither transmembrane regions nor a signal peptide but is homologous to the UDP-sugar binding C-terminal domain of Murg and the Cps14g beta4-galactosyltransferase. Both the eukaryotic proteins are encoded by essential genes in Saccharomyces cerevisiae, and down-regulation of either causes growth retardation, reduced N-glycosylation of carboxypeptidase Y, and accumulation of dolichyl-PP-GlcNAc. In vitro studies demonstrate that these proteins are required for transfer of [3H]GlcNAc from UDP-[3H]GlcNAc onto dolichyl-PP-GlcNAc. To conclude, two gene products showing homology to bacterial glycosyltransferases are required for the second step in dolichyl-PP-oligosaccharide biosynthesis.

Purification and characterization of the bacterial MraY translocase catalyzing the first membrane step of peptidoglycan biosynthesis

The MraY translocase catalyzes the first membrane step of bacterial cell wall peptidoglycan synthesis (i.e. the transfer of the phospho-N-acetylmuramoyl-pentapeptide motif onto the undecaprenyl phosphate carrier lipid), a reversible reaction yielding undecaprenylpyrophosphoryl-N-acetylmuramoyl-pentapeptide (lipid intermediate I). This essential integral membrane protein, which is considered as a very promising target for the search of new antibacterial compounds, has thus far been clearly underexploited due to its intrinsic refractory nature to overexpression and purification. We here report conditions for the high level overproduction and for the first time the purification to homogeneity of milligram quantities of MraY protein. The kinetic parameters and effects of pH, salts, cations, and detergents on enzyme activity are described, taking the Bacillus subtilis MraY translocase as a model.

Acceptor specificity and inhibition of the bacterial cell-wall glycosyltransferase MurG

A continuous fluorescence coupled enzyme assay was developed to study the acceptor specificity of the glycosyltransferase MurG toward different lipid I analogues with various substituents replacing the undecaprenyl moiety. It was found that most lipid I analogues are accepted as substrates and, amongst these, the saturated C14 analogue exhibits the best activity. This substrate was used to evaluate the inhibition activity of such antibiotics as moenomycin, vancomycin, and two chlorobiphenyl vancomycin derivatives. A vancomycin derivative with a chlorobiphenyl moiety on the aglycon section was identified as a potent inhibitor of MurG.

Membrane interaction of the glycosyltransferase MurG: a special role for cardiolipin

MurG is a peripheral membrane protein that is one of the key enzymes in peptidoglycan biosynthesis. The crystal structure of Escherichia coli MurG (S. Ha, D. Walker, Y. Shi, and S. Walker, Protein Sci. 9:1045-1052, 2000) contains a hydrophobic patch surrounded by basic residues that may represent a membrane association site. To allow investigation of the membrane interaction of MurG on a molecular level, we expressed and purified MurG from E. coli in the absence of detergent. Surprisingly, we found that lipid vesicles copurify with MurG. Freeze fracture electron microscopy of whole cells and lysates suggested that these vesicles are derived from vesicular intracellular membranes that are formed during overexpression. This is the first study which shows that overexpression of a peripheral membrane protein results in formation of additional membranes within the cell. The cardiolipin content of cells overexpressing MurG was increased from 1 +/- 1 to 7 +/- 1 mol% compared to nonoverexpressing cells. The lipids that copurify with MurG were even further enriched in cardiolipin (13 +/- 4 mol%). MurG activity measurements of lipid I, its natural substrate, incorporated in pure lipid vesicles showed that the MurG activity is higher for vesicles containing cardiolipin than for vesicles with phosphatidylglycerol. These findings support the suggestion that MurG interacts with phospholipids of the bacterial membrane. In addition, the results show a special role for cardiolipin in the MurG-membrane interaction.

Chemistry and biology of the ramoplanin family of peptide antibiotics

The peptide antibiotic ramoplanin factor A2 is a promising clinical candidate for treatment of Gram-positive bacterial infections that are resistant to antibiotics such as glycopeptides, macrolides, and penicillins. Since its discovery in 1984, no clinical or laboratory-generated resistance to this antibiotic has been reported. The mechanism of action of ramoplanin involves sequestration of peptidoglycan biosynthesis Lipid intermediates, thus physically occluding these substrates from proper utilization by the late-stage peptidoglycan biosynthesis enzymes MurG and the transglycosylases (TGases). Ramoplanin is structurally related to two cell wall active lipodepsipeptide antibiotics, janiemycin, and enduracidin, and is functionally related to members of the lantibiotic class of antimicrobial peptides (mersacidin, actagardine, nisin, and epidermin) and glycopeptide antibiotics (vancomycin and teicoplanin). Peptidomimetic chemotherapeutics derived from the ramoplanin sequence may find future use as antibiotics against vancomycin-resistant Enterococcus faecium (VRE), methicillin-resistant Staphylococcus aureus (MRSA), and related pathogens. Here we review the chemistry and biology of the ramoplanins including its discovery, structure elucidation, biosynthesis, antimicrobial activity, mechanism of action, and total synthesis.

