Shared lipid phosphate carrier in the biosynthesis of teichoic acid and peptidoglycan

Characterization of Ssc, an N-acetylgalactosamine-containing Staphylococcus aureus surface polysaccharide

Whole genome sequencing has revealed that the genome of Staphylococcus aureus possesses an uncharacterized 5-gene operon (SAOUHSC_00088-00092 in strain 8325 genome) that encodes factors with functions related to polysaccharide biosynthesis and export, indicating the existence of a new extracellular polysaccharide species. We designate this locus as ssc for staphylococcal surface carbohydrate. We found that the ssc genes were weakly expressed and highly repressed by the global regulator MgrA. To characterize Ssc, Ssc was heterologously expressed in Escherichia coli and extracted by heat treatment. Ssc was also conjugated to AcrA from Campylobacter jejuni in E. coli using protein glycan coupling technology (PGCT). Analysis of the heat-extracted Ssc and the purified Ssc-AcrA glycoconjugate by tandem mass spectrometry revealed that Ssc is likely a polymer consisting of N-acetylgalactosamine. We further demonstrated that the expression of the ssc genes in S. aureus affected phage adsorption and susceptibility, suggesting that Ssc is surface-exposed.

IMPORTANCE

Surface polysaccharides play crucial roles in the biology and virulence of bacterial pathogens. Staphylococcus aureus produces four major types of polysaccharides that have been well-characterized. In this study, we identified a new surface polysaccharide containing N-acetylgalactosamine (GalNAc). This marks the first report of GalNAc-containing polysaccharide in S. aureus. Our discovery lays the groundwork for further investigations into the chemical structure, surface location, and role in pathogenesis of this new polysaccharide.

Imbalance of peptidoglycan biosynthesis alters the cell surface charge of Listeria monocytogenes

The bacterial cell wall is composed of a thick layer of peptidoglycan and cell wall polymers, which are either embedded in the membrane or linked to the peptidoglycan backbone and referred to as lipoteichoic acid (LTA) and wall teichoic acid (WTA), respectively. Modifications of the peptidoglycan or WTA backbone can alter the susceptibility of the bacterial cell towards cationic antimicrobials and lysozyme. The human pathogen Listeria monocytogenes is intrinsically resistant towards lysozyme, mainly due to deacetylation and O-acetylation of the peptidoglycan backbone via PgdA and OatA. Recent studies identified additional factors, which contribute to the lysozyme resistance of this pathogen. One of these is the predicted ABC transporter, EslABC. An eslB mutant is hyper-sensitive towards lysozyme, likely due to the production of thinner and less O-acetylated peptidoglycan. Using a suppressor screen, we show here that suppression of eslB phenotypes could be achieved by enhancing peptidoglycan biosynthesis, reducing peptidoglycan hydrolysis or alterations in WTA biosynthesis and modification. The lack of EslB also leads to a higher negative surface charge, which likely stimulates the activity of peptidoglycan hydrolases and lysozyme. Based on our results, we hypothesize that the portion of cell surface exposed WTA is increased in the eslB mutant due to the thinner peptidoglycan layer and that latter one could be caused by an impairment in UDP-N-acetylglucosamine (UDP-GlcNAc) production or distribution.

A Defective Undecaprenyl Pyrophosphate Synthase Induces Growth and Morphological Defects That Are Suppressed by Mutations in the Isoprenoid Pathway of Escherichia coli

The peptidoglycan exoskeleton shapes bacteria and protects them against osmotic forces, making its synthesis the target of many current antibiotics. Peptidoglycan precursors are attached to a lipid carrier and flipped from the cytoplasm into the periplasm to be incorporated into the cell wall. In Escherichia coli, this carrier is undecaprenyl phosphate (Und-P), which is synthesized as a diphosphate by the enzyme undecaprenyl pyrophosphate synthase (UppS). E. coli MG1655 exhibits wild-type morphology at all temperatures, but one of our laboratory strains (CS109) was highly aberrant when grown at 42°C. This strain contained mutations affecting the Und-P synthetic pathway genes uppS, ispH, and idi Normal morphology was restored by overexpressing uppS or by replacing the mutant (uppS31) with the wild-type allele. Importantly, moving uppS31 into MG1655 was lethal even at 30°C, indicating that the altered enzyme was highly deleterious, but growth was restored by adding the CS109 versions of ispH and idi Purified UppS^W31R was enzymatically defective at all temperatures, suggesting that it could not supply enough Und-P during rapid growth unless suppressor mutations were present. We conclude that cell wall synthesis is profoundly sensitive to changes in the pool of polyisoprenoids and that isoprenoid homeostasis exerts a particularly strong evolutionary pressure.IMPORTANCE Bacterial morphology is determined primarily by the overall structure of the semirigid macromolecule peptidoglycan. Not only does peptidoglycan contribute to cell shape, but it also protects cells against lysis caused by excess osmotic pressure. Because it is critical for bacterial survival, it is no surprise that many antibiotics target peptidoglycan biosynthesis. However, important gaps remain in our understanding about how this process is affected by peptidoglycan precursor availability. Here, we report that a mutation altering the enzyme that synthesizes Und-P prevents cells from growing at high temperatures and that compensatory mutations in enzymes functioning upstream of uppS can reverse this phenotype. The results highlight the importance of Und-P metabolism for maintaining normal cell wall synthesis and shape.

