Envelope-bound N-acetylmuramyl-L-alanine amidase of Escherichia coli K 12. Purification and properties of the enzyme

Peptidoglycan hydrolases of Escherichia coli

The review summarizes the abundant information on the 35 identified peptidoglycan (PG) hydrolases of Escherichia coli classified into 12 distinct families, including mainly glycosidases, peptidases, and amidases. An attempt is also made to critically assess their functions in PG maturation, turnover, elongation, septation, and recycling as well as in cell autolysis. There is at least one hydrolytic activity for each bond linking PG components, and most hydrolase genes were identified. Few hydrolases appear to be individually essential. The crystal structures and reaction mechanisms of certain hydrolases having defined functions were investigated. However, our knowledge of the biochemical properties of most hydrolases still remains fragmentary, and that of their cellular functions remains elusive. Owing to redundancy, PG hydrolases far outnumber the enzymes of PG biosynthesis. The presence of the two sets of enzymes acting on the PG bonds raises the question of their functional correlations. It is difficult to understand why E. coli keeps such a large set of PG hydrolases. The subtle differences in substrate specificities between the isoenzymes of each family certainly reflect a variety of as-yet-unidentified physiological functions. Their study will be a far more difficult challenge than that of the steps of the PG biosynthesis pathway.

Beta-lactam antibiotics: from antibiosis to resistance and bacteriology

This review focuses on the era of antibiosis that led to a better understanding of bacterial morphology, in particular the cell wall component peptidoglycan. This is an effort to take readers on a tour de force from the concept of antibiosis, to the serendipity of antibiotics, evolution of beta-lactam development, and the molecular biology of antibiotic resistance. These areas of research have culminated in a deeper understanding of microbiology, particularly in the area of bacterial cell wall synthesis and recycling. In spite of this knowledge, which has enabled design of new even more effective therapeutics to combat bacterial infection and has provided new research tools, antibiotic resistance remains a worldwide health care problem.

Specific labeling of peptidoglycan precursors as a tool for bacterial cell wall studies

Because of its importance for bacterial cell survival, the bacterial cell wall is an attractive target for new antibiotics in a time of great demand for new antibiotic compounds. Therefore, more knowledge about the diverse processes related to bacterial cell wall synthesis is needed. The cell wall is located on the exterior of the cell and consists mainly of peptidoglycan, a large macromolecule built up from a three-dimensional network of aminosugar strands interlinked with peptide bridges. The subunits of peptidoglycan are synthesized inside the cell before they are transported to the exterior in order to be incorporated into the growing peptidoglycan. The high flexibility of the cell wall synthesis machinery towards unnatural derivatives of these subunits enables research on the bacterial cell wall using labeled compounds. This review highlights the high potential of labeled cell wall precursors in various areas of cell wall research. Labeled precursors can be used in investigating direct cell wall-antibiotic interactions and in cell wall synthesis and localization studies. Moreover, these compounds can provide a powerful tool in the elucidation of the cell wall proteome, the "wallosome," and thus, might provide new targets for antibiotics.

Biochemical characterization and physiological properties of Escherichia coli UDP-N-acetylmuramate:L-alanyl-gamma-D-glutamyl-meso-diaminopimelate ligase

The UDP-N-acetylmuramate:L-alanyl-gamma-D-glutamyl-meso-diaminopimelate ligase (murein peptide ligase [Mpl]) is known to be a recycling enzyme allowing reincorporation into peptidoglycan (murein) of the tripeptide L-alanyl-gamma-D-glutamyl-meso-diaminopimelate released during the maturation and constant remodeling of this bacterial cell wall polymer that occur during cell growth and division. Mpl adds this peptide to UDP-N-acetylmuramic acid, thereby providing an economical additional source of UDP-MurNAc-tripeptide available for de novo peptidoglycan biosynthesis. The Mpl enzyme from Escherichia coli was purified to homogeneity as a His-tagged form, and its kinetic properties and parameters were determined. Mpl was found to accept tri-, tetra-, and pentapeptides as substrates in vitro with similar efficiencies, but it accepted the dipeptide L-Ala-D-Glu and L-Ala very poorly. Replacement of meso-diaminopimelic acid by L-Lys resulted in a significant decrease in the catalytic efficacy. The effects of disruption of the E. coli mpl gene and/or the ldcA gene encoding the LD-carboxypeptidase on peptidoglycan metabolism were investigated. The differences in the pools of UDP-MurNAc peptides and of free peptides between the wild-type and mutant strains demonstrated that the recycling activity of Mpl is not restricted to the tripeptide and that tetra- and pentapeptides are also directly reused by this process in vivo. The relatively broad substrate specificity of the Mpl ligase indicates that it is an interesting potential target for antibacterial compounds.

