Amidation and Cross-Linking of the Enzymatically Synthesized Peptidoglycan of Bacillus stearothermophilus

AsnB Mediates Amidation of Meso-Diaminopimelic Acid Residues in the Peptidoglycan of Listeria monocytogenes and Affects Bacterial Surface Properties and Host Cell Invasion

A mutant of Listeria monocytogenes ScottA with a transposon in the 5' untranslated region of the asnB gene was identified to be hypersensitive to the antimicrobial t-cinnamaldehyde. Here, we report the functional characterization of AsnB in peptidoglycan (PG) modification and intracellular infection. While AsnB of Listeria is annotated as a glutamine-dependent asparagine synthase, sequence alignment showed that this protein is closely related to a subset of homologs that catalyze the amidation of meso-diaminopimelic acid (mDAP) residues in the peptidoglycan of other bacterial species. Structural analysis of peptidoglycan from an asnB mutant, compared to that of isogenic wild-type (WT) and complemented mutant strains, confirmed that AsnB mediates mDAP amidation in L. monocytogenes. Deficiency in mDAP amidation caused several peptidoglycan- and cell surface-related phenotypes in the asnB mutant, including formation of shorter but thicker cells, susceptibility to lysozyme, loss of flagellation and motility, and a strong reduction in biofilm formation. In addition, the mutant showed reduced invasion of human epithelial JEG-3 and Caco-2 cells. Analysis by immunofluorescence microscopy revealed that asnB inactivation abrogated the proper display at the listerial surface of the invasion protein InlA, which normally gets cross-linked to mDAP via its LPXTG motif. Together, this work shows that AsnB of L. monocytogenes, like several of its homologs in related Gram-positive bacteria, mediates the amidation of mDAP residues in the peptidoglycan and, in this way, affects several cell wall and cell surface-related properties. It also for the first time implicates the amidation of peptidoglycan mDAP residues in cell wall anchoring of InlA and in bacterial virulence.

Reconstitution of peptidoglycan cross-linking leads to improved fluorescent probes of cell wall synthesis

The peptidoglycan precursor, Lipid II, produced in the model Gram-positive bacterium Bacillus subtilis differs from Lipid II found in Gram-negative bacteria such as Escherichia coli by a single amidation on the peptide side chain. How this difference affects the cross-linking activity of penicillin-binding proteins (PBPs) that assemble peptidoglycan in cells has not been investigated because B. subtilis Lipid II was not previously available. Here we report the synthesis of B. subtilis Lipid II and its use by purified B. subtilis PBP1 and E. coli PBP1A. While enzymes from both organisms assembled B. subtilis Lipid II into glycan strands, only the B. subtilis enzyme cross-linked the strands. Furthermore, B. subtilis PBP1 catalyzed the exchange of both d-amino acids and d-amino carboxamides into nascent peptidoglycan, but the E. coli enzyme only exchanged d-amino acids. We exploited these observations to design a fluorescent d-amino carboxamide probe to label B. subtilis PG in vivo and found that this probe labels the cell wall dramatically better than existing reagents.

In vitro reconstitution of peptidoglycan assembly from the Gram-positive pathogen Streptococcus pneumoniae

Understanding the molecular basis of bacterial cell wall assembly is of paramount importance in addressing the threat of increasing antibiotic resistance worldwide. Streptococcus pneumoniae presents a particularly acute problem in this respect, as it is capable of rapid evolution by homologous recombination with related species. Resistant strains selected by treatment with β-lactams express variants of the target enzymes that do not recognize the drugs but retain their activity in cell wall building, despite the antibiotics being mimics of the natural substrate. Until now, the crucial transpeptidase activity that is inhibited by β-lactams was not amenable to in vitro investigation with enzymes from Gram-positive organisms, including streptococci, staphylococci, or enterococci pathogens. We report here for the first time the in vitro assembly of peptidoglycan using recombinant penicillin-binding proteins from pneumococcus and the precursor lipid II. The two required enzymatic activities, glycosyl transferase for elongating glycan chains and transpeptidase for cross-linking stem-peptides, were observed. Most importantly, the transpeptidase activity was dependent on the chemical nature of the stem-peptide. Amidation of the second residue glutamate into iso-glutamine by the recently discovered amido-transferase MurT/GatD is required for efficient cross-linking of the peptidoglycan.

Mycobacteriophage Ms6 LysA: a peptidoglycan amidase and a useful analytical tool

Since the peptidoglycan isolated from Mycobacterium spp. is refractory to commercially available murolytic enzymes, possibly due to the presence of various modifications found on this peptidoglycan, the utility of a mycobacteriophage-derived murolytic enzyme was assessed for an analysis of peptidoglycan from mycobacteria. We cloned, expressed, and purified the lysA gene product, a protein with homology to known peptidoglycan-degrading amidases, from bacteriophage Ms6. The recombinant protein was shown to cleave the bond between l-Ala and d-muramic acid of muramyl pentapeptide and to release up to 70% of the diaminopimelic acid present in the isolated mycobacterial cell wall. In contrast to lysozyme, which, in culture, inhibits the growth of both Mycobacterium smegmatis and Mycobacterium tuberculosis, LysA had no effect on the growth of either species. However, the enzyme is useful for solubilizing the peptide chains of isolated mycobacterial peptidoglycan for analysis. The data indicate that the stem peptides from M. smegmatis are heavily amidated, containing few free carboxylic acids, regardless of the cross-linking status.

