Molecular interplay of murein synthases and murein hydrolases in Escherichia coli

Semisynthesis of a Bacterium with Non-canonical Cell-Wall Cross-Links

The cell wall is an elaborate framework of peptidoglycan that serves to protect the bacterium against osmotic challenge. This exoskeleton is composed of repeating saccharides covalently cross-linked by peptide stems. The general structure of the cell wall is widely conserved across diverse Gram-negative bacteria. To begin to explore the biological consequence of introducing non-canonical cross-links into the cell wall of Escherichia coli, we generated a bacterium where up to 31% of the cell-wall cross-links are formed by a non-enzymatic reaction between a sulfonyl fluoride and an amino group. Bacteria with these non-canonical cell-wall cross-links achieve a high optical density in culture, divide and elongate successfully, and display no loss of outer membrane integrity. This work represents a first step in the design of bacteria with non-canonical "synthetic" cell walls.

Slt, MltD, and MltG of Pseudomonas aeruginosa as Targets of Bulgecin A in Potentiation of β-Lactam Antibiotics

The interplay between the activities of lytic transglycosylases (LTs) and penicillin-binding proteins (PBPs) is critical for the health of the bacterial cell wall. Bulgecin A (a natural-product inhibitor of LTs) potentiates the activity of β-lactam antibiotics (inhibitors of PBPs), underscoring this intimate mechanistic interdependence. Bulgecin A in the presence of an appropriate β-lactam causes bulge deformation due to the formation of aberrant peptidoglycan at the division site of the bacterium. As Pseudomonas aeruginosa, a nefarious human pathogen, has 11 LT paralogs, the answer as to which LT activity correlates with β-lactam potentiation is important and is currently unknown. Growth of P. aeruginosa PAO1 strains harboring individual transposon-insertion mutants at each of the 11 genes for LTs, in the presence of the β-lactam antibiotic ceftazidime or meropenem, implicated the gene products of slt, mltD, and mltG (of the 11), in bulge formation and potentiation. Hence, the respective enzymes would be the targets of inhibition by bulgecin A, which was indeed documented. We further demonstrated by imaging in real time and by SEM that cell lysis occurs by the structural failure of this bulge. Upon removal of the β-lactam antibiotic prior to lysis, P. aeruginosa experiences delayed recovery from the elongation and bulge phenotype in the presence of bulgecin A. These observations argue for a collaborative role for the target LTs in the repair of the aberrant cell wall, the absence of activities of which in the presence of bulgecin A results in potentiation of the β-lactam antibiotic.

The "hole" story of predatory outer-membrane vesicles

All Gram-negative bacteria release membrane vesicles. These vesicles contain a cargo of proteins and enzymes that include one or more autolysins. Autolysins are a group of enzymes with specificity for the different linkages within peptidoglycan sacculi that if uncontrolled cause bacteriolysis. This minireview, written in honor and memory of Terry Beveridge, presents an overview of autolytic activity and focuses on Beveridge's important original observations regarding predatory membrane vesicles and their associated autolysin cargo.

Lytic transglycosylases: concinnity in concision of the bacterial cell wall

The lytic transglycosylases (LTs) are bacterial enzymes that catalyze the non-hydrolytic cleavage of the peptidoglycan structures of the bacterial cell wall. They are not catalysts of glycan synthesis as might be surmised from their name. Notwithstanding the seemingly mundane reaction catalyzed by the LTs, their lytic reactions serve bacteria for a series of astonishingly diverse purposes. These purposes include cell-wall synthesis, remodeling, and degradation; for the detection of cell-wall-acting antibiotics; for the expression of the mechanism of cell-wall-acting antibiotics; for the insertion of secretion systems and flagellar assemblies into the cell wall; as a virulence mechanism during infection by certain Gram-negative bacteria; and in the sporulation and germination of Gram-positive spores. Significant advances in the mechanistic understanding of each of these processes have coincided with the successive discovery of new LTs structures. In this review, we provide a systematic perspective on what is known on the structure-function correlations for the LTs, while simultaneously identifying numerous opportunities for the future study of these enigmatic enzymes.