A MurG assay which utilises a synthetic analogue of lipid I

A standard assay for the MurG enzyme using a lipid I analogue [MurNAc(N(epsilon)-dansylpentapeptide)-pyrophosphoryl (R,S)-alpha-dihydroheptaprenol] and radioactive UDP-N-acetylglucosamine was set up. A high concentration (35%) of dimethylsulfoxide was necessary for maximal activity. Separation and quantitation were accomplished by reverse-phase high performance liquid chromatography (HPLC) in isocratic conditions and on-line radioactivity detection, thereby providing a rapid and accurate assay. The kinetic parameters of the MurG reaction were determined; the reaction was shown to also catalyse the reverse reaction at a measurable rate. A lipid I analogue containing dihydroundecaprenol as the prenyl chain turned out to be a poor MurG substrate, presumably owing to aggregation.

Glycogen synthase: towards a minimum catalytic unit?

Classically, alpha-1,4-glucan synthases have been divided into two families, animal/fungal glycogen synthases (GS) and bacterial/plant starch synthases (G(S)S), according to differences in sequence, sugar donor specificity and regulatory mechanisms. Detailed sequence analysis, predicted secondary structure comparison and threading analysis show that these two families are structurally related and that some domains of GSs were acquired to meet regulatory requirements. Archaeal G(S)S present structural and functional features that are conserved in one, the other or both families. Therefore, they are the link between GS and G(S)S and harbor the minimal sequence and structural features that constitute the minimum catalytic unit of the alpha-1,4-glucan synthase superfamily.

A multitarget assay for inhibitors of membrane-associated steps of peptidoglycan biosynthesis

Peptidoglycan synthesis begins in the cytoplasm with the condensation of UDP-N-acetyl glucosamine (UDP-GlcNAc) and phosphoenolpyruvate catalyzed by UDP-N-acetylglucosamine enolpyruvoyl transferase. UDP-GlcNAc is also utilized as substrate for the glycosyltransferase MurG, a membrane-bound enzyme that catalyzes the production of lipid II. Membranes from Escherichia coli cells overproducing MurG support peptidoglycan formation at a rate approximately fivefold faster than membranes containing wild-type levels of MurG. Conditions have been optimized for the production of large amounts of membranes with increased levels of MurG, allowing the development of an assay suitable for high-throughput screening of large compound libraries. The quality of the purified membranes was assessed by electron microscopy and also by testing cross-linked peptidoglycan production. Moreover, kinetic studies allowed the determination of optimal concentrations of the substrates and membranes to be utilized for maximum sensitivity of the assay. Using a 96-well assay format, the IC50 values for vancomycin, tunicamycin, flavomycin, and bacitracin were 1.1 microM, 0.01 microg/ml, 0.03 microg/ml, and 0.7 microg/ml, respectively.

Intrinsic lipid preferences and kinetic mechanism of Escherichia coli MurG

MurG, the last enzyme involved in the intracellular phase of peptidoglycan synthesis, is a membrane-associated glycosyltransferase that couples N-acetyl glucosamine to the C4 hydroxyl of a lipid-linked N-acetyl muramic acid derivative (lipid I) to form the beta-linked disaccharide (lipid II) that is the minimal subunit of peptidoglycan. Lipid I is anchored to the bacterial membrane by a 55 carbon undecaprenyl chain. Because this long lipid chain impedes kinetic analysis of MurG, we have been investigating alternative substrates containing shortened lipid chains. We now describe the intrinsic lipid preferences of MurG and show that the optimal substrate for MurG in the absence of membranes is not the natural substrate. Thus, while the undecaprenyl carrier lipid may be critical for certain steps in the biosynthetic pathway to peptidoglycan, it is not required-in fact, is not preferred-by MurG. Using synthetic substrate analogues and products containing different length lipid chains, as well as a synthetic dead-end acceptor analogue, we have also shown that MurG follows a compulsory ordered Bi Bi mechanism in which the donor sugar binds first. This information should facilitate obtaining crystals of MurG with substrates bound, an important goal because MurG belongs to a major superfamily of NDP-glycosyltransferases for which no structures containing intact substrates have yet been solved.

Synthesis of P(1)-Citronellyl-P(2)-alpha-D-pyranosyl pyrophosphates as potential substrates for the E. coli undecaprenyl-pyrophosphoryl-N-acetylglucoseaminyl transferase MurG

P(1)-Citronellyl-P(2)-alpha-D-pyranosyl pyrophosphates containing alpha-D-N-acetylglucoseaminyl, alpha-D-glucosyl, and alpha-D-N-acetylmuramyl carbohydrates were synthesized and used in substrate specificity studies of the Escherichia coli MurG enzyme. Oxalyl chloride activation of citronellyl phosphate for coupling to alpha-D-pyranose-1-phosphates resulted in markedly improved yields over traditional Khorana-Moffatt and diphenyl chlorophosphate activation strategies.

