Isolation of a mutant of Escherichia coli lacking penicillin-sensitive D-alanine carboxypeptidase IA

Divergent Effects of Peptidoglycan Carboxypeptidase DacA on Intrinsic β-Lactam and Vancomycin Resistance

Vancomycin and β-lactams are clinically important antibiotics that inhibit the formation of peptidoglycan cross-links, but their binding targets are different. The binding target of vancomycin is d-alanine-d-alanine (d-Ala-d-Ala), whereas that of β-lactam is penicillin-binding proteins (PBPs). In this study, we revealed the divergent effects of peptidoglycan (PG) carboxypeptidase DacA on vancomycin and β-lactam resistance in Escherichia coli and Bacillus subtilis. The deletion of DacA induced sensitivity to most β-lactams, whereas it induced strong resistance toward vancomycin. Notably, both phenotypes did not have a strong association with ld-transpeptidases, which are necessary for the formation of PG 3-3 cross-links and covalent bonds between PG and an Lpp outer membrane (OM) lipoprotein. Vancomycin resistance was induced by an increased amount of decoy d-Ala-d-Ala residues within PG, whereas β-lactam sensitivity was associated with physical interactions between DacA and PBPs. The presence of an OM permeability barrier strongly strengthened vancomycin resistance, but it significantly weakened β-lactam sensitivity. Collectively, our results revealed two distinct functions of DacA, which involved inverse modulation of bacterial resistance to clinically important antibiotics, β-lactams and vancomycin, and presented evidence for a link between DacA and PBPs.

IMPORTANCE Bacterial PG hydrolases play important roles in various aspects of bacterial physiology, including cytokinesis, PG synthesis, quality control of PG, PG recycling, and stress adaptation. Of all the PG hydrolases, the role of PG carboxypeptidases is poorly understood, especially regarding their impacts on antibiotic resistance. We have revealed two distinct functions of PG carboxypeptidase DacA with respect to antibiotic resistance. The deletion of DacA led to sensitivity to most β-lactams, while it caused strong resistance to vancomycin. Our study provides novel insights into the roles of PG carboxypeptidases in the regulation of antibiotic resistance and a potential clue for the development of a drug to improve the clinical efficacy of β-lactam antibiotics.

A large displacement of the SXN motif of Cys115-modified penicillin-binding protein 5 from Escherichia coli

Penicillin-binding proteins (PBPs), which are the lethal targets of beta-lactam antibiotics, catalyse the final stages of peptidoglycan biosynthesis of the bacterial cell wall. PBP 5 of Escherichia coli is a D-alanine CPase (carboxypeptidase) that has served as a useful model to elucidate the catalytic mechanism of low-molecular-mass PBPs. Previous studies have shown that modification of Cys115 with a variety of reagents results in a loss of CPase activity and a large decrease in the rate of deacylation of the penicilloyl-PBP 5 complex [Tamura, Imae and Strominger (1976) J. Biol. Chem. 251, 414-423; Curtis and Strominger (1978) J. Biol. Chem. 253, 2584-2588]. The crystal structure of wild-type PBP 5 in which Cys115 fortuitously had formed a covalent adduct with 2-mercaptoethanol was solved at 2.0 A (0.2 nm) resolution, and these results provide a structural rationale for how thiol-directed reagents lower the rate of deacylation. When compared with the structure of the unmodified wild-type enzyme, a major change in the architecture of the active site is observed. The two largest differences are the disordering of a loop comprising residues 74-90 and a shift in residues 106-111, which results in the displacement of Ser110 of the SXN active-site motif. These results support the developing hypothesis that the SXN motif of PBP 5, and especially Ser110, is intimately involved in the catalytic mechanism of deacylation.

Crystal Structure of Wild-type Penicillin-binding Protein 5 from Escherichia coli

Penicillin-binding protein 5 (PBP 5) of Escherichia coli functions as a d-alanine carboxypeptidase (CPase), cleaving d-alanine from the C terminus of cell wall peptides. Like all PBPs, PBP 5 forms a covalent acyl-enzyme complex with beta-lactam antibiotics; however, PBP 5 is distinguished by its high rate of deacylation of the acylenzyme complex (t(1/2) approximately 10 min). A Gly105 --> Asp mutation in PBP 5 markedly impairs deacylation with only minor effects on acylation, and abolishes CPase activity. We have determined the three-dimensional structure of a soluble form of wild-type PBP 5 at 1.85-A resolution and have also refined the structure of the G105D mutant form of PBP 5 to 1.9-A resolution. Comparison of the two structures reveals that the major effect of the mutation is to disorder a loop comprising residues 74-90 that sits atop the SXN motif of the active site. Deletion of the 74-90 loop in wild-type PBP 5 markedly diminished the deacylation rate of penicillin G with a minimal impact on acylation, and abolished CPase activity. These effects were very similar to those observed in the G105D mutant, reinforcing the idea that this mutation causes disordering of the 74-90 loop. Mutation of two consecutive serines within this loop, which hydrogen bond to Ser110 and Asn112 in the SXN motif, had marked effects on CPase activity, but not beta-lactam antibiotic binding or hydrolysis. These data suggest a direct role for the SXN motif in deacylation of the acyl-enzyme complex and imply that the functioning of this motif is modulated by the 74-90 loop.