Activation by Allostery in Cell-Wall Remodeling by a Modular Membrane-Bound Lytic Transglycosylase from Pseudomonas aeruginosa

Bacteria grow and divide without loss of cellular integrity. This accomplishment is notable, as a key component of their cell envelope is a surrounding glycopeptide polymer. In Gram-negative bacteria this polymer-the peptidoglycan-grows by the difference between concurrent synthesis and degradation. The regulation of the enzymatic ensemble for these activities is poorly understood. We report herein the structural basis for the control of one such enzyme, the lytic transglycosylase MltF of Pseudomonas aeruginosa. Its structure comprises two modules: an ABC-transporter-like regulatory module and a catalytic module. Occupancy of the regulatory module by peptidoglycan-derived muropeptides effects a dramatic and long-distance (40 Å) conformational change, occurring over the entire protein structure, to open its active site for catalysis. This discovery of the molecular basis for the allosteric control of MltF catalysis is foundational to further study of MltF within the complex enzymatic orchestration of the dynamic peptidoglycan.

Biochemical characterization of the major N-acetylmuramidase from Lactobacillus buchneri

Bacterial cell wall hydrolases are essential for peptidoglycan remodelling in regard to bacterial cell growth and division. In this study, peptidoglycan hydrolases (PGHs) of different Lactobacillus buchneri strains were investigated. First, the genome sequence of L. buchneri CD034 and L. buchneri NRRL B-30929 was analysed in silico for the presence of PGHs. Of 23 putative PGHs with different predicted hydrolytic specificities, the glycosyl hydrolase family 25 domain-containing homologues LbGH25B and LbGH25N from L. buchneri CD034 and NRRL B-30929, respectively, were selected and characterized in detail. Zymogram analysis confirmed hydrolysing activity on bacterial cell walls for both enzymes. Subsequent reversed-phase HPLC and MALDI-TOF MS analysis of the peptidoglycan breakdown products from L. buchneri strains CD034 and NRRL B-30929, and from Lactobacillus rhamnosus GG, which served as a reference, revealed that LbGH25B and LbGH25N have N-acetylmuramidase activity. Both enzymes were identified as cell wall-associated proteins by means of immunofluorescence microscopy and cellular fractionation, as well as by the ability of purified recombinant LbGH25B and LbGH25N to bind to L. buchneri cell walls in vitro. Moreover, similar secondary structures mainly composed of β-sheets and nearly identical thermal stabilities with T m values around 49 °C were found for the two N-acetylmuramidases by far-UV circular dichroism spectroscopy. The functional and structural data obtained are discussed and compared to related PGHs. In this study, a major N-acetylmuramidase from L. buchneri was characterized in detail for the first time.

Bacterial actin MreB forms antiparallel double filaments

Filaments of all actin-like proteins known to date are assembled from pairs of protofilaments that are arranged in a parallel fashion, generating polarity. In this study, we show that the prokaryotic actin homologue MreB forms pairs of protofilaments that adopt an antiparallel arrangement in vitro and in vivo. We provide an atomic view of antiparallel protofilaments of Caulobacter MreB as apparent from crystal structures. We show that a protofilament doublet is essential for MreB's function in cell shape maintenance and demonstrate by in vivo site-specific cross-linking the antiparallel orientation of MreB protofilaments in E. coli. 3D cryo-EM shows that pairs of protofilaments of Caulobacter MreB tightly bind to membranes. Crystal structures of different nucleotide and polymerisation states of Caulobacter MreB reveal conserved conformational changes accompanying antiparallel filament formation. Finally, the antimicrobial agents A22/MP265 are shown to bind close to the bound nucleotide of MreB, presumably preventing nucleotide hydrolysis and destabilising double protofilaments.DOI: http://dx.doi.org/10.7554/eLife.02634.001.