Tunicamycin Mediated Inhibition of Wall Teichoic Acid Affects Staphylococcus aureus and Listeria monocytogenes Cell Morphology, Biofilm Formation and Virulence

The emergence of bacterial resistance to therapeutic antibiotics limits options for treatment of common microbial diseases. Subinhibitory antibiotics dosing, often aid in the emergence of resistance, but its impact on pathogen's physiology and pathogenesis is not well understood. Here we investigated the effect of tunicamycin, a cell wall teichoic acid (WTA) biosynthesis inhibiting antibiotic at the subinhibitory dosage on Staphylococcus aureus and Listeria monocytogenes physiology, antibiotic cross-resistance, biofilm-formation, and virulence. Minimum inhibitory concentration (MIC) of tunicamycin to S. aureus and L. monocytogenes was 20-40 μg/ml and 2.5-5 μg/ml, respectively, and the subinhibitory concentration was 2.5-5 μg/ml and 0.31-0.62 μg/ml, respectively. Tunicamycin pre-exposure reduced cellular WTA levels by 18-20% and affected bacterial cell wall ultrastructure, cell membrane permeability, morphology, laser-induced colony scatter signature, and bacterial ability to form biofilms. It also induced a moderate level of cross-resistance to tetracycline, ampicillin, erythromycin, and meropenem for S. aureus, and ampicillin, erythromycin, vancomycin, and meropenem for L. monocytogenes. Pre-treatment of bacterial cells with subinhibitory concentrations of tunicamycin also significantly reduced bacterial adhesion to and invasion into an enterocyte-like Caco-2 cell line, which is supported by reduced expression of key virulence factors, Internalin B (InlB) and Listeria adhesion protein (LAP) in L. monocytogenes, and a S. aureus surface protein A (SasA) in S. aureus. Tunicamycin-treated bacteria or the bacterial WTA preparation suppressed NF-κB and inflammatory cytokine production (TNFα, and IL-6) from murine macrophage cell line (RAW 264.7) indicating the reduced WTA level possibly attenuates an inflammatory response. These results suggest that at the subinhibitory dosage, tunicamycin-mediated inhibition of WTA biosynthesis interferes with cell wall structure, pathogens infectivity and inflammatory response, and ability to form biofilms but promotes the development of antibiotic cross-resistance.

Interrupting Biosynthesis of O Antigen or the Lipopolysaccharide Core Produces Morphological Defects in Escherichia coli by Sequestering Undecaprenyl Phosphate

Undecaprenyl phosphate (Und-P) is a member of the family of essential polyprenyl phosphate lipid carriers and in the Gram-negative bacterium Escherichia coli is required for synthesizing the peptidoglycan (PG) cell wall, enterobacterial common antigen (ECA), O antigen, and colanic acid. Previously, we found that interruption of ECA biosynthesis indirectly alters PG synthesis by sequestering Und-P via dead-end intermediates, causing morphological defects. To determine if competition for Und-P was a more general phenomenon, we determined if O-antigen intermediates caused similar effects. Indeed, disrupting the synthesis of O antigen or the lipopolysaccharide core oligosaccharide induced cell shape deformities, which were suppressed by preventing the initiation of O-antigen biosynthesis or by manipulating Und-P metabolism. We conclude that accumulation of O-antigen intermediates alters PG synthesis by sequestering Und-P. Importantly, many previous experiments addressed the physiological functions of various oligosaccharides and glycoconjugates, but these studies employed mutants that accumulate deleterious intermediates. Thus, conclusions based on these experiments must be reevaluated to account for possible indirect effects of Und-P sequestration.

IMPORTANCE

Bacteria use long-chain isoprenoids like undecaprenyl phosphate (Und-P) as lipid carriers to assemble numerous glycan polymers that comprise the cell envelope. In any one bacterium, multiple oligosaccharide biosynthetic pathways compete for a common pool of Und-P, which means that disruptions in one pathway may produce secondary consequences that affect the others. Using the Gram-negative bacterium Escherichia coli as a model, we demonstrate that interruption of the biogenesis of O antigen, a major outer membrane component, indirectly impairs peptidoglycan synthesis by sequestering Und-P into dead-end intermediates. These results strongly argue that the functions of many Und-P-utilizing pathways must be reevaluated, because much of our current understanding is based on experiments that did not control for these unintended secondary effects.

