Mode of action of pesticin: N-acetylglucosaminidase activity.

Homogeneous preparations of pesticin, a bacteriocin produced by Yersinia pestis, neither significantly inhibited net synthesis of deoxyribonucleic acid, ribonucleic acid, or protein in Escherichia coli phi nor caused detectable degradation of deoxyribonucleic acid in vivo. Accordingly, its mode of action does not resemble that of colicin E2 as suggested by others. However, incorporation of cell wall-specific label ([14C]diaminopimelic acid) into trichloroacetic acid-insoluble material of growing cells was inhibited by pesticin which also promoted release of such radioactivity from both resting cells and purified mureinlipoprotein. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of reaction mixtures containing appropriately labeled mureinlipoprotein showed that [3H]N-acetylglucosamine comigrated either with [14C]diaminopimelic acid in the murein peptide or with [14C]isoleucine of the Braun lipoprotein. As judged by these findings and pesticin-dependent release of reducing equivalents but not 4-hydroxy-2-acetamido sugars, the bacteriocin possesses N-acetylglucosaminidase activity. Hydrolysis of murein-lipoprotein occurred over a broad pH, with an optimum of 4.7. Mureinlipoproteins from a variety of pesticin-sensitive and -resistant organisms were hydrolyzed by the bacteriocin, indicating that its antibacterial specificity resides at the level of absorption.     

The N-acetyl-D-glucosamine kinase of Escherichia coli and its role in murein recycling

N-acetyl-D-glucosamine (GlcNAc) is a major component of bacterial cell wall murein and the lipopolysaccharide of the outer membrane. During growth, over 60% of the murein of the side wall is degraded, and the major products, GlcNAc-anhydro-N-acetylmuramyl peptides, are efficiently imported into the cytoplasm and cleaved to release GlcNAc, anhydro-N-acetylmuramic acid, murein tripeptide (L-Ala-D-Glu-meso-diaminopimelic acid), and D-alanine. Like murein tripeptide, GlcNAc is readily recycled, and this process was thought to involve phosphorylation, since GlcNAc-6-phosphate (GlcNAc-6-P) is efficiently used to synthesize murein or lipopolysaccharide or can be metabolized by glycolysis. Since the gene for GlcNAc kinase had not been identified, in this work we purified GlcNAc kinase (NagK) from Escherichia coli cell extracts and identified the gene by determining the N-terminal sequence of the purified kinase. A nagK deletion mutant lacked phosphorylated GlcNAc in its cytoplasm, and the cell extract of the mutant did not phosphorylate GlcNAc, indicating that NagK is the only GlcNAc kinase expressed in E. coli. Unexpectedly, GlcNAc did not accumulate in a nagK nagEBACD mutant, though both GlcNAc and GlcNAc-6-P accumulate in the nagEBACD mutant, suggesting the existence of an alternative pathway (presumably repressed by GlcNAc-6-P) that reutilizes GlcNAc without the involvement of NagK.     

Novel type of murein transglycosylase in Escherichia coli.

The purification and properties of a novel type of murein transglycosylase from Escherichia coli are described. The purified enzyme appears as a single band on sodium dodecyl sulfate-polyacrylamide gels and has an apparent molecular weight of approximately 65,000 as estimated by gel filtration and gel electrophoresis. It degrades pure murein sacculi from E. coli almost completely into low-molecular-weight products. The two prominent muropeptide fragments in the digest are the disaccharide-tripeptide N-acetylglucosamine-N-acetylmuramic acid-L-alanine-D-iso-glutamic acid-meso-diaminopimelic acid and the corresponding disaccharide-tetrapeptide N-acetylglucosamine-N-acetylmuramic acid-L-alanine-D-iso-glutamic acid-meso-diaminopimelic acid-D-alanine. The unique feature of these compounds is that the disaccharide has no reducing end group and that the muramic acid residue possesses an internal 1 leads to 6 anhydro linkage. The new lytic enzyme is designated as a murein: murein transglycosylase. Its possible role in the rearrangement of murein during cell growth and division is discussed.     

Pesticin-dependent generation of somotically stable spheroplast-like structures.