Nod1 participates in the innate immune response to Pseudomonas aeruginosa

The mammalian innate immune system recognizes pathogen-associated molecular patterns through pathogen recognition receptors. Nod1 has been described recently as a cytosolic receptor that detects specifically diaminopimelate-containing muropeptides from Gram-negative bacteria peptidoglycan. In the present study we investigated the potential role of Nod1 in the innate immune response against the opportunistic pathogen Pseudomonas aeruginosa. We demonstrate that Nod1 detects the P. aeruginosa peptidoglycan leading to NF-kappaB activation and that this activity is diminished in epithelial cells expressing a dominant-negative Nod1 construct or in mouse embryonic fibroblasts from Nod1 knock-out mice infected with P. aeruginosa. Finally, we demonstrate that the cytokine secretion kinetics and bacterial killing are altered in Nod1-deficient cells infected with P. aeruginosa in the early stages of infection.

Peptidoglycan molecular requirements allowing detection by Nod1 and Nod2

Nod1 and Nod2 are mammalian proteins implicated in the intracellular detection of pathogen-associated molecular patterns. Recently, naturally occurring peptidoglycan (PG) fragments were identified as the microbial motifs sensed by Nod1 and Nod2. Whereas Nod2 detects GlcNAc-MurNAc dipeptide (GM-Di), Nod1 senses a unique diaminopimelate-containing GlcNAc-MurNAc tripeptide muropeptide (GM-TriDAP) found mostly in Gram-negative bacterial PGs. Because Nod1 and Nod2 detect similar yet distinct muropeptides, we further analyzed the molecular sensing specificity of Nod1 and Nod2 toward PG fragments. Using a wide array of natural or modified muramyl peptides, we show here that Nod1 and Nod2 have evolved divergent strategies to achieve PG sensing. By defining the PG structural requirements for Nod1 and Nod2 sensing, this study reveals how PG processing and modifications, either by host or bacterial enzymes, may affect innate immune responses.

Involvement of N-acetylmuramyl-L-alanine amidases in cell separation and antibiotic-induced autolysis of Escherichia coli

N-acetylmuramyl-L-alanine amidases are widely distributed among bacteria. However, in Escherichia coli, only one periplasmic amidase has been described until now, which is suggested to play a role in murein recycling. Here, we report that three amidases, named AmiA, B and C, exist in E. coli and that they are involved in splitting of the murein septum during cell division. Moreover, the amidases were shown to act as powerful autolytic enzymes in the presence of antibiotics. Deletion mutants in amiA, B and C were growing in long chains of unseparated cells and displayed a tolerant response to the normally lytic combination of aztreonam and bulgecin. Isolated murein sacculi of these chain-forming mutants showed rings of thickened murein at the site of blocked septation. In vitro, these murein ring structures were digested more slowly by muramidases than the surrounding murein. In contrast, when treated with the amidase AmiC or the endopeptidase MepA, the rings disappeared, and gaps developed at these sites in the murein sacculi. These results are taken as evidence that highly stressed murein cross-bridges are concentrated at the site of blocked cell division, which, when cleaved, result in cracking of the sacculus at this site. As amidase deletion mutants accumulate trimeric and tetrameric cross-links in their murein, it is suggested that these structures mark the division site before cleavage of the septum.

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.

Interference with murein turnover has no effect on growth but reduces beta-lactamase induction in Escherichia coli

Physiological studies of a mutant of Escherichia coli lacking the three lytic transglycosylases Slt70, MltA, and MltB revealed that interference with murein turnover can prevent AmpC beta-lactamase induction. The triple mutant, although growing normally, shows a dramatically reduced rate of murein turnover. Despite the reduction in the formation of low-molecular-weight murein turnover products, neither the rate of murein synthesis nor the amount of murein per cell was increased. This might be explained by assuming that during growth in the absence of the major lytic transglycosylases native murein strands are excised by the action of endopeptidases and directly reused without further breakdown to muropeptides. The reduced rate of murein turnover could be correlated with lowered cefoxitin-induced expression of beta-lactamase, present on a plasmid carrying the ampC and ampR genes from Enterobacter cloacae. Overproduction of MltB stimulated beta-lactamase induction, whereas specific inhibition of Slt70 by bulgecin repressed ampC expression. Thus, specific inhibitors of lytic transglycosylases can increase the potency of penicillins and cephalosporins against bacteria inducing AmpC-like beta-lactamases.

Growth of the stress-bearing and shape-maintaining murein sacculus of Escherichia coli

To withstand the high intracellular pressure, the cell wall of most bacteria is stabilized by a unique cross-linked biopolymer called murein or peptidoglycan. It is made of glycan strands [poly-(GlcNAc-MurNAc)], which are linked by short peptides to form a covalently closed net. Completely surrounding the cell, the murein represents a kind of bacterial exoskeleton known as the murein sacculus. Not only does the sacculus endow bacteria with mechanical stability, but in addition it maintains the specific shape of the cell. Enlargement and division of the murein sacculus is a prerequisite for growth of the bacterium. Two groups of enzymes, hydrolases and synthases, have to cooperate to allow the insertion of new subunits into the murein net. The action of these enzymes must be well coordinated to guarantee growth of the stress-bearing sacculus without risking bacteriolysis. Protein-protein interaction studies suggest that this is accomplished by the formation of a multienzyme complex, a murein-synthesizing machinery combining murein hydrolases and synthases. Enlargement of both the multilayered murein of gram-positive and the thin, single-layered murein of gram-negative bacteria seems to follow an inside-to-outside growth strategy. New material is hooked in a relaxed state underneath the stress-bearing sacculus before it becomes inserted upon cleavage of covalent bonds in the layer(s) under tension. A model is presented that postulates that maintenance of bacterial shape is achieved by the enzyme complex copying the preexisting murein sacculus that plays the role of a template.