Differentiated roles for MreB-actin isologues and autolytic enzymes in Bacillus subtilis morphogenesis

Cell morphogenesis in most bacteria is governed by spatiotemporal growth regulation of the peptidoglycan cell wall layer. Much is known about peptidoglycan synthesis but regulation of its turnover by hydrolytic enzymes is much less well understood. Bacillus subtilis has a multitude of such enzymes. Two of the best characterized are CwlO and LytE: cells lacking both enzymes have a lethal block in cell elongation. Here we show that activity of CwlO is regulated by an ABC transporter, FtsEX, which is required for cell elongation, unlike cell division as in Escherichia coli. Actin-like MreB proteins are thought to play a key role in orchestrating cell wall morphogenesis. B. subtilis has three MreB isologues with partially differentiated functions. We now show that the three MreB isologues have differential roles in regulation of the CwlO and LytE systems and that autolysins control different aspects of cell morphogenesis. The results add major autolytic activities to the growing list of functions controlled by MreB isologues in bacteria and provide new insights into the different specialized functions of essential cell wall autolysins.

Positioning cell wall synthetic complexes by the bacterial morphogenetic proteins MreB and MreD

In Caulobacter crescentus, intact cables of the actin homologue, MreB, are required for the proper spatial positioning of MurG which catalyses the final step in peptidoglycan precursor synthesis. Similarly, in the periplasm, MreC controls the spatial orientation of the penicillin binding proteins and a lytic transglycosylase. We have now found that MreB cables are required for the organization of several other cytosolic murein biosynthetic enzymes such as MraY, MurB, MurC, MurE and MurF. We also show these proteins adopt a subcellular pattern of localization comparable to MurG, suggesting the existence of cytoskeletal-dependent interactions. Through extensive two-hybrid analyses, we have now generated a comprehensive interaction map of components of the bacterial morphogenetic complex. In the cytosol, this complex contains both murein biosynthetic enzymes and morphogenetic proteins, including RodA, RodZ and MreD. We show that the integral membrane protein, MreD, is essential for lateral peptidoglycan synthesis, interacts with the precursor synthesizing enzymes MurG and MraY, and additionally, determines MreB localization. Our results suggest that the interdependent localization of MreB and MreD functions to spatially organize a complex of peptidoglycan precursor synthesis proteins, which is required for propagation of a uniform cell shape and catalytically efficient peptidoglycan synthesis.

Molecular control of bacterial death and lysis

Although the phenomenon of bacterial cell death and lysis has been studied for over 100 years, the contribution of these important processes to bacterial physiology and development has only recently been recognized. Contemporary study of cell death and lysis in a number of different bacteria has revealed that these processes, once thought of as being passive and unregulated, are actually governed by highly complex regulatory systems. An emerging paradigm in this field suggests that, analogous to programmed cell death in eukaryotes, regulated cell death and lysis in bacteria play an important role in both developmental processes, such as competence and biofilm development, and the elimination of damaged cells, such as those irreversibly injured by environmental or antibiotic stress. Further study in this exciting field of bacterial research may provide new insight into the potential evolutionary link between control of cell death in bacteria and programmed cell death (apoptosis) in eukaryotes.

Bacterial peptidoglycan (murein) hydrolases

Most bacteria have multiple peptidoglycan hydrolases capable of cleaving covalent bonds in peptidoglycan sacculi or its fragments. An overview of the different classes of peptidoglycan hydrolases and their cleavage sites is provided. The physiological functions of these enzymes include the regulation of cell wall growth, the turnover of peptidoglycan during growth, the separation of daughter cells during cell division and autolysis. Specialized hydrolases enlarge the pores in the peptidoglycan for the assembly of large trans-envelope complexes (pili, flagella, secretion systems), or they specifically cleave peptidoglycan during sporulation or spore germination. Moreover, peptidoglycan hydrolases are involved in lysis phenomena such as fratricide or developmental lysis occurring in bacterial populations. We will also review the current view on the regulation of autolysins and on the role of cytoplasm hydrolases in peptidoglycan recycling and induction of beta-lactamase.