Homogenous pesticin, a bacteriocin produced by Yersinia pestis, promoted rapid dose-dependent killing of Escherichia coli phi but permitted residual generation of cell mass. Both growing cells and those blocked in net synthesis of nucleic acids or protein were converted by pesticin to osmotically stable spheroplast-like forms. Morphology and viability of cells starved for fermentable carbohydrate were not affected by pesticin. Similar spheroplast-like structures were formed from sensitive cells of Yersinia pseudotuberculosis, Yersinia enterocolitica, and Y. pestis.     

Periplasmic location of the pesticin immunity protein suggests inactivation of pesticin in the periplasm.

The pesticin activity and immunity genes on plasmid pPCP1 of Yersinia pestis were sequenced. They encoded proteins of 40 kDa (pesticin) and 16 kDa (immunity protein); the latter was found in the periplasm. The location of the immunity protein suggests that imported pesticin is inactivated in the periplasm before it hydrolyzes murein. Pesticin contains a TonB box close to the N-terminal end that is identical to the TonB box of colicin B. The DNA sequences flanking the pesticin determinant were highly homologous to those flanking the colicin 10 determinant. It is proposed that through these highly homologous DNA sequences, genes encoding bacteriocins may be exchanged between plasmids by recombination. In the case of pesticin, recombination may have destroyed the lysis gene, of which only a rudimentary fragment exists on pPCP1.     

Spheroplasts of plague microbe strains from the Transcaucasian uplands and their capacity for pesticin synthesis]

Spherical formations of the plague microbe strains from the Transcaucasian Upland, I plague microbe strain of the sandwort variety and I strain of the marmot variety were obtained under the effect of lithium chloride. They had the remains of the cell wall, were viable, sensitive to osmotic shock, preserved sensitivity to the specific bacteriophage and pesticins. All this was evident of isolation of the spheroplasts of the plague microbe. The spheroplasts showed a capacity for pesticin production. The pesticin synthesis by the spheroplasts of the plague causative agent from the Transcaucasian Upland increased with an increase in the content of lithium chloride in the medium. The largest inhibiiton zones were observed, when 0.7-0.8 per cent of lithium chloride were present in the medium. In the spheroplasts of the plague causative agent from the Mountain Altai (the marmot variety) the pesticin synthesis was inhibited with an increase in the content of lithium chloride in the medium. The activity spectrum of the pesticins of the spheroplasts of the plague causative agent from the Transcaucasian Upland and the spheroplasts of the strains of the marmot and sandwort varieties was broader than that of the rod-like forms of these strains. The indicator properties were found in the strains of the plague microbe of the marmot and sandwort varieties with respect to the pesticins of the spheroplasts of the sel-like producing organisms and organisms from the Transcaucasian Upland.     

Colicin M is an inhibitor of murein biosynthesis.

Colicin M inhibited the incorporation of DL + meso-2,6-diamino[3,4,5-3H]pimelic acid into the murein (peptidoglycan) of growing cells of Escherichia coli W7 dap lys. The inhibition of the UDP-N-acetylmuramyl pentapeptide-dependent incorporation of UDP-N-acetyl-D-[U-14C]glucosamine into isolated cell envelopes indicated interference with a late step of murein biosynthesis. After the inhibition of murein biosynthesis, cells lysed, and they released lysis products of murein. In vitro, the murein biosynthesis of colicin M-tolerant mutants (tolM) was inhibited by colicin M. Therefore, tolerance is probably conferred by an impaired uptake of an altered fixation close to the target site and not by a mutation of the target itself. Preliminary studies with beta-lactam antibiotics and with mutants in penicillin-binding proteins did not reveal a specific enzymatic step inhibited by colicin M. The unique action among the colicins renders colicin M a potentially useful tool for studying murein biosynthesis.     

Lipoprotein synthesis in Escherichia coli spheroplasts: accumulation of lipoprotein in cytoplasmic membrane.

Synthesis of cell envelope proteins was studied in ethylenediaminetetraacetic acid-lysozyme spheroplasts of Escherichia coli ML30. The rate of incorporation of [3H]arginine into proteins in spheroplasts was about 30% of that of intact cells. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of proteins synthesized in spheroplasts revealed the preferential synthesis of five polypeptides, one of which has been identified as the free form of murein lipoprotein. Lipoprotein synthesized in spheroplasts was found to be of same molecular size as that of mature lipoprotein. No prolipoprotein was observed even with a short pulse-labeling with [3H]arginine. On the other hand, significant accumulation of newly synthesized lipoprotein in the cytoplasmic membrane fraction of spheroplasts was observed. These results suggest that the processing of prolipoprotein occurs in the cytoplasmic membrane fraction of the cell envelope.     