Identification of the mpl gene encoding UDP-N-acetylmuramate: L-alanyl-gamma-D-glutamyl-meso-diaminopimelate ligase in Escherichia coli and its role in recycling of cell wall peptidoglycan

A gene, mpl, encoding UDP-N-acetylmuramate:L-alanyl-gamma-D-glutamyl-meso-diaminopimelat e ligase was recognized by its amino acid sequence homology with murC as the open reading frame yjfG present at 96 min on the Escherichia coli map. The existence of such an enzymatic activity was predicted from studies indicating that reutilization of the intact tripeptide L-alanyl-gamma-D-glutamyl-meso-diaminopimelate occurred and accounted for well over 30% of new cell wall synthesis. Murein tripeptide ligase activity could be demonstrated in crude extracts, and greatly increased activity was produced when the gene was cloned and expressed under control of the trc promoter. A null mutant totally lacked activity but was viable, showing that the enzyme is not essential for growth.

From growth to autolysis: the murein hydrolases in Escherichia coli

Murein hydrolases cleave bonds in the bacterial exoskeleton, the murein (peptidoglycan) sacculus, a covalently closed bag-shaped polymer made of glycan strands that are crosslinked by peptides. During growth and division of a bacterial cell, these enzymes are involved in the controlled metabolism of the murein sacculus. Murein hydrolases are believed to function as pacemaker enzymes for the enlargement of the murein sacculus since opening of bonds in the murein net is needed to allow the insertion of new subunits into the sacculus. Furthermore, they are responsible for splitting the septum during cell division. The murein turnover products that are released during growth are further degraded by these (1 --> 6)-anhydromuramic acid derivatives by an intramolecular transglycosylation reaction.

Over-production, purification and properties of the uridine-diphosphate-N-acetylmuramate:L-alanine ligase from Escherichia coli

The UDP-N-acetylmuramate:L-alanine ligase of Escherichia coli was over-produced in strains harbouring recombinant plasmids bearing the murC gene under the control of the lac or trc promoter. Plasmid pAM1005, in which the promoter and ribosome-binding site region of murC were removed and in which the gene was directly under the control of promoter trc, led to a 2000-fold amplification of the L-alanine-adding activity after induction by isopropyl-thio-beta-D-galactopyranoside. The murC gene product was visualized as a 50-kDa protein accounting for approximately 50% of the cell protein. A two-step purification led to 1 g of a homogeneous protein from an 18-1 culture. The N-terminal sequence of the purified protein correlated with the nucleotide sequence of the murC gene. The presence of 2-mercaptoethanol and glycerol was essential for the stability of the enzyme. The Km values for UDP-N-acetylmuramic acid, L-alanine and ATP/Mg2+ were estimated at 100, 20 and 450 microM, respectively. Under the optimal in vitro conditions a turnover number of 928 min-1 was calculated and a copy number/cell of 600 could be roughly estimated. The specificity of the enzyme for its substrates was investigated with various analogues. The enzyme also catalysed the reverse reaction.

The negative regulator of beta-lactamase induction AmpD is a N-acetyl-anhydromuramyl-L-alanine amidase

Construction of a malE-ampD gene fusion allowed purification of biologically active fusion protein by affinity chromatography. The cloned malE-ampD gene fusion complemented a chromosomal ampD mutation. Purified MalE-AmpD fusion protein was found to have murein amidase activity with a pronounced specificity for 1,6-anhydromuropeptides, the characteristic murein turnover products in Escherichia coli. Being a N-acetyl-anhydromuranmyl-L-alanine amidase AmpD is likely to be involved in recycling of the turnover products. It is suggested that the negative regulatory effect of AmpD is due to the hydrolysis of anhydro-muropeptides which may function as signals for beta-lactamase induction.

Analysis of the sodium dodecyl sulfate-stable peptidoglycan autolysins of select gram-negative pathogens by using renaturing polyacrylamide gel electrophoresis