Activities and regulation of peptidoglycan synthases

Peptidoglycan (PG) is an essential component in the cell wall of nearly all bacteria, forming a continuous, mesh-like structure, called the sacculus, around the cytoplasmic membrane to protect the cell from bursting by its turgor. Although PG synthases, the penicillin-binding proteins (PBPs), have been studied for 70 years, useful in vitro assays for measuring their activities were established only recently, and these provided the first insights into the regulation of these enzymes. Here, we review the current knowledge on the glycosyltransferase and transpeptidase activities of PG synthases. We provide new data showing that the bifunctional PBP1A and PBP1B from Escherichia coli are active upon reconstitution into the membrane environment of proteoliposomes, and that these enzymes also exhibit DD-carboxypeptidase activity in certain conditions. Both novel features are relevant for their functioning within the cell. We also review recent data on the impact of protein-protein interactions and other factors on the activities of PBPs. As an example, we demonstrate a synergistic effect of multiple protein-protein interactions on the glycosyltransferase activity of PBP1B, by its cognate lipoprotein activator LpoB and the essential cell division protein FtsN.

Antagonism screen for inhibitors of bacterial cell wall biogenesis uncovers an inhibitor of undecaprenyl diphosphate synthase

Drug combinations are valuable tools for studying biological systems. Although much attention has been given to synergistic interactions in revealing connections between cellular processes, antagonistic interactions can also have tremendous value in elucidating genetic networks and mechanisms of drug action. Here, we exploit the power of antagonism in a high-throughput screen for molecules that suppress the activity of targocil, an inhibitor of the wall teichoic acid (WTA) flippase in Staphylococcus aureus. Well-characterized antagonism within the WTA biosynthetic pathway indicated that early steps would be sensitive to this screen; however, broader interactions with cell wall biogenesis components suggested that it might capture additional targets. A chemical screening effort using this approach identified clomiphene, a widely used fertility drug, as one such compound. Mechanistic characterization revealed the target was the undecaprenyl diphosphate synthase, an enzyme that catalyzes the synthesis of a polyisoprenoid essential for both peptidoglycan and WTA synthesis. The work sheds light on mechanisms contributing to the observed suppressive interactions of clomiphene and in turn reveals aspects of the biology that underlie cell wall synthesis in S. aureus. Further, this effort highlights the utility of antagonistic interactions both in high-throughput screening and in compound mode of action studies. Importantly, clomiphene represents a lead for antibacterial drug discovery.

An antibiotic that inhibits a late step in wall teichoic acid biosynthesis induces the cell wall stress stimulon in Staphylococcus aureus

Wall teichoic acids (WTAs) are phosphate-rich, sugar-based polymers attached to the cell walls of most Gram-positive bacteria. In Staphylococcus aureus, these anionic polymers regulate cell division, protect cells from osmotic stress, mediate host colonization, and mask enzymatically susceptible peptidoglycan bonds. Although WTAs are not required for survival in vitro, blocking the pathway at a late stage of synthesis is lethal. We recently discovered a novel antibiotic, targocil, that inhibits a late acting step in the WTA pathway. Its target is TarG, the transmembrane component of the ABC transporter (TarGH) that exports WTAs to the cell surface. We examined here the effects of targocil on S. aureus using transmission electron microscopy and gene expression profiling. We report that targocil treatment leads to multicellular clusters containing swollen cells displaying evidence of osmotic stress, strongly induces the cell wall stress stimulon, and reduces the expression of key virulence genes, including dltABCD and capsule genes. We conclude that WTA inhibitors that act at a late stage of the biosynthetic pathway may be useful as antibiotics, and we present evidence that they could be particularly useful in combination with beta-lactams.

Synthetic lethal compound combinations reveal a fundamental connection between wall teichoic acid and peptidoglycan biosyntheses in Staphylococcus aureus

Methicillin resistance in Staphylococcus aureus depends on the production of mecA, which encodes penicillin-binding protein 2A (PBP2A), an acquired peptidoglycan transpeptidase (TP) with reduced susceptibility to β-lactam antibiotics. PBP2A cross-links nascent peptidoglycan when the native TPs are inhibited by β-lactams. Although mecA expression is essential for β-lactam resistance, it is not sufficient. Here we show that blocking the expression of wall teichoic acids (WTAs) by inhibiting the first enzyme in the pathway, TarO, sensitizes methicillin-resistant S. aureus (MRSA) strains to β-lactams even though the β-lactam-resistant transpeptidase, PBP2A, is still expressed. The dramatic synergy between TarO inhibitors and β-lactams is noteworthy not simply because strategies to overcome MRSA are desperately needed but because neither TarO nor the activities of the native TPs are essential in MRSA strains. The "synthetic lethality" of inhibiting TarO and the native TPs suggests a functional connection between ongoing WTA expression and peptidoglycan assembly in S. aureus. Indeed, transmission electron microscopy shows that S. aureus cells blocked in WTA synthesis have extensive defects in septation and cell separation, indicating dysregulated cell wall assembly and degradation. Our studies imply that WTAs play a fundamental role in S. aureus cell division and raise the possibility that synthetic lethal compound combinations may have therapeutic utility for overcoming antibiotic-resistant bacterial infections.