Premature targeting of cell division proteins to midcell reveals hierarchies of protein interactions involved in divisome assembly

In order to divide, the bacterium Escherichia coli must assemble a set of at least 10 essential proteins at the nascent division site. These proteins localize to midcell according to a linear hierarchy, suggesting that cell division proteins are added to the nascent divisome in strict sequence. We previously described a method, 'premature targeting', which allows us to target a protein directly to the division site independently of other cell division proteins normally required for its localization at midcell. By systematically applying this method to probe the recruitment of and associations among late cell division proteins, we show that this linear assembly model is likely incorrect. Rather, we find that the assembly of most of the late proteins can occur independently of 'upstream' proteins. Further, most late proteins, when prematurely targeted to midcell, can back-recruit upstream proteins in the reverse of the predicted pathway. Together these observations indicate that the late proteins, with the notable exception of the last protein in the pathway, FtsN, are associated in a hierarchical set of protein complexes. Based on these observations we present a revised model for assembly of the E. coli division apparatus.

In Vitro Murein (Peptidoglycan) Synthesis by Dimers of the Bifunctional Transglycosylase-Transpeptidase PBP1B from Escherichia coli

PBP1B is a major bifunctional murein (peptidoglycan) synthase catalyzing transglycosylation and transpeptidation reactions in Escherichia coli. PBP1B has been shown to form dimers in vivo. The K(D) value for PBP1B dimerization was determined by surface plasmon resonance. The effect of the dimerization of PBP1B on its activities was studied with a newly developed in vitro murein synthesis assay with radioactively labeled lipid II precursor as substrate. Under conditions at which PBP1B dimerizes, the enzyme synthesized murein with long glycan strands (>25 disaccharide units) and with almost 50% of the peptides being part of cross-links. PBP1B was also capable of synthesizing trimeric muropeptide structures. Tri-, tetra-, and pentapeptide compounds could serve as acceptors in the PBP1B-catalyzed transpeptidation reaction.

A novel firmicute protein family related to the actinobacterial resuscitation-promoting factors by non-orthologous domain displacement

BACKGROUND

In Micrococcus luteus growth and resuscitation from starvation-induced dormancy is controlled by the production of a secreted growth factor. This autocrine resuscitation-promoting factor (Rpf) is the founder member of a family of proteins found throughout and confined to the actinobacteria (high G + C Gram-positive bacteria). The aim of this work was to search for and characterise a cognate gene family in the firmicutes (low G + C Gram-positive bacteria) and obtain information about how they may control bacterial growth and resuscitation.

RESULTS

In silico analysis of the accessory domains of the Rpf proteins permitted their classification into several subfamilies. The RpfB subfamily is related to a group of firmicute proteins of unknown function, represented by YabE of Bacillus subtilis. The actinobacterial RpfB and firmicute YabE proteins have very similar domain structures and genomic contexts, except that in YabE, the actinobacterial Rpf domain is replaced by another domain, which we have called Sps. Although totally unrelated in both sequence and secondary structure, the Rpf and Sps domains fulfil the same function. We propose that these proteins have undergone "non-orthologous domain displacement", a phenomenon akin to "non-orthologous gene displacement" that has been described previously. Proteins containing the Sps domain are widely distributed throughout the firmicutes and they too fall into a number of distinct subfamilies. Comparative analysis of the accessory domains in the Rpf and Sps proteins, together with their weak similarity to lytic transglycosylases, provide clear evidence that they are muralytic enzymes.

CONCLUSIONS

The results indicate that the firmicute Sps proteins and the actinobacterial Rpf proteins are cognate and that they control bacterial culturability via enzymatic modification of the bacterial cell envelope.