For the first time, peptidoglycan autolysins from cellular fractions derived from sonicated cultures of Pseudomonas aeruginosa PAO1, Escherichia coli W7, Klebsiella pneumoniae CWK2, and Proteus mirabilis 19 were detected and partially characterized by zymogram analysis. Purified murein sacculi from P. aeruginosa PAO1 were incorporated into a sodium dodecyl sulfate (SDS)-polyacrylamide gel at a concentration of 0.05% (wt/vol) to serve as a substrate for the separated autolysins. At least 11 autolysin bands of various intensities with M(r)s ranging between 17,000 and 122,000 were detected in each of the homogenated cultures. Some of the autolysins of the four bacteria had similar M(r)s. The zymogram analysis was used to show that a number of the autolysins from E. coli were inhibited by the heavy metals Hg2+ and Cu2+, at 1 and 10 mM, respectively, high ionic strengths, and reagents known to affect the packing of lipopolysaccharides. The activity of an autolysin with an M(r) of 65,000 was also impaired by penicillin G, whereas it was enhanced by gentamicin. A preliminary screen to determine the relationship between penicillin-binding proteins (PBPs) and autolysins was carried out by using a dual assay in which radiolabelled penicillin V bands were visualized on an autolysin zymogram. Radiolabelled bands corresponding to PBPs 3, 4, 5, and 6 from E. coli and P. aeruginosa; PBPs 3, 4, and 6 from Proteus mirabilis; and PBP 6 from K. pneumoniae degraded the murein sacculi in the gels and were presumed to have autolytic activity, although the possibility of two distinct enzymes, each with one of the activities, comigrating in the SDS-polyacrylamide gels could not be excluded. Some radiolabelled bands possessed an Mr of <34,000 and coincided with similar low-Mr autolysin bands.

Substrate specificity of Escherichia coli LD-carboxypeptidase on biosynthetically modified muropeptides

Escherichia coli murein can be biosynthetically modified. Amino acids at positions 3 and 4 (m-diaminopimelic acid and D-alanine, respectively) on the peptide moieties can be changed under appropriate growth conditions. The activity of E. coli LD-carboxypeptidase on biosynthetically modified substrates has been studied in vitro. The enzyme hydrolysed all tested disaccharide-tetrapeptide monomeric muropeptides modified at position 4. Monomers with m-lanthionine, but not with L-ornithine, instead of m-diaminopimelic acid at position 3 were accepted. However, neither cross-linked muropeptides nor macromolecular murein were substrates for the reaction. Our observations argue against a direct effect of LD-carboxypeptidase on macromolecular murein metabolism.

Coordinate regulation of murein peptidase activity and AmpC beta-lactamase synthesis in Escherichia coli

In the periplasmic space of Escherichia coli, the (L)-m-A2pm-(D)-m-A2pm peptide, the lipoprotein, and the AmpC beta-lactamase are controlled by growth rate. To explain this coordinate regulation, it is proposed that the AmpC protein functions as an LD-endopeptidase in addition to its known function as a beta-lactamase. As LD-peptides, DD-peptides and beta-lactams are structurally similar, LD-peptidases may belong to the larger family of DD-peptidases and serine beta-lactamases. In contrast to E. coli, many related bacteria possess an inducible AmpC protein. Several gene systems necessary for AmpC induction are known to affect various aspects of peptidoglycan metabolism. It is proposed that AmpC induction occurs indirectly via a recyclable cell wall peptide.

Expression of the iap gene coding for protein p60 of Listeria monocytogenes is controlled on the posttranscriptional level

Expression of the iap gene of Listeria monocytogenes encoding a major extracellular protein (p60) was analyzed. Different start sites for transcription of the iap gene were identified by primer extension analysis in L. monocytogenes and in a recombinant Escherichia coli clone. The mutant RIII of L. monocytogenes represents a member of the frequently occurring L. monocytogenes R mutants, which form cell chains and produce greatly reduced amounts of p60. However, the concentrations of iap-specific mRNA were similar in mutant RIII and the wild-type strain. The introduction of additional copies of the iap gene from wild-type L. monocytogenes led to an equal increase of iap mRNA in both strains, but overexpression of protein p60 was only observed in the wild-type strain. The nucleotide sequences of both iap genes and their 5' noncoding regions were identical in all parts that are essential for efficient transcription of the iap gene, translation of the iap-specific mRNA, and transport of the p60 protein. These data suggest that the expression of the iap gene in L. monocytogenes is controlled on the posttranscriptional level by a specific factor that is defective in mutant RIII.

Salt-mediated multicell formation in Deinococcus radiodurans

The highly radiation-resistant tetracoccal bacterium Deinococcus radiodurans exhibited a reversible multi-cell-form transition which depended on the NaCl concentration in the medium. In response to 0.8% NaCl addition into the medium, the pair/tetrad (designated 2/4) cells in a young culture grew and divided but did not separate and became 8-, 16-, and 32-cell units successively. In exponential growth phase, the cells divided in a 16/32 pattern. Potassium ions were equally effective as Na+ in mediating this multicell-formation effect; Mg2+, Li+, and Ca2+ also worked but produced less multiplicity. This effect appears to be species specific. This-section micrographs revealed that in a 16/32-cell unit, eight 2/4 cells were encased in an orderly manner within a large peripheral wall, showing five cycles of septation. Our results suggest the presence of a salt-sensitive mechanism for controlling cell separation in D. radiodurans.