Substrate specificity and membrane topology of Escherichia coli PgpB, an undecaprenyl pyrophosphate phosphatase

The synthesis of the lipid carrier undecaprenyl phosphate (C(55)-P) requires the dephosphorylation of its precursor, undecaprenyl pyrophosphate (C(55)-PP). The latter lipid is synthesized de novo in the cytosol and is also regenerated after its release from the C(55)-PP-linked glycans in the periplasm. In Escherichia coli the dephosphorylation of C(55)-PP was shown to involve four integral membrane proteins, BacA, and three members of the type 2 phosphatidic acid phosphatase family, PgpB, YbjG, and YeiU. Here, the PgpB protein was purified to homogeneity, and its phosphatase activity was examined. This enzyme was shown to catalyze the dephosphorylation of C(55)-PP with a relatively low efficiency compared with diacylglycerol pyrophosphate and farnesyl pyrophosphate (C(15)-PP) lipid substrates. However, the in vitro C(55)-PP phosphatase activity of PgpB was specifically enhanced by different phospholipids. We hypothesize that the phospholipids are important determinants to ensure proper conformation of the atypical long axis C(55) carrier lipid in membranes. Furthermore, a topological analysis demonstrated that PgpB contains six transmembrane segments, a large periplasmic loop, and the type 2 phosphatidic acid phosphatase signature residues at a periplasmic location.

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.

Periplasmic phosphorylation of lipid A is linked to the synthesis of undecaprenyl phosphate

One-third of the lipid A found in the Escherichia coli outer membrane contains an unsubstituted diphosphate unit at position 1 (lipid A 1-diphosphate). We now report an inner membrane enzyme, LpxT (YeiU), which specifically transfers a phosphate group to lipid A, forming the 1-diphosphate species. (32)P-labelled lipid A obtained from lpxT mutants do not produce lipid A 1-diphosphate. In vitro assays with Kdo(2)-[4'-(32)P]lipid A as the acceptor shows that LpxT uses undecaprenyl pyrophosphate as the substrate donor. Inhibition of lipid A 1-diphosphate formation in wild-type bacteria was demonstrated by sequestering undecaprenyl pyrophosphate with the cyclic polypeptide antibiotic bacitracin, providing evidence that undecaprenyl pyrophosphate serves as the donor substrate within whole bacteria. LpxT-catalysed phosphorylation is dependent upon transport of lipid A across the inner membrane by MsbA, a lipid A flippase, indicating a periplasmic active site. In conclusion, we demonstrate a novel pathway in the periplasmic modification of lipid A that is directly linked to the synthesis of undecaprenyl phosphate, an essential carrier lipid required for the synthesis of various bacterial polymers, such as peptidoglycan.

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.

CpsE from type 2 Streptococcus pneumoniae catalyzes the reversible addition of glucose-1-phosphate to a polyprenyl phosphate acceptor, initiating type 2 capsule repeat unit formation

The majority of the 90 capsule types made by the gram-positive pathogen Streptococcus pneumoniae are assembled by a block-type mechanism similar to that utilized by the Wzy-dependent O antigens and capsules of gram-negative bacteria. In this mechanism, initiation of repeat unit formation occurs by the transfer of a sugar to a lipid acceptor. In S. pneumoniae, this step is catalyzed by CpsE, a protein conserved among the majority of capsule types. Membranes from S. pneumoniae type 2 strain D39 and Escherichia coli containing recombinant Cps2E catalyzed incorporation of [14C]Glc from UDP-[14C]Glc into a lipid fraction in a Cps2E-dependent manner. The Cps2E-dependent glycolipid product from both membranes was sensitive to mild acid hydrolysis, suggesting that Cps2E was catalyzing the formation of a polyprenyl pyrophosphate Glc. Addition of exogenous polyprenyl phosphates ranging in size from 35 to 105 carbons to D39 and E. coli membranes stimulated Cps2E activity. The stimulation was due, in part, to utilization of the exogenous polyprenyl phosphates as an acceptor. The glycolipid product synthesized in the absence of exogenous polyprenyl phosphates comigrated with a 60-carbon polyprenyl pyrophosphate Glc. When 10 or 100 microM UMP was added to reaction mixtures containing D39 membranes, Cps2E activity was inhibited 40% and 80%, respectively. UMP, which acted as a competitive inhibitor of UDP-Glc, also stimulated Cps2E to catalyze the reverse reaction, with synthesis of UDP-Glc from the polyprenyl pyrophosphate Glc. These data indicated that Cps2E was catalyzing the addition of Glc-1-P to a polyprenyl phosphate acceptor, likely undecaprenyl phosphate.