Reconstitution of binding protein dependent ribose transport in spheroplasts derived from a binding protein negative escherichia coli K12 mutant and from salmonella typhimurium

Highly purified ribose-binding protein from Escherichia coli has been used to reconstitute a binding-protein-dependent ribose transport in spheroplasts derived from a binding-protein-deficient mutant of E coli K 12, and in spheroplasts derived from Salmonella typhimurium. The cross-species reconstitution was nearly as efficient as the reconstitution of the E coli strain from which the binding protein was derived. Antibody raised against the ribose binding protein completely prevented reconstitution, whereas it had no effect on whole cells. The reconstitution procedure has been improved by generating spheroplasts from cells grown in a rich medium and by reducing the background uptake in spheroplasts through a special washing procedure. Rapid purification of ribose binding protein by high pressure liquid chromatography is also described.     

Murein segregation in Escherichia coli.

Peptidoglycan (murein) segregation has been studied by means of a new labeling method. The method relies on the ability of Escherichia coli cells to incorporate D-Cys into macromolecular murein. The incorporation depends on a periplasmic amino acid exchange reaction. At low concentrations, D-Cys is innocuous to the cell. The distribution of modified murein in purified sacculi can be traced and visualized by immunodetection of the -SH groups by fluorescence and electron microscopy techniques. Analysis of murein segregation in wild-type and cell division mutant strains revealed that murein in polar caps is metabolically inert and is segregated in a conservative fashion. Elongation of the sacculus apparently occurs by diffuse insertion of precursors over the cylindrical part of the cell surface. At the initiation of cell division, there is a FtsZ-dependent localized activation of murein synthesis at the potential division sites. Penicillin-binding protein 3 and the products of the division genes ftsA and ftsQ are dispensable for the activation of division sites. As a consequence, under restrictive conditions ftsA,ftsI,or ftsQ mutants generate filamentous sacculi with rings of all-new murein at the positions where septa would otherwise develop.     

Murein structure and lack of DD- and LD-carboxypeptidase activities in Caulobacter crescentus

High-pressure liquid chromatography of a muramidase digest of murein sacculi from Caulobacter crescentus showed that the absence of D-alanine carboxypeptidase activity in the cells was reflected by a very high content of pentapeptide in the murein. Approximately half of the pentapeptide side chains were shown to contain glycine, which replaced D-alanine as the terminal amino acid.     

The effect of osmotic shock on the accessibility of the murein layer of exponentially growing Escherichia coli to lysozyme.

The restricted access of lysozyme to the murein layer of exponential phase Escherichia coli is enhanced considerably by osmotic shock. When cells suspended in Tris/EDTA/sucrose are diluted 11-fold in water or 10 mM EDTA in the presence of lysozyme, their susceptibility to lysozyme increases by a factor of 50--100, for both Escherichia coli JC411 and W3110, grown to the early exponential phase in unsuppleneted or supplemented minimal media, and in Brain Heart Infusion. Since an 11-fold dilution causes lysis of lysozyme spheroplasts, the effects of a 2-fold dilution have also been investigated. A 2-fold dilution of cell suspended in TrisEDTA/sucrose still increases their susceptibility to lysozyme by a factor of 10--50, but the resulting spheroplasts remain intact. EDTA is necessary to permit lysozyme access to the murein layer during the dilution, which is ineffective in the presence of 5 mM MgCl2. These results are discussed in terms of the formation of lysozyme spheroplasts from young Escherichia coli.     

Murein (peptidoglycan) binding property of the essential cell division protein FtsN from Escherichia coli