Purification and characterization of N-acetylmuramoyl-L-alanine amidase from human serum

Purification to homogeneity of the N-acetylmuramoyl-L-alanine amidase (mucopeptide amidohydrolase, EC 3.5.1.28) from human serum has been achieved with a high yield. By molecular sieving chromatography, a molecular weight of 120,000-130,000 has been found for the native enzyme. Polyacrylamide gel electrophoresis under native conditions gave a unique band of Mr = 125,000. The same technique performed under denaturing conditions revealed that the protein is a dimer composed of one subunit of Mr = 57,000 and another of Mr = 70,000. In isoelectrofocalization assays, the amidase behaved as an acidic protein. Ethylenediaminetetraacetate inhibited the enzyme activity; the Mg2+ requirement was confirmed. The simultaneous presence of sulfhydryl groups and disulfide bonds in the protein was evidenced by the inhibitions produced by different thiol-blocking reagents and by several thiol-bearing substances. Direct measurements established the presence of two accessible thiol groups and the occurrence of nine disulfide bonds per protein molecule. Studies of substrate hydrolyzing capacities showed a marked preference for the muramoyl tripeptide derived from the Escherichia coli or Bacillus cereus mureins, the disaccharide tetrapeptide and the bis disaccharide tetra-tetrapeptide from E. coli were also good substrates. Activities on small muropeptides of other composition are also reported. Whole (insoluble) peptidoglycans representing the main bacterial chemotypes were submitted to the enzyme action; although with weak specific activities, the human amidase was nevertheless able to release soluble peptides from some of them. A bacteriolytic capacity on some microorganisms cannot be excluded. Results are discussed and the human enzyme is compared to presently known microbial muramoyl amidases.

Sequence analysis, transcriptional organization, and insertional mutagenesis of the envA gene of Escherichia coli

The Escherichia coli cell permeability-cell separation gene envA and the region immediately downstream were sequenced. The envA gene consisted of 305 codons which encoded a 34-kilodalton polypeptide that lacked a signal sequence and hydrophobic membrane-spanning regions. The envA1 mutation was determined to be a missense mutation in codon 19 resulting in a change in the amino acid sequence from histidine to tyrosine. Located 299 base pairs downstream of the envA gene was an unidentified open reading frame consisting of 147 codons. This open reading frame was followed by an additional open reading frame starting 59 base pairs further downstream and corresponded to the secA gene. A transcription terminator was located just downstream of envA on a fragment that contained a sequence corresponding to a typical rho-independent terminator. Transcription of envA and the upstream fts genes terminated at this terminator and was probably uncoupled from the downstream genes, including secA. Gene disruption experiments indicated that the envA gene was an essential gene.

Nature of the interactions involved in the lipid-protein complexes of the Escherichia coli N-acetylmuramoyl-L-alanine amidase

Depending on its concentration, phosphatidylglycerol, one of the three main Escherichia coli phospholipid species, is able to activate or inactivate the E. coli murein amidase (N-acetylmuramoyl-L-alanine amidase, EC 3.5.1.28) (Vanderwinkel, E. and De Vlieghere, M. (1985) Biochim. Biophys. Acta 838, 54-59). The mechanisms underlying the modulation of this enzyme activity were studied by analyzing the effects of cations, polycationic molecules, various surfactants and amphiphilic water-soluble compounds. K+, Mg2+ and polyamines were all able to prevent completely the enzyme inactivation produced by millimolar order concentration of phosphatidylglycerol. The efficiencies of the ionic species tested were in the order K+ less than Mg2+ = putrescine less than spermidine less than spermine. The kinetics of the counteraction processes were all sigmoidal. By contrast, the activation of the murein amidase produced by phosphatidylglycerol in micromolar concentration appeared to be insensitive to the ionic strength of the medium. Surfactants and amphiphilic molecules differing in their polar head and hydrophobic tail were found to activate the enzyme at various degrees for concentrations below their critical micellar concentration. The non-ionic surfactants were the most potent activators and remarkably mimicked the phosphatidylglycerol activation. The enzyme activation process appeared to require only a hydrophobic solvation shell around the protein. All kinetic data supported our previous interpretation of the phosphatidylglycerol-enzyme interactions in terms of multisite non-allosteric theory.

Transglycosylase and endopeptidase participate in the degradation of murein during autolysis of Escherichia coli

The cell wall degradation products released from Escherichia coli during autolysis triggered by cephaloridine or trichloroacetic acid were isolated and characterized. Murein was selectively lost from the disaccharide tetrapeptides and the bisdisaccharide tetrapeptide components. Two major autolytic products accounted for more than 85% of the released material. Compound 1 (60 to 80% of released material) was a disaccharide tetrapeptide monomer containing a 1,6-anhydromuramic acid residue. Compound 2 (15 to 30% of released material) was a mixture of a tritripeptide and a tritetrapeptide without hexosamines. Taken together the findings suggest that autolytic cell wall degradation in E. coli is selective and involves the activity of both the hydrolytic transglycosylase and an endopeptidase. Upon release, at least some of the wall components were also exposed to the activity of the N-acetylmuramic acid-L-alanine amidase.

Release of cell wall peptides into culture medium by exponentially growing Escherichia coli

Escherichia coli W7 cells were found to release three different muropeptides into the culture medium: tetrapeptide (L-Ala-D-Glu-meso-diaminopimelic acid-D-Ala), tripeptide (L-Ala-D-Glu-meso-diaminopimelic acid), and a previously undescribed dipeptide (meso-diaminopimelic acid-D-Ala). From the rate of release of these three peptides, it was calculated that 6 to 8% of the murein in the sacculus was lost per generation.