A continuum of anionic charge: structures and functions of D-alanyl-teichoic acids in gram-positive bacteria

Teichoic acids (TAs) are major wall and membrane components of most gram-positive bacteria. With few exceptions, they are polymers of glycerol-phosphate or ribitol-phosphate to which are attached glycosyl and D-alanyl ester residues. Wall TA is attached to peptidoglycan via a linkage unit, whereas lipoteichoic acid is attached to glycolipid intercalated in the membrane. Together with peptidoglycan, these polymers make up a polyanionic matrix that functions in (i) cation homeostasis; (ii) trafficking of ions, nutrients, proteins, and antibiotics; (iii) regulation of autolysins; and (iv) presentation of envelope proteins. The esterification of TAs with D-alanyl esters provides a means of modulating the net anionic charge, determining the cationic binding capacity, and displaying cations in the wall. This review addresses the structures and functions of D-alanyl-TAs, the D-alanylation system encoded by the dlt operon, and the roles of TAs in cell growth. The importance of dlt in the physiology of many organisms is illustrated by the variety of mutant phenotypes. In addition, advances in our understanding of D-alanyl ester function in virulence and host-mediated responses have been made possible through targeted mutagenesis of dlt. Studies of the mechanism of D-alanylation have identified two potential targets of antibacterial action and provided possible screening reactions for designing novel agents targeted to D-alanyl-TA synthesis.

Polymerization of mycobacterial arabinogalactan and ligation to peptidoglycan

The cell wall of Mycobacterium spp. consists predominately of arabinogalactan chains linked at the reducing ends to peptidoglycan via a P-GlcNAc-(alpha1-3)-Rha linkage unit (LU) and esterified to a variety of mycolic acids at the nonreducing ends. Several aspects of the biosynthesis of this complex have been defined, including the initial formation of the LU on a polyprenyl phosphate (Pol-P) molecule followed by the sequential addition of galactofuranosyl (Galf) units to generate Pol-P-P-LU-(Galf)1,2,3, etc. and Pol-P-P-LU-galactan, catalyzed by a bifunctional galactosyltransferase (Rv3808c) capable of adding alternating 5- and 6-linked Galf units. By applying cell-free extracts of Mycobacterium smegmatis, containing cell wall and membrane fragments, and differential labeling with UDP-[14C]Galp and recombinant UDP-Galp mutase as the source of [14C]Galf for galactan biosynthesis and 5-P-[14C]ribosyl-P-P as a donor of [14C]Araf for arabinan synthesis, we now demonstrate sequential synthesis of the simpler Pol-P-P-LU-(Galf)n glycolipid intermediates followed by the Pol-P-P-LU-arabinogalactan and, finally, ligation of the P-LU-arabinogalactan to peptidoglycan. This first time demonstration of in vitro ligation of newly synthesized P-LU-arabinogalactan to newly synthesized peptidoglycan is a necessary forerunner to defining the genetics and enzymology of cell wall polymer-peptidoglycan ligation in Mycobacterium spp. and examining this step as a target for new antibacterial drugs.

The biology of colicin M

This communication summarizes our present knowledge of colicin M, an unusual member of the colicin group. The gene encoding colicin M, cma, has been sequenced and the protein isolated and purified. With a deduced molecular size of 29,453 Da, colicin M is the smallest of the known colicins. The polypeptide can be divided into functional domains for cell surface receptor binding, uptake into the cell, and killing activity. To kill, the colicin must enter from outside the cell. Colicin M blocks the biosynthesis of both peptidoglycan and O-antigen by inhibiting regeneration of the bactoprenyl-P carrier lipid. Autolysis occurs as a secondary effect following inhibition of peptidoglycan synthesis. Colicin M is the only colicin known to have such a mechanism of action. Immunity to this colicin is mediated by the cmi gene product, a protein of 13,890 Da. This cytoplasmic membrane protein confers immunity by binding to and thus neutralizing the colicin. Cmi shares properties with both immunity proteins of the pore-forming and the cytoplasmically active colicins. Genes for the colicin and immunity protein are found next to each other, but in opposite orientation, on pColM plasmids. The mechanism of colicin M release is not known.

Role of lipid intermediate(s) in the synthesis of serogroup B Neisseria meningitidis capsular polysaccharide

Neisseria meningitidis serogroup B strain M986 was examined for the involvement of lipid intermediate(s) participating in the biosynthesis of the sialic acid capsular polysaccharide. The addition of exogenous undecaprenyl phosphate, phosphatidylethanolamine, or phosphatidylglycerol to particulate membranes, in the presence of cytidine 5'-monophosphosialic acid, resulted in the stimulation of sialyltransferase activity specifically by undecaprenyl phosphate. Sialyltransferase activity, after delipidation of particulate membrane proteins, was specifically reconstituted by undecaprenyl phosphate. After the addition of 14C-labeled cytidine 5'-monophosphosialic acid to particulate membranes, the level of labeled lipid intermediate(s), extracted by chloroform-methanol (2:1), increased up to a maximum level between 3.75 and 5.0 min, which subsequently decreased to a lower steady-state level. Pulse-chase experiments revealed a transient, solvent-extractable, lipid-linked component. The extracted N-acetylneuraminic acid was in polymeric form. Sequential oxidation and reduction of the extracted radioactivity followed by neuraminidase treatment revealed an average degree of polymerization of four or five N-acetylneuraminic acid residues. Bacitracin-sensitive peptidoglycan was synthesized in vitro by particulate membranes. Cross-competition experiments between peptidoglycan and capsular polysaccharide synthesis by preincubation of precursors of one pathway during synthesis of the other revealed a competitive effect for a common component. This component was believed to be a common pool of undecaprenyl phosphate. A model for the production and regulation of the capsular polysaccharide is proposed.