Investigations into the mechanisms used by the C-terminal anchors of Escherichia coli penicillin-binding proteins 4, 5, 6 and 6b for membrane interaction

Escherichia coli low molecular mass penicillin-binding proteins (PBPs) include PBP4, PBP5, PBP6 and PBP6b. Evidence suggests that these proteins interact with the inner membrane via C-terminal amphiphilic alpha-helices. Nonetheless, the membrane interactive mechanisms utilized by the C-terminal anchors of PBP4 and PBP6b show differences to those utilized by PBP5 and PBP6. Here, hydrophobic moment-based analyses have predicted that, in contrast to the PBP4 and PBP6b C-termini, those of PBP5 and PBP6 are candidates to form oblique orientated alpha-helices. Consistent with these predictions, Fourier transform infrared spectroscopy (FTIR) has shown that peptide homologs of the PBP4 and PBP5 C-terminal regions, P4 and P5, respectively, both possessed the ability to adopt alpha-helical structure in the presence of lipid. However, whereas P4 appeared to show a preference for interaction with the surface regions of dimyristoylglycerophosphoethanolamine and dimyristoylglycerophosphoglycerol membranes, P5 appeared to show deep penetration of both these latter membranes and dimyristoylglycerophosphocholine membranes. Based on these results, we have suggested that in contrast to the membrane anchoring of the PBP4 and PBP6b C-terminal alpha-helices, the PBP5 and PBP6 C-terminal alpha-helices may possess hydrophobicity gradients and penetrate membranes in an oblique orientation.

Fast lysis of Escherichia coli filament cells requires differentiation of potential division sites

Periodic activation of zonal peptidoglycan (murein) synthesis at division sites in Escherichia coli has been reported recently. Zonal synthesis is responsible for septum formation, whereas elongation of the cell sacculus is performed by diffuse insertion of precursors. Zonal synthesis can be triggered in ftsA, ftsQ and ftsI (pbpB) division mutants growing as filaments at the restrictive temperature, but not in ftsZ mutant strains. The lytic response to beta-lactams of cells able or unable to periodically trigger a zonal mode of murein synthesis could be substantially different. Therefore, we investigated the response to the bacteriolytic beta-lactam cefsulodin of ftsZ and ftsI mutants growing at the restrictive (42 degrees C) temperature. The ftsI cells lysed early and quickly after addition of the antibiotic. Sacculi of lysed cells were transversely cut in a very sharp way. In contrast the ftsZ strain lysed late and slowly after addition of the antibiotic and sacculi showed a generalized weakening of the murein network and extended breaks with a frayed appearance. No transversely cut sacculi comparable to those seen in the ftsI samples were found. Our results strongly support that beta-lactam-induced lysis occurs preferentially at division sites because of the activation of zonal murein synthesis at the initiation of septation.

Differential responses of Escherichia coli cells expressing cytoplasmic domain mutants of penicillin-binding protein 1b after impairment of penicillin-binding proteins 1a and 3

Penicillin-binding protein 1b (PBP1b) is the major high-molecular-weight PBP in Escherichia coli. Although it is coded by a single gene, it is usually found as a mixture of three isoforms which vary with regard to the length of their N-terminal cytoplasmic tail. We show here that although the cytoplasmic tail seems to play no role in the dimerization of PBP1b, as was originally suspected, only the full-length protein is able to protect the cells against lysis when both PBP1a and PBP3 are inhibited by antibiotics. This suggests a specific role for the full-length PBP1b in the multienzyme peptidoglycan-synthesizing complex that cannot be fulfilled by either PBP1a or the shorter PBP1b proteins. Moreover, we have shown by alanine-stretch-scanning mutagenesis that (i) residues R(11) to G(13) are major determinants for correct translocation and folding of PBP1b and that (ii) the specific interactions involving the full-length PBP1b can be ascribed to the first six residues at the N-terminal end of the cytoplasmic domain. These results are discussed in terms of the interactions with other components of the murein-synthesizing complex.