Murein hydrolase (N-acetyl-muramyl-L-alanine amidase) in human serum

An enzyme was identified in human serum which unlike lysozyme cleaved the amide bond between N-acetyl-muramic acid and L-alanine of the peptide side chain of the rigid layer (murein) of Escherichia coli. The N-acetyl-muramyl-L-alanine amidase released all of the peptide side chains including those to which the lipoprotein is bound. A portion of the peptide side chains of the Micrococcus lysodeikticus murein was also hydrolysed from the polysaccharide chains. E. coli, M. lysodeikticus, Bacillus subtilis and Staphylococcus aureus were not killed by the amidase. Treatment of E. coli with EDTA or osmotic shock rendered the cells sensitive to the amidase and they were killed. Possible biological functions of the amidase are discussed. The enzyme was separated from lysozyme in human serum. Gel permeation chromatography indicated a molecular weight of the active enzyme of 82,000 while gel electrophoresis in the presence of sodium dodecyl sulfate revealed a molecular weight of 75,000. Thus, the enzyme probably consists of a single polypeptide chain. Incubation with neuraminidase rendered the amidase more basic suggesting the release of sialic acid residues. The modified glycoprotein disclosed an increased activity to murein. Enzyme activity was inhibited by p-chloromercuribenzene sulfonate and ethyleneglycol-bis(2-aminomethyl) tetraacetate (EGTA) at 1 and 0.2 mM concentration, respectively, whereas EDTA up to 5 mM was without effect. The amidase was also inactivated by agents that reduce disulfide bridges.

Transcriptional organization within an Escherichia coli cell division gene cluster: direction of transcription of the cell separation gene envA

A cluster of at least 14 genes, each concerned with some aspect of cell envelope growth, morphogenesis, or function, is located at 2 min on the genetic map of Escherichia coli. We located the envA cell division gene and its promoter within the cluster and determined the direction of transcription of the gene by constructing fusions between its promoter and the galK coding sequence. In addition, we identified the promoter of a possible new gene lying between envA and the secA gene. We also present evidence from gene fusion studies which shows the direction of transcription of the ftsZ(sulB) division gene. The direction of transcription is the same for all three promoters and is the same as that of all other cluster genes for which this is known. We discuss the significance of this observation, together with the fact that every gene examined in sufficient detail within the cluster appears to have its own promoter and to be able to be expressed from isolated cloned fragments. Using a novel variable-copy plasmid vector, we demonstrate that the DNA fragment containing the envA gene is not stably maintained in multiple copies. The construction of two independent, nonoverlapping deletions allows us to conclude that the envA product itself is responsible for this effect.

Mutation of the N-acetylmuramyl-L-alanine amidase gene of Escherichia coli K-12

Mutants of Escherichia coli with very low N-acetylmuramyl-L-alanine amidase activity were isolated. The gene amiA responsible for most of this enzyme activity was mapped at 51 min on the E. coli chromosome, with the most plausible gene order assumed to be amiA pts(H or I) purC. The mutant phenotype was recessive and physiologically undiscernible.

Temperature-sensitive autolysis-defective mutants of Escherichia coli

Two independently isolated temperature-sensitive autolysis-defective mutants of Escherichia coli LD5 (thi lysA dapD) were characterized. The mutants were isolated by screening the survivors of a three-step enrichment process involving sequential treatments with bactericidal concentrations of D-cycloserine, benzyl-penicillin, and D-cycloserine at 42 degrees C. Cultures of the mutants underwent autolysis during beta-lactam treatment, D-cycloserine treatment, or diaminopimelic acid deprivation at 30 degrees C. The same treatments at 42 degrees C inhibited growth but did not induce lysis of the mutants. The minimum inhibitory concentrations of selected beta-lactam antibiotics and D-cycloserine were identical for the parent and mutant strains at both 30 and 42 degrees C. Both mutants failed to form colonies at 42 degrees C, and both gave rise to spontaneous temperature-resistant revertants. The revertants exhibited the normal lytic response when treated with D-cycloserine and beta-lactams or when deprived of diaminopimelic acid at 42 degrees C. The basis for the autolysis-defective phenotype of these mutants could not be determined. However, a nonspecific in vitro assay for peptidoglycan hydrolase activity in cell-free extracts indicated that both mutants were deficient in a peptidoglycan hydrolase. Both mutations were localized to the 56- to 61-min region of the E. coli chromosome by F' complementation.