Isolation and characterization of soluble peptidoglycan from several strains of Streptococcus faecium

Two phenotypically autolysis-deficient strains of Streptococcus faecium ATCC 9790 were shown to produce high-molecular-weight, soluble, linear, uncross-linked peptidoglycan when incubated with benzylpenicillin in a wall medium which permits cell wall synthesis (wall thickening) but not balanced growth. This high-molecular-weight s-peptidoglycan was shown to have a molecular weight of 46,000 to 54,000, lack peptide cross-links, and be virtually devoid of accessory wall polymers. It was hydrolyzed by hen egg white lysozyme and the endogenous, autolytic N-acetylmuramidase of S. faecium, but was not attacked by proteinases. Chemical analyses of the polymer are consistent with the following structure, where n is the number of repeating disaccharide units: (formula; see text).

Production and release of peptidoglycan and wall teichoic acid polymers in pneumococci treated with beta-lactam antibiotics

Autolysin-defective pneumococci treated with inhibitory concentrations of penicillin and other beta-lactam antibiotics continued to produce non-cross-linked peptidoglycan and cell wall teichoic acid polymers, the majority of which were released into the surrounding medium. The released cell wall polymers were those synthesized by the pneumococci after the addition of the antibiotics. The peptidoglycan and wall teichoic acid chains released were not linked to one another; they could be separated by affinity chromatography on an agarose-linked phosphorylcholine-specific myeloma protein column. Omission of choline, a nutritional requirement and component of the pneumococcal teichoic acid, from the medium inhibited both teichoic acid and peptidoglycan synthesis and release. These observations are discussed in terms of plausible mechanisms for the coordination between the biosynthesis of peptidoglycan and cell wall teichoic acids.

Peptidoglycan synthesis by partly autolyzed cells of Bacillus subtilis W23

Partly autolyzed, osmotically stabilized cells of Bacillus subtilis W23 synthesized peptidoglycan from the exogenously supplied nucleotide precursors UDP-N-acetylglucosamine and UDP-N-acetylmuramyl pentapeptide. Freshly harvested cells did not synthesize peptidoglycan. The peptidoglycan formed was entirely hydrolyzed by N-acetylmuramoylhydrolase, and its synthesis was inhibited by the antibiotics bacitracin, vancomycin, and tunicamycin. Peptidoglycan formation was optimal at 37 degrees C and pH 8.5, and the specific activity of 7.0 nmol of N-acetylglucosamine incorporated per mg of membrane protein per h at pH 7.5 was probably decreased by the action of endogenous wall autolysins. No cross-linked peptidoglycan was formed. In addition, a lysozyme-resistant polymer was also formed from UDP-N-acetylglucosamine alone. Peptidoglycan synthesis was inhibited by trypsin and p-chloromercuribenzenesulfonic acid, and we conclude that it occurred at the outer surface of the membrane. Although phospho-N-acetylmuramyl pentapeptide translocase activity was detected on the outside surface of the membrane, no transphosphorylation mechanism was observed for the translocation of UDP-N-acetylglucosamine. Peptidoglycan was similarly formed with partly autolyzed preparations of B. subtilis NCIB 3610, B. subtilis 168, B. megaterium KM, and B. licheniformis ATCC 9945. Intact protoplasts of B. subtilis W23 did not synthesize peptidoglycan from externally supplied nucleotides although the lipid intermediate was formed which was inhibited by tunicamycin and bacitracin. It was therefore considered that the lipid cycle had been completed, and the absence of peptidoglycan synthesis was believed to be due to the presence of lysozyme adhering to the protoplast membrane. The significance of these results and similar observations for teichoic acid synthesis (Bertram et al., J. Bacteriol. 148:406-412, 1981) is discussed in relation to the translocation of bacterial cell wall polymers.

Teichoic and teichuronic acids: biosynthesis, assembly, and location

Images

Attachment of the main chain to the linkage unit in biosynthesis of teichoic acids

The main chain of teichoic acids can be assembled in cell-free membrane preparations by the transfer of residues from the appropriate nucleotide precursors to an incompletely characterized amphiphilic molecule, lipoteichoic acid carrier (LTC). However, in the cell wall, the main chain is attached to peptidoglycan through a linkage unit which is synthesized independently. It is believed that, in these cell-free systems, lipid intermediates carrying linkage units are also able to accept residues directly from nucleotide precursors to build up the main chain. In this paper, we have shown that the main chain attached to LTC was transferred from LTC to lipids containing the linkage unit. Thus, in these systems, there appear to be two routes to the biosynthesis of teichoic acid-linkage unit complexes, one by direct assembly of the main chain on linkage unit lipids and the other by transfer of the preassembled main chain from LTC to the linkage unit. It was also shown that linkage unit lipids from different organisms were interchangeable and that these were used for polymer synthesis by Bacillus subtilis 3610, in which the teichoic acid is a poly(glycerol phosphate).