Antibacterial activity of synthetic analogues based on the disaccharide structure of moenomycin, an inhibitor of bacterial transglycosylase

Moenomycin is a natural product glycolipid that inhibits the growth of a broad spectrum of Gram-positive bacteria. In Escherichia coli, moenomycin inhibits peptidoglycan synthesis at the transglycosylation stage, causes accumulation of cell-wall intermediates, and leads to lysis and cell death. However, unlike Esc. coli, where 5-6 log units of killing are observed, 0-2 log units of killing occurred when Gram-positive bacteria were treated with similar multiples of the MIC. In addition, bulk peptidoglycan synthesis in intact Gram-positive cells was resistant to the effects of moenomycin. In contrast, synthetic disaccharides based on the moenomycin disaccharide core structure were identified that were bactericidal to Gram-positive bacteria, inhibited cell-wall synthesis in intact cells, and were active on both sensitive and vancomycin-resistant enterococci. These disaccharide analogues do not inhibit the formation of N:-acetylglucosamine-ss-1, 4-MurNAc-pentapeptide-pyrophosphoryl-undecaprenol (lipid II), but do inhibit the polymerization of lipid II into peptidoglycan in Esc. coli. In addition, cell growth was required for bactericidal activity. The data indicate that synthetic disaccharide analogues of moenomycin inhibit cell-wall synthesis at the transglycosylation stage, and that their activity on Gram-positive bacteria differs from moenomycin due to differential targeting of the transglycosylation process. Inhibition of the transglycosylation process represents a promising approach to the design of new antibacterial agents active on drug-resistant bacteria.

Crystal structure of Escherichia coli lytic transglycosylase Slt35 reveals a lysozyme-like catalytic domain with an EF-hand

BACKGROUND

Lytic transglycosylases are bacterial muramidases that catalyse the cleavage of the beta- 1,4-glycosidic bond between N-acetylmuramic acid (MurNAc) and N-acetylglucosamine (GlcNAc) in peptidoglycan with concomitant formation of a 1,6-anhydrobond in the MurNAc residue. These muramidases play an important role in the metabolism of the bacterial cell wall and might therefore be potential targets for the rational design of antibacterial drugs. One of the lytic transglycosylases is Slt35, a naturally occurring soluble fragment of the outer membrane bound lytic transglycosylase B (MltB) from Escherichia coli.

RESULTS

The crystal structure of Slt35 has been determined at 1.7 A resolution. The structure reveals an ellipsoid molecule with three domains called the alpha, beta and core domains. The core domain is sandwiched between the alpha and beta domains. Its fold resembles that of lysozyme, but it contains a single metal ion binding site in a helix-loop-helix module that is surprisingly similar to the eukaryotic EF-hand calcium-binding fold. Interestingly, the Slt35 EF-hand loop consists of 15 residues instead of the usual 12 residues. The only other prokaryotic proteins with an EF-hand motif identified so far are the D-galactose-binding proteins. Residues from the alpha and core domains form a deep groove where the substrate fragment GlcNAc can be bound.

CONCLUSIONS

The three-domain structure of Slt35 is completely different from the Slt70 structure, the only other lytic transglycosylase of known structure. Nevertheless, the core domain of Slt35 closely resembles the fold of the catalytic domain of Slt70, despite the absence of any obvious sequence similarity. Residue Glu162 of Slt35 is in an equivalent position to Glu478, the catalytic acid/base of Slt70. GlcNAc binds close to Glu162 in the deep groove. Moreover, mutation of Glu162 into a glutamine residue yielded a completely inactive enzyme. These observations indicate the location of the active site and strongly support a catalytic role for Glu162.

High resolution crystal structures of the Escherichia coli lytic transglycosylase Slt70 and its complex with a peptidoglycan fragment