N-acetylmuramoyl-L-alanine amidase of Escherichia coli K12. Possible physiological functions

Various experiments were carried out in an attempt to determine the possible physiological function of the N-acetylmuramoyl-L-alanine amidase purified from Escherichia coli K12 on the basis of its activity on N-acetylmuramoyl-L-alanyl-D-gamma-glutamyl-meso-diaminopimelic acid [MurNAc-LAla-DGlu(msA2pm)]. A Km value of 0.04 mM was determined with this substrate. Specificity studies revealed that compounds with a MurNAc-LAla linkage are the most probable substrates of this enzyme in vivo. Purified amidase had no effect on purified peptidoglycan and only low levels (1-2.5%) of cleaved MurNAc-LAla linkages were detected in peptidoglycan isolated from normally growing cells. However, the action of the amidase in vivo on peptidoglycan was clearly detectable during autolysis. The amidase activity of cells treated by osmotic shock, ether or toluene, as well as that of mutants with altered outer membrane composition was investigated. Attempts to reveal a transfer reaction catalysed by amidase were unsuccessful. Furthermore, by its location and specificity, amidase was clearly not involved in the formation of UDP-MurNAc. The possibility that it might be functioning in vivo as a hydrolase degrading exogeneous peptidoglycan fragments in the periplasma was substantiated by the fact that MurNAc itself and MurNAc-peptides could sustain growth of E. coli as sole carbon and nitrogen sources. Finally, out of 200 thermosensitive mutants examined for altered amidase activity, only two strains had less than 50% of the normal level of activity, whereas ten strains were found to possess more than 50%. In fact, two of the overproducers encountered presented a 4-5-fold increase in activity.

Partial purification and characterization of N-acetylmuramyl-L-alanine amidase from human and mouse serum

The enzyme N-acetylmuramyl-L-alanine amidase (mucopeptide amidohydrolase, EC 3.5.1.28) has been detected in human, mouse, rabbit, bovine and sheep sera. A method for detection of amidase activity using [14C]peptidoglycan monomer as the substrate has been developed. Partial purification of human and mouse amidase was achieved by gel chromatography on Bio-Gel A-1.5 m, DEAE-Sephadex A-50 and Sephadex G-100. Both amidase preparations exhibited maximal activity at pH 9.0 in Tris-HCl buffer and required Mg2+ for maximal activity. Following digestion of peptidoglycan monomer, GlcNAc-MurNAc-L-Ala-D-isoglutaminyl-meso-diaminopimelyl-D-Ala-D-Ala, the disaccharide GlcNAc-MurNAc and the corresponding pentapeptide L-Ala-D-isoglutaminyl-meso-diaminopimelyl-D-Ala-D-Ala were formed and subsequently isolated and chemically characterized. The enzyme therefore acts as an N-acetylmuramyl-L-alanine amidase by cleaving the bond between N-acetylmuramic acid and L-alanine. The glycan linked, peptide-not-cross-linked peptidoglycan dimer was also shown to be a substrate for human and mouse amidase.

Activity of N-acetylmuramoyl-L-alanine amidase in phospholipidic environments

A purified preparation of N-acetylmuramoyl-L-alanine amidase (EC 3.5.1.28), a murein hydrolase from Escherichia coli, was found to lose its activity during incubation in the presence of bacterial phospholipid suspensions. Whether it was co-dispersed with the phospholipids or added to sonicated phospholipid suspension, the enzyme was inhibited (or inactivated) from the first minutes of incubation at 37 degree C. As phosphatidylglycerol/cardiolipin ratio of the phospholipid suspension as increased (all other things being equal), a further decrease of amidase activity was observed. The highest losses of activity were found after co-dispersion of the enzyme and the substrate together with the phospholipids, the resulting suspension being formed of larger multilayered vesicles, as revealed by electron microscopy. In these conditions, the effect on enzyme activity was only partially accounted for by the proportion of the enzyme that was entrapped in the vesicles. The entrapment capacity of the enzyme (using a 35S-labelled enzyme preparation) and of the substrate (3H-labelled) by the multilamellar phospholipidic vesicles did not significantly change as a function of their relative content of phosphatidylglycerol and cardiolipin. The possible physiological meaning of the results is discussed is connection with our previous data and with other works related to membranous phospholipid distribution and to septum formation control in bacteria.

Autolysis of Escherichia coli

Autolysis of unwashed exponential-phase Escherichia coli cells was efficiently promoted by first submitting them to a quick downshock with distilled water before an upshock with 0.5 M sodium acetate, pH 6.5. The association of these two osmotic shocks had a remarkable synergistic effect and led to significant decreases in turbidity and viability. Different factors influencing the rate of cell lysis were examined. A close correlation was established between autolysis and the degradation of peptidoglycan. Both phenomena were induced by the same shock treatment, followed similar kinetics, and were efficiently blocked by addition of divalent cations. Cell lysis was also inducible by a shock treatment with 10(-3) M ethylenediaminetetraacetic acid or ethylene glycol-bis(beta-aminoethyl ether)-N,N-tetraacetic acid and blocked by the addition of divalent cations.

Escherichia coli murein transglycosylase. Purification by affinity chromatography and interaction with polynucleotides

Escherichia coli murein transglycosylase, a potential autolysin which splits the sugar chains of the murein sacculus, was rapidly purified from a crude cell extract by sequential chromatography on columns of blue Sepharose and poly(U)-Sepharose. In accordance with the binding to blue Sepharose and poly(U)-Sepharose, the transglycosylase is inhibited by Cibacron blue F3G-A, the affinity ligand of blue Sepharose, and also by polynucleotides, the latter, however, with varying efficiency. Among the polynucleotides tested, single-stranded DNA was found to be one of the most potent inhibitors. When bound to a blue Sepharose column, transglycosylase could be displaced from the column with single-stranded DNA. Taken together, these results point to a polynucleotide binding area on the transglycosylase molecule. Some aspects of the blue Sepharose affinity chromatography and the possible biological significance of the transglycosylase are discussed.