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.

Mycobacterial lipid II is composed of a complex mixture of modified muramyl and peptide moieties linked to decaprenyl phosphate

Structural analysis of compounds identified as lipid I and II from Mycobacterium smegmatis demonstrated that the lipid moiety is decaprenyl phosphate; thus, M. smegmatis is the first bacterium reported to utilize a prenyl phosphate other than undecaprenyl phosphate as the lipid carrier involved in peptidoglycan synthesis. In addition, mass spectrometry showed that the muropeptides from lipid I are predominantly N-acetylmuramyl-L-alanine-D-glutamate-meso-diaminopimelic acid-D-alanyl-D-alanine, whereas those isolated from lipid II form an unexpectedly complex mixture in which the muramyl residue and the pentapeptide are modified singly and in combination. The muramyl residue is present as N-acetylmuramic acid, N-glycolylmuramic acid, and muramic acid. The carboxylic functions of the peptide side-chains of lipid II showed three types of modification, with the dominant one being amidation. The preferred site for amidation is the free carboxyl group of the meso-diaminopimelic acid residue. Diamidated species were also observed. The carboxylic function of the terminal D-alanine of some molecules is methylated, as are all three carboxylic acid functions of other molecules. This study represents the first structural analysis of mycobacterial lipid I and II and the first report of extensive modifications of these molecules. The observation that lipid I was unmodified strongly suggests that the lipid II intermediates of M. smegmatis are substrates for a variety of enzymes that introduce modifications to the sugar and amino acid residues prior to the synthesis of peptidoglycan.

The prevalence and mechanisms of vancomycin resistance in Staphylococcus aureus

The emergence of Staphylococcus aureus resistant to vancomycin has caused considerable concern. Such strains are currently rare, although they have been isolated from many areas of the world. Considerable controversy surrounds strains of S. aureus displaying heterogeneous resistance to vancomycin regarding their definition and methods for detection. This has led to considerable variance in estimates of prevalence (0-1.3%-20% in Japan) and has hindered efforts to define the clinical relevance of these strains. The mechanism of resistance involves a complex reorganization of cell wall metabolism, leading to a grossly thickened cell wall with reduced peptidoglycan cross-linking. There may be many different ways in which strains achieve this endpoint. Current knowledge and theories are summarized.

Contribution of a thickened cell wall and its glutamine nonamidated component to the vancomycin resistance expressed by Staphylococcus aureus Mu50

Staphylococcus aureus Mu50, which has reduced susceptibility to vancomycin, has a remarkably thickened cell wall with an increased proportion of glutamine nonamidated muropeptides. In addition, Mu50 had enhanced glutamine synthetase and L-glutamine D-fructose-6-phosphate aminotransferase activities, which are involved in the cell-wall peptidoglycan synthesis pathway. Furthermore, significantly increased levels of incorporation of (14)C-labeled D-glucose into the cell wall was observed in Mu50. Unlike a femC mutant S. aureus strain, increased levels of production of nonamidated muropeptides in Mu50 was not caused by lower levels of glutamine synthetase activity but was considered to be due to the glutamine depletion caused by increased glucose utilization by the cell to biosynthesize increased amounts of peptidoglycan. After the cells were allowed to synthesize cell wall in the absence or presence of glucose and glutamine, cells with different cell-wall thicknesses and with cell walls with different levels of cross-linking were prepared, and susceptibility testing of these cells demonstrated a strong correlation between the cell-wall thickness and the degree of vancomycin resistance. Affinity trapping of vancomycin molecules by the cell wall and clogging of the outer layers of peptidoglycan by bound vancomycin molecules were considered to be the mechanism of vancomycin resistance of Mu50. The reduced cross-linking and the increased affinity of binding to vancomycin of the Mu50 cell wall presumably caused by the increased proportion of nonamidated muropeptides may also contribute to the resistance to some extent.

A novel glycan polymerase that synthesizes uncross-linked peptidoglycan in Escherichia coli

A simple and efficient procedure to assay peptidoglycan synthesis in vitro was established. By this procedure, a novel activity for glycan polymerization in Escherichia coli was found in the fraction containing no detectable penicillin-binding protein (PBP). This polymerase activity was relatively insensitive to moenomycin, showed requirement for Ca2+ or Mn2+ but not for Mg2+, and led to production of uncross-linked glycan chains. These properties distinguished the glycan polymerase from the activities shown by the fractions containing PBPs. The glycan polymerase catalyzing polymerization of glycan units from lipid intermediates was purified and identified as a protein of 34 kDa.