The 70 kDa soluble lytic transglycosylase (Slt70) from Escherichia coli is an exo-muramidase, that catalyses the cleavage of the glycosidic bonds between N -acetylmuramic acid and N -acetylglucosamine residues in peptidoglycan, the main structural component of the bacterial cell wall. This cleavage is accompanied by the formation of a 1,6-anhydro bond between the C1 and O6 atoms in the N -acetylmuramic acid residue (anhMurNAc). Crystallographic studies at medium resolution revealed that Slt70 is a multi-domain protein consisting of a large ring-shaped alpha-superhelix with on top a catalytic domain, which resembles the fold of goose-type lysozyme. Here we report the crystal structures of native Slt70 and of its complex with a 1,6-anhydromuropeptide solved at nominal resolutions of 1.65 A and 1.90 A, respectively. The high resolution native structure reveals the details on the hydrogen bonds, electrostatic and hydrophobic interactions that stabilise the catalytic domain and the alpha-superhelix. The building-block of the alpha-superhelix is an "up-down-up-down" four-alpha-helix bundle involving both parallel and antiparallel helix pairs. Stabilisation of the fold is provided through an extensive packing of apolar atoms, mostly from leucine and alanine residues. It lacks, however, an internal consensus sequence that characterises other super-secondary helical folds like the beta-helix in pectate lyase or the (beta-alpha)-helix in the ribonuclease inhibitor. The 1, 6-anhydromuropeptide product binds in a shallow groove adjacent to the peptidoglycan-binding groove of the catalytic domain. The groove is formed by conserved residues at the interface of the catalytic domain and the alpha-superhelix. The structure of the Slt70-1, 6-anhydromuropeptide complex confirms the presence of a specific binding-site for the peptide moieties of the peptidoglycan and it substantiates the notion that Slt70 starts the cleavage reaction at the anhMurNAc end of the peptidoglycan.

Disulfide bridges are not involved in penicillin-binding protein 1b dimerization in Escherichia coli

PBP1b can be found as a dimer in Escherichia coli. Previous results suggested that dimerization involved the cysteine(s) in an intermolecular disulfide bond. We show that either deletion mutants or a mutant without cysteines is fully active and still binds penicillin and that the latter can also form dimers.

Morphogenesis of Escherichia coli

The shape of Escherichia coli is strikingly simple compared to those of higher eukaryotes. In fact, the end result of E. coli morphogenesis is a cylindrical tube with hemispherical caps. It is argued that physical principles affect biological forms. In this view, genes code for products that contribute to the production of suitable structures for physical factors to act upon. After introduction of a physical model, the discussion is focused on the shape-maintaining (peptidoglycan) layer of E. coli. This is followed by a detailed analysis of the structural relationship of the cellular interior to the cytoplasmic membrane. A basic theme of this review is that the transcriptionally active nucleoid and the cytoplasmic translation machinery form a structural continuity with the growing cellular envelope. An attempt has been made to show how this dynamic relationship during the cell cycle affects cell polarity and how it leads to cell division.

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.

Bacterial cell division

Bacteria usually divide by building a central septum across the middle of the cell. This review focuses on recent results indicating that the tubulin-like FtsZ protein plays a central role in cytokinesis as a major component of a contractile cytoskeleton. Assembly of this cytoskeletal element abutting the membrane is a key point for regulation. The characterization of FtsZ homologues in Mycoplasmas, Archaea, and chloroplasts implies that the constriction mechanism is conserved and that FtsZ can constrict in the absence of peptidoglycan synthesis. In most Eubacteria, the internal cytoskeleton must also regulate synthesis of septal peptidoglycan. The Escherichia coli septum-specific penicillin-binding protein 3 (PBP3) forms a complex with other enzymes involved in murein metabolism, suggesting a centrally located transmembrane complex capable of splicing multiple new strands of peptidoglycan into the cell wall. Important questions remain about the spatial and temporal control of bacterial division.

The bimodular G57-V577 polypeptide chain of the class B penicillin-binding protein 3 of Escherichia coli catalyzes peptide bond formation from thiolesters and does not catalyze glycan chain polymerization from the lipid II intermediate

Because the specificity profile of the membrane anchor-free G57-V577 penicillin-binding protein 3 (PBP3) of Escherichia coli for a large series of beta-lactam antibiotics is similar to that of the full-size membrane-bound PBP, the truncated PBP is expected to adopt the native folded conformation. The truncated PBP3 functions as a thiolesterase. In aqueous media and in the presence of millimolar concentrations of a properly structured amino compound, it catalyzes the aminolysis of the thiolester until completion, suggesting that the penicillin-binding module of PBP3 is designed to catalyze transpeptidation reactions. In contrast, the truncated PBP3 is devoid of glycan polymerization activity on the E. coli lipid II intermediate, suggesting that the non-penicillin-binding module of PBP3 is not a transglycosylase.