Enzymes synthesizing and hydrolyzing murein in Escherichia coli. Topographical distribution over the cell envelope

Envelopes from regions of the cell which in vivo show very little, if any, murein synthesis were isolated using the minicell-producing strain P678-54. Envelopes from minicells, representing in fact cell ends, were able to synthesize murein and to carry out transpeptidation in vitro; also all four murein hydrolase activities tested, carboxypeptidase, endopeptidase, amidase and transglycosylase, were found to be present. The specific activities of the murein synthesizing and degrading enzymes in envelopes derived from cell poles and from actively growing cells were similar. The topological distribution of murein-synthesizing enzymes and of murein hydrolases over the cell envelope is discussed.

Evidence for a role of N-acetylmuramyl-L-alanine amidase in septum separation in Escherichia coli

Septum formation and septum separation have been studied in a chain-forming mutant of Escherichia coli K-12 bearing the envA mutation and its parental strain. In comparison to the wild type, the mutant showed a sixfold reduction in the specific activity of the enzyme, N-acetylmuramyl-L-alanine amidase (EC 3.5.1.28), part of which was associated to the outer membrane. Genetic as well as physiological suppression of chain formation resulted in an increase in amidase activity. The addition of N-acetylmuramyl-L-alanyl-D-glutamyl-meso-diaminopimelic acid to growing wild-type cells and to cells bearing the envA mutation caused an inhibition of cell separation and an increased frequency of visible septa. The kinetics of septum formation and separation was followed in chains by the use of ampicillin and nalidixic acid. The latter drug inhibited initiation of new septa but allowed preformed ones to go to cell separation at a rate corresponding to that of steady-state growing cells. Ampicillin treatment, on the other hand, resulted in a more rapid decrease in the frequency of septa. The disparate effects of ampicillin and nalidixic acid were not explained by a difference in amidase activity but could be due to an inhibitory effect of ampicillin on a septal peptidoglycan fusing activity.

Protease I from Escherichia coli. Some physicochemical properties and substrate specificity

Protease I, a periplasmic endopeptidase from Escherichia coli has been further purified by a modified procedure. While the purified protein consists of a single polypeptide chain of about 21000 daltons, its molecular weight in dilute salt solution was estimated to be near 43000, suggesting that the enzyme has a marked tendency to dimerize. It has only one disulphide bond and is very sensitive to urea. In agreement with previous evidence of a chymotrypsin-like specificity, hydrolytic assays of various p-nitrophenyl esters of N-substituted amino acids showed that phenylalanine and tyrosine derivatives are the best substrates for the enzyme. The Km(app) for N-benzoyloxycarbonyl-L-tyrosin-p-nitrophenyl ester at pH 7.5 In 100 mM sodium phosphate buffer at 25 degrees C was found to be 0.2 mM. In contrast to chymotrypsin, protease I is unable to hydrolyse N-acetyl-L-phenylalanine ethyl ester and its tyrosine analogue. Moreover, the enzyme appears devoid of amidase activity and exhibits a low activity upon polypeptides. At 37 degrees C, it cleaves the carboxymethylated B-chain of bovine insulin at four points: Phe25-Tyr26, Phe24-Phe25, Leu15-Tyr16 and Ser9-His10. From a detailed study of peptides bonds hydrolyzed, it was concluded that protease I has a stringent requirement for both residues forming the scissile bond, and appears to possess an extended hydrophobic binding site.

Activity of three murein hydrolases during the cell division cycle of Escherichia coli K-12 as measured in toluene-treated cells

The specific activities of three murein hydrolases, carboxypeptidase I, carboxypeptidase II, and amidase were studied with respect to cell division in toluene-treated cells of Escherichia coli K-12. Carboxypeptidase I and amidase activities were constant throughout the division cycle in cells of D11/lac+pro+. Detectable carboxypeptidase II activity varied and was highest at the time of division by a factor of three. Carboxypeptidase II specific activity was also correlated with cell division in BUG 6, a temperature-sensitive mutant (J.N Reeve, D.J. Groves, and D.J. Clark, 1970). Fifteen minutes after shifting BUG 6 from 42 C (nondividing conditions) to 32 C (dividing conditions), there was a rapid resumption of cell division, accompanied by a 10-fold increase in the specific activity of carboxypeptidase II. These results demonstrate a correlation between detectable carboxypeptidase II activity and cell division as reflected by activity in toluene-treated cells. The subcellular location of carboxypeptidase II, a soluble enzyme was found to be periplasmic since it was released by tris(hydroxymethyl)-aminomethane-ethylenediaminetetraacetate treatment and osmotic shock, two methods known to release periplasmic enzymes.