Membrane lipoteichoic acid of Streptococcus pyogenes and its stabilized L-form and the effect of two antibiotics upon its cellular content

Membrane lipoteichoic acid continues to be synthesized by an osmotically fragile, stabilized L-form of Streptococcus pyogenes. Chromatographic and electrophoretic comparisons indicate that the lipid componenent of lipoteichoic acid in this L-form and its parental streptococcus is glycerophosphoryldiglucosyl diglyceride and not phosphatidylkojibiosyl diglyceride. Based upon dry weight determinations, the yield of lipoteichoic acid from the L-form is 0.19%, as compared with 0.97% from the streptococcus. When grown with bacitracin the L-form contains the same amount of teichoic acid as when grown without this antibiotic; however, its lipoteichoic acid content is reduced by 85%. Similarly, the L-form grown with novobiocin for 10 h contains only 17% of the teichoic acid found in control cells.

A proposed pathway of plasma glycoprotein synthesis

Plasma glycoprotein synthesis in the liver occurs in a stepwise fashion. The first sugar, N-acetyl-glucosamine, is attached to the protein during the growth of the polypeptide chain on the membrane-bound ribosomes. Subsequent carbohydrates are incorporated after the completion of the protein in the lumen of the endoplasmic reticulum and Golgi apparatus. The reactions are carried out by enzymes strongly bound to the membranes. Because the glycosylation reaction occurs in the interior of the cytoplasmic tubules a permeability problem for the nucleotide sugar exists. Recent studies indicate that sugar-lipids are formed on the cytoplasmic site of the membrane and these complexes transfer the sugars across the membrane. Experimental evidence for this pathway is presented in this article.

The control of synthesis of bacterial cell walls. Interaction in the synthesis of nucleotide precursors

Phosphoenolpyruvate-UDP-N-acetylglucosamine enolpyruvyltransferase, UDP-N-acetylglucosamine pyrophosphorylase and CDP-glycerol pyrophosphorylase activities were demonstrated in soluble extracts from Bacillus licheniformis A.T.C.C. 9945. The effect of various nucleotides, sugar nucleotides and sugar phosphates on the nucleotide pyrophosphorylases was investigated. UDP-N-acetylglucosamine pyrophosphorylase was inhibited by UDP-MurAc-pentapeptide (UDP-N-acetylmuramyl-l-alanyl-d-glutamyl- meso-diaminopimelyl-d-alanyl -d-alanine) and CDP-glycerol. CDP-glycerol pyrophosphorylase was inhibited by UDP-MurAc-pentapeptide and stimulated by UDP-N-acetylglucosamine. Interaction between a precursor of one cell-wall polymer and an enzyme involved in the synthesis of a precursor of a second polymer has therefore been demonstrated. The possible role of such interaction in the control of bacterial cell-wall synthesis is discussed. Of the other compounds investigated mono- and di-nucleotides were shown to be inhibitory, indicating that nucleotide pyrophosphorylase activities may be influenced by the energy charge of the cell.

Identification of prenol intermediates of wall biosynthesis in growing cells of Lactobacillus plantarum

The incorporation of (14)C-mevalonic acid by Lactobacillus plantarum predominantly into C(55) prenol made it possible to determine the distribution of (14)C-prenol between all its derivatives. In logarithmic-phase cells, 25% of the prenol was free, 31% was as monophosphate, 4% as pyrophosphate, 12% as peptidoglycan precursor, and 28% as glyco-phospho-prenol. The glyco-phospho-prenol contained rhamnose, and probably glucose, galactose, and ribitol phosphate, and it may, therefore, be involved in polysaccharide and teichoic acid biosynthesis. The proportion of free prenol increased, up to 73%, as the cell culture aged. Free prenol was also formed when cells were incubated in buffer. The free prenol was readily reutilized when cells were returned to growth medium.

Teichoic acid of a stabilized L-form of Streptococcus pyogenes

A stabilized L-form of Streptococcus pyogenes continues to synthesize glycerol teichoic acid. This polymer was obtained from S. pyogenes and its L-form, treated in identical fashion, and compared. Highly purified glycerol teichoic acid from only the L-form was found to be devoid of d-alanine and to have a shorter chain length. Otherwise, the glycerol teichoic acid from these two organisms was found to be a 1,3-phosphodiester-linked glycerophosphate polymer substituted with d-glucose. Evidence is presented that most, if not all, of the glycerol teichoic acid in this streptococcus lies between the wall and membrane. A possible need for the continued synthesis of a minute amount of glycerol teichoic acid by this L-form for survival is discussed in terms of the known function of teichoic acids in bacteria.