The binding of the essential cell division protein FtsN of Escherichia coli to the murein (peptidoglycan) sacculus was studied. Soluble truncated variants of FtsN, including the complete periplasmic part of the protein as well as a variant containing only the C-terminal 77 amino acids, did bind to purified murein sacculi isolated from wild-type cells. FtsN variants lacking this C-terminal region showed reduced or no binding to murein. Binding of FtsN was severely reduced when tested against sacculi isolated either from filamentous cells with blocked cell division or from chain-forming cells of a triple amidase mutant. Binding experiments with radioactively labeled murein digestion products revealed that the longer murein glycan strands (>25 disaccharide units) showed a specific affinity to FtsN, but neither muropeptides, peptides, nor short glycan fragments bound to FtsN. In vivo FtsN could be cross-linked to murein with the soluble disulfide bridge containing cross-linker DTSSP. Less FtsN, but similar amounts of OmpA, was cross-linked to murein of filamentous or of chain-forming cells compared to levels in wild-type cells. Expression of truncated FtsN variants in cells depleted in full-length FtsN revealed that the presence of the C-terminal murein-binding domain was not required for cell division under laboratory conditions. FtsN was present in 3,000 to 6,000 copies per cell in exponentially growing wild-type E. coli MC1061. We discuss the possibilities that the binding of FtsN to murein during cell division might either stabilize the septal region or might have a function unrelated to cell division.     

Effects of lemongrass oil on the morphological characteristics and peptidoglycan synthesis of Escherichia coli cells

The antibacterial effect of lemongrass oil, obtained from the aerial part of Cymbopogon citratus, on cells of Escherichia coli was investigated by electron microscopy and by measuring cell wall formation. Two strains of E. coli K-12 were used, one of which required diaminopimelic acid in the growth medium for its murein formation. Lemongrass oil was found to elicit morphological changes like filamentation, inhibition of septum formation, spheroplast formation, production of 'blisters', 'bulges' or mesosomes, as well as lysis and development of abnormally shaped cells. The incorporation of radioactively labelled diaminopimelic acid into the cell wall murein of strain W7, was inhibited by lemongrass oil in a dose dependent way. The sequence of changes induced by lemongrass oil on bacterial cell morphology and also its interference with murein synthesis in E. coli cells were interpreted to involve the penicillin binding proteins PBP 2 and PBP 3.     

N-acetylmuramyl-L-alanine amidase of Vi phage III.

1. A lytic enzyme was isolated from Vi phage III-induced lysate of Salmonella typhi, and purified about 200-fold by chromatography on IRC-50, CM-cellulose, and Sephadex G-75 columns. 2. Both E. coli B murein and muropeptide C6 were digested on incubation with the lytic enzyme. The main product of murein and muropeptide C6 digestion is identical with tetrapeptide Ala-Glu-DAP-Ala. The release of amino groups during digestion was not accompanied by the appearance of either reducing groups or hexosamines. 3. It is concluded that Vi phage III-induced lytic enzyme is N-acetylmuramyl-L-alanine amidase, which cleaves the amide bond between N-acetylmuramic acid and L-alanine.     

Expression,purification, and characterization of a biologically active bovine enterokinase catalytic subunit in Escherichia coli

Enterokinase (EC 3.4.21.9) is a serine proteinase in the duodenum that exhibits specificity for the sequence (Asp)(4)-Lys. It converts trypsinogen to trypsin. Its high specificity for the recognition site makes enterokinase (EK) a useful tool for in vitro cleavage of fusion proteins. cDNA encoding the catalytic chain of Chinese bovine enterokinase was cloned and its encoding amino acid sequence is identical to the previously reported sequence although there are two one-base mutations which do not change the encoded amino acid. The EK catalytic subunit cDNA was cloned into plasmid pET32a, and fused downstream to the fusion partner thioredoxin (Trx) and the following DDDDK enterokinase recognition sequence. The recombinant bovine enterokinase catalytic subunit was expressed in Escherichia coli BL21(DE3), and most products existed in soluble form. After an in vivo autocatalytic cleavage of the recombinant Trx-EK catalytic domain fusion protein, intact, biologically active EK catalytic subunit was released from the fusion protein. The recombinant intact EK catalytic subunit was purified to homogeneity with a specific activity of 720 AUs/mg protein through ammonium sulfate precipitation, DEAE chromatography, and gel filtration. The purified intact EK catalytic subunit has a K(m) of 0.17 mM, and K(cat) is 20.8s(-1). From 100 ml flask culture, 4.3 mg pure active EK catalytic subunits were obtained.     