Katanosin B and plusbacin A(3), inhibitors of peptidoglycan synthesis in methicillin-resistant Staphylococcus aureus

Both katanosin B and plusbacin A(3) are naturally occurring cyclic depsipeptide antibiotics containing a lactone linkage. They showed strong antibacterial activity against methicillin-resistant Staphylococcus aureus and VanA-type vancomycin-resistant enterococci, with MICs ranging from 0.39 to 3.13 microg/ml, as well as against other gram-positive bacteria. They inhibited the incorporation of N-acetylglucosamine, a precursor of cell wall synthesis, into peptidoglycan of S. aureus whole cells at concentrations close to their MICs. In vitro studies with a wall-membrane particulate fraction of S. aureus showed that katanosin B and plusbacin A(3) inhibited the formation of lipid intermediates, with 50% inhibitory concentrations (IC(50)s) of 2.2 and 2.3 microg/ml, respectively, and inhibited the formation of nascent peptidoglycan, with IC(50)s of 0.8 and 0.4 microg/ml, respectively. Vancomycin, a well-known inhibitor of transglycosylation, did not inhibit the formation of lipid intermediates but did inhibit the formation of nascent peptidoglycan, with an IC(50) of 4.1 microg/ml. Acetyl-Lys-D-Ala-D-Ala, an analog of the terminus of the lipid intermediates, effectively suppressed the inhibition of transglycosylation by vancomycin, but did not suppress those by katanosin B and plusbacin A(3). These results indicate that the antibacterial activity of katanosin B and plusbacin A(3) is due to blocking of transglycosylation and its foregoing steps of cell wall peptidoglycan synthesis via a mechanism differing from that of vancomycin.

Interim report on genomics of Escherichia coli

We present a summary of recent progress in understanding Escherichia coli K-12 gene and protein functions. New information has come both from classical biological experimentation and from using the analytical tools of functional genomics. The content of the E. coli genome can clearly be seen to contain elements acquired by horizontal transfer. Nevertheless, there is probably a large, stable core of >3500 genes that are shared among all E. coli strains. The gene-enzyme relationship is examined, and, in many cases, it exhibits complexity beyond a simple one-to-one relationship. Also, the E. coli genome can now be seen to contain many multiple enzymes that carry out the same or closely similar reactions. Some are similar in sequence and may share common ancestry; some are not. We discuss the concept of a minimal genome as being variable among organisms and obligatorily linked to their life styles and defined environmental conditions. We also address classification of functions of gene products and avenues of insight into the history of protein evolution.

Topological analysis of the MraY protein catalysing the first membrane step of peptidoglycan synthesis

The two-dimensional membrane topology of the Escherichia coli and Staphylococcus aureus MraY transferases, which catalyse the formation of the first lipid intermediate of peptidoglycan synthesis, was established using the beta-lactamase fusion system. All 28 constructed mraY-blaM fusions produced hybrid proteins. Analysis of the ampicillin resistance of the strains with hybrids led to a common topological model possessing 10 transmembrane segments, five cytoplasmic domains and six periplasmic domains including the N- and C-terminal ends. The agreement between the topologies of E. coli and S. aureus, their agreement to a fair extent with predicted models and a number of features arising from the comparative analysis of 25 orthologue sequences strongly suggested the validity of the model for all eubacterial MraYs. The primary structure of the 10 transmembrane segments diverged among orthologues, but they retained their hydrophobicity, number and size. The similarity of the sequences and distribution of the five cytoplasmic domains in both models, as well as their conservation among the MraY orthologues, clearly suggested their possible involvement in substrate recognition and in the catalytic process. Complementation tests showed that only fusions with untruncated mraY restored growth. It was noteworthy that S. aureus MraY was functional in E. coli. An increased MraY transferase activity was observed only with the untruncated hybrids from both organisms.

Cell wall targets in methicillin-resistant staphylococci

Multiresistant staphylococci pose an alarmingly growing problem, especially in serious hospital infections. The recent emergence of strains with reduced susceptibility against vancomycin, the last remaining drug effective against methicillin (multi) resistant Staphylococcus aureus, highlights the urgent need for new antimicrobial agents and new therapeutic regimen. Previously, new drugs were discovered exclusively in bacterial whole cell growth assays. Today's more rational approach depends on the identification of suitable target genes and proteins. These should be bacteria-specific and essential for growth either in vitro or in vivo. Targets within cell wall synthesis and remodeling pathways might be particularly attractive because the bacterial cell wall is a unique structure occurring only in prokaryots; many of the antibiotics in use today have confirmed its 'drugability'. However, several potential targets within this field have not yet been exploited successfully for anti-staphylococcal therapy and some were discovered only recently. After a short summary of known potential targets a set of genes involved in the pentaglycine interpeptide bridge formation of the staphylococcal cell wall will be introduced as interesting targets to combat multiresistant staphylococcal infections. Copyright 1999 Harcourt Publishers LtdCopyright DUMMY.