RNA synthesis and turnover during density-inhibited growth and encystment of Acanthamoeba castellanii

Alterations in transcription that precede and accompany encystment (E) of suspension grown A. castellanii have been investigated. Comparative studies were performed on cells undergoing spontaneous E in high density stationary phase cultures or after experimental induction of E at low cell densities by deprivation of nutrients in exponential growth. Onset of growth deceleration at high cell densities was accompanied by an increase in the cellular RNA. The maximum RNA content occurred in cells at stationary phase and subsequently declined with the appearance of cysts in the cultures. On the contrary, the RNA content in cells whose growth was immediately terminated by experimental E induction remained at a constant exponential level through 5 h postinduction and then began to decline shortly before the appearance of cysts. The mature cyst formed in stationary phase cultures and after experimental E induction contained an equivalent amount of RNA ( approximately 50% of the exponential value). Comparison of the kinetics of [(3)H]uridine incorporation demonstrated that there was an abrupt reduction in the rate of uridine incorporation into RNA with onset of growth deceleration or after growth termination in experimental E induction. The reduced incorporation of uridine into RNA could not be attributed to to a reduced uptake of the isotope by the cells or an altered capacity of the cells to phosphorylate uridine. Uridine continued to be incorporated into RNA at a reduced rate in cells throughout growth deceleration, in stationary phase, and up to 12 h postexperimental induction. Considered together, these results indicate that a buildup in RNA is not necessary for induction of encystment in acanthamoeba. The accumulated RNA in stationary phase cells appears to be due to the greater reduction in the growth rate than in transcription and the absence of RNA turnover in cells during growth deceleration. Initiation of RNA turnover appears to accompany growth termination and induction of E. The results further demonstrate that the regulation of the rate of transcription is closely coordinated with the control of growth and encystment in acanthamoeba.

Isolation of the lipid intermediate in peptidoglycan biosynthesis from Escherichia coli

The lipid intermediate in peptidoglycan biosynthesis was isolated from Escherichia coli strain W and characterized as C(55)-isoprenyl-pyrophosphoryl N-acetylmuramyl(-pentapeptide)-N-acetyl-glucosamine.

Biosynthesis of the wall teichoic acid in Bacillus licheniformis

1. The biosynthesis of the wall teichoic acid, poly(glycerol phosphate glucose), has been studied with a particulate membrane preparation from Bacillus licheniformis A.T.C.C. 9945. The precursor CDP-glycerol supplies glycerol phosphate residues, whereas UDP-glucose supplies only glucose to the repeating structure of the polymer. 2. Synthesis proceeds through polyprenol phosphate derivatives, and chemical studies and pulse-labelling techniques show that the first intermediate is the phosphodiester, glucose polyprenol monophosphate. CDP-glycerol donates a glycerol phosphate residue to this to give a second intermediate, (glycerol phosphate glucose phosphate) polyprenol. 3. The glucose residue in the lipid intermediates has the beta configuration, and chain extension in the synthesis of polymer occurs by transglycosylation with inversion of anomeric configuration at two stages.

The mechanism of wall synthesis in bacteria. The organization of enzymes and isoprenoid phosphates in the membrane

1. The synthesis of peptidoglycan and teichoic acids by cell-free preparations from Bacillus licheniformis A.T.C.C. 9945 and Bacillus subtilis N.C.T.C. 3610 has been studied under a variety of conditions. 2. It was shown that poly(glycerol phosphate) is synthesized through a lipid intermediate, and it is concluded from this and other work that all major bacterial wall polymers are formed in a similar manner through such intermediates. 3. Close interrelation between the synthesis of peptidoglycan and teichoic acids was demonstrated, and inhibition studies confirm that the polyprenol phosphate molecules participating in the synthesis of peptidoglycan are shared with the systems that synthesize teichoic acids. 4. Nucleotides for the synthesis of one polymer are inhibitory towards synthesis of the other, and these effects can be enhanced or diminished by preincubation of the enzyme system with appropriate nucleotide precursors. 5. It is concluded that the return of undecaprenol phosphate to a common pool occurs only after the completion of polymer chains, and not after each cycle in the attachment of individual repeating units. This and other observations support a model for bacterial wall synthesis in which the multi-enzyme systems for each polymer are closely aligned in the membrane, with a molecule of undecaprenol phosphate located between them in a manner that enables it to be shared. The general mechanisms of wall synthesis and its control are discussed.

Lipid intermediates in the biosynthesis of the wall teichoic acid in Staphylococcus lactis 13

1. Particulate enzyme systems have been prepared from Staphylococcus lactis I3 which effect the synthesis of wall teichoic acid (a polymer containing a repeating unit in which d-glycerol 1-phosphate is attached to the 4-position on N-acetylglucosamine 1-phosphate) from the nucleotide precursors CDP-glycerol and UDP-N-acetylglucosamine. By using nucleotides labelled with (32)P and (14)C it has been shown that the synthesis proceeds via lipid intermediates. 2. Two intermediates have been found. In one of these N-acetylglucosamine 1-phosphate is present, whereas in the other the repeating unit of the teichoic acid occurs. 3. The simultaneous formation of the teichoic acid, a poly-(N-acetylglucosamine 1-phosphate) and an unidentified lipid, together with the poor ability of most particulate systems to synthesize polymer and the instability of the lipid intermediates themselves, have interfered with pulse-labelling experiments. Nevertheless, the biosynthetic sequence has been elucidated. It is concluded that the intermediates are derivatives of undecaprenol phosphate.