Electron Microscopy and Viability of Lysostaphin-induced Staphylococcal Spheroplasts, Protoplast-like Bodies, and Protoplasts

The cell walls of a selected isolate of Staphylococcus aureus FDA 209P were observed undergoing progressive disintegration when exposed to lysostaphin (1 unit/ml) in 24% NaCl solution. Electron micrographs of ultrathin sections of test cells after exposure to lysostaphin for 2 min showed only superficial evidence of lytic damage. However, an average of 89% of these cells were osmotically fragile, and 21% were damaged beyond their capacity to regenerate cell walls and to grow as normal staphylococci. The 68% (average) of the osmotically fragile cells which retained the capacity to revert to normal staphylococci were designated spheroplasts. Neither perforations of the cell walls nor separation of the cell walls from the plasma membranes were observed in the micrographs of these 2-min spheroplasts. Thus, it appears that the osmotic fragility of these and possibly all lysostaphin-induced staphylococcal spheroplasts results from the hydrolysis of a critical number of the pentapeptide cross-linkages of the murein of the cell wall. Electron micrographs of cells exposed to lysostaphin for 5 to 10 min showed perforations and more extensive damage, including the separation of walls from the plasma membranes and the disintegration of large sections of the walls. Smaller numbers of spheroplasts (21 and 8%) were recovered from these 5- and 10-min preparations; those recovered probably represent cells which were attacked more slowly than the majority by the lytic enzyme. The nonrevertible, osmotically fragile cells that retained segments of cell wall were designated protoplast-like bodies. After 20-min exposure to lysostaphin, all of the cell wall was digested away from most of the cells, and true staphylococcal protoplasts were produced. These lysostaphin-induced, osmotically fragile forms appear to have different osmotic properties from the staphylococcal "protoplasts" reported by other investigators and should serve as the basis for a variety of fundamental investigations.     

Escherichia coli produces a cytoplasmic alpha-amylase, AmyA.

In the gap between two closely linked flagellar gene clusters on the Escherichia coli and Salmonella typhimurium chromosomes (at about 42 to 43 min on the E. coli map), we found an open reading frame whose sequence suggested that it encoded an alpha-amylase; the deduced amino acid sequences in the two species were 87% identical. The strongest similarities to other alpha-amylases were to the excreted liquefying alpha-amylases of bacilli, with > 40% amino acid identity; the N-terminal sequence of the mature bacillar protein (after signal peptide cleavage) aligned with the N-terminal sequence of the E. coli or S. typhimurium protein (without assuming signal peptide cleavage). Minicell experiments identified the product of the E. coli gene as a 56-kDa protein, in agreement with the size predicted from the sequence. The protein was retained by spheroplasts rather than being released with the periplasmic fraction; cells transformed with plasmids containing the gene did not digest extracellular starch unless they were lysed; and the protein, when overproduced, was found in the soluble fraction. We conclude that the protein is cytoplasmic, as predicted by its sequence. The purified protein rapidly digested amylose, starch, amylopectin, and maltodextrins of size G6 or larger; it also digested glycogen, but much more slowly. It was specific for the alpha-anomeric linkage, being unable to digest cellulose. The principal products of starch digestion included maltotriose and maltotetraose as well as maltose, verifying that the protein was an alpha-amylase rather than a beta-amylase. The newly discovered gene has been named amyA. The natural physiological role of the AmyA protein is not yet evident.     

Characterization of the cell wall murein of Thiobacillus versutus

Amino acid analysis of pure murein isolated from cells of Thiobacillus versutus grown in complex medium revealed the typical constituents of most mureins from gram-negative cells, i.e. muramic acid, glucosamine, glutamic acid, alanine and diaminopimelic acid in molecular ratio of 0.58: 0.79: 1.0: 1.76:1.07, respectively. The presence of glycine and leucine was also demonstrated (0.20 and 0.08 compared to glutamic acid). Glycine was also present in the murein of cells grown in chemically defined synthetic medium. The crosslinkage of T. versutus murein was approximately 36% --much higher than for most other gram-negative species. High pressure liquid chromatography analysis of muropeptide composition following muramidase digestion of T. versutus murein revealed essentially the same pattern as for Escherichia coli under similar conditions of digestion and separation with, however, some differences in the minor peaks.     

Evidence for diffuse growth of the cylindrical portion of the Escherichia coli murein sacculus.

High-resolution autoradiography of thin sections of Escherichia coli cells whose murein was pulse-labeled with [3H]diaminopimelic acid after a period of diaminopimelic acid deprivation indicated that elongation of the murein sacculus occurs by a multisite (diffuse) process. Upon chasing, radioactivity in polar murein was stable, whereas radioactivity in cylindrical murein was reduced, indicating that diffuse intercalation of new murein occurred during cell elongation. Elongation and septation were shown to be overlapping processes.