Penicillin binding protein 5 affects cell diameter, contour, and morphology of Escherichia coli

Although general physiological functions have been ascribed to the high-molecular-weight penicillin binding proteins (PBPs) of Escherichia coli, the low-molecular-weight PBPs have no well-defined biological roles. When we examined the morphology of a set of E. coli mutants lacking multiple PBPs, we observed that strains expressing active PBP 5 produced cells of normal shape, while mutants lacking PBP 5 produced cells with altered diameters, contours, and topological features. These morphological effects were visible in untreated cells, but the defects were exacerbated in cells forced to filament by inactivation of PBP 3 or FtsZ. After filamentation, cellular diameter varied erratically along the length of individual filaments and many filaments exhibited extensive branching. Also, in general, the mean diameter of cells lacking PBP 5 was significantly increased compared to that of cells from isogenic strains expressing active PBP 5. Expression of cloned PBP 5 reversed the effects observed in DeltadacA mutants. Although deletion of PBP 5 was required for these phenotypes, the absence of additional PBPs magnified the effects. The greatest morphological alterations required that at least three PBPs in addition to PBP 5 be deleted from a single strain. In the extreme cases in which six or seven PBPs were deleted from a single mutant, cells and cell filaments expressing PBP 5 retained a normal morphology but cells and filaments lacking PBP 5 were aberrant. In no case did mutation of another PBP produce the same drastic morphological effects. We conclude that among the low-molecular-weight PBPs, PBP 5 plays a principle role in determining cell diameter, surface uniformity, and overall topology of the peptidoglycan sacculus.

Escherichia coli mutants lacking all possible combinations of eight penicillin binding proteins: viability, characteristics, and implications for peptidoglycan synthesis

The penicillin binding proteins (PBPs) synthesize and remodel peptidoglycan, the structural component of the bacterial cell wall. Much is known about the biochemistry of these proteins, but little is known about their biological roles. To better understand the contributions these proteins make to the physiology of Escherichia coli, we constructed 192 mutants from which eight PBP genes were deleted in every possible combination. The genes encoding PBPs 1a, 1b, 4, 5, 6, and 7, AmpC, and AmpH were cloned, and from each gene an internal coding sequence was removed and replaced with a kanamycin resistance cassette flanked by two res sites from plasmid RP4. Deletion of individual genes was accomplished by transferring each interrupted gene onto the chromosome of E. coli via lambda phage transduction and selecting for kanamycin-resistant recombinants. Afterwards, the kanamycin resistance cassette was removed from each mutant strain by supplying ParA resolvase in trans, yielding a strain in which a long segment of the original PBP gene was deleted and replaced by an 8-bp res site. These kanamycin-sensitive mutants were used as recipients in further rounds of replacement mutagenesis, resulting in a set of strains lacking from one to seven PBPs. In addition, the dacD gene was deleted from two septuple mutants, creating strains lacking eight genes. The only deletion combinations not produced were those lacking both PBPs 1a and 1b because such a combination is lethal. Surprisingly, all other deletion mutants were viable even though, at the extreme, 8 of the 12 known PBPs had been eliminated. Furthermore, when both PBPs 2 and 3 were inactivated by the beta-lactams mecillinam and aztreonam, respectively, several mutants did not lyse but continued to grow as enlarged spheres, so that one mutant synthesized osmotically resistant peptidoglycan when only 2 of 12 PBPs (PBPs 1b and 1c) remained active. These results have important implications for current models of peptidoglycan biosynthesis, for understanding the evolution of the bacterial sacculus, and for interpreting results derived by mutating unknown open reading frames in genome projects. In addition, members of the set of PBP mutants will provide excellent starting points for answering fundamental questions about other aspects of cell wall metabolism.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Cloning and characterization of the ponA gene encoding penicillin-binding protein 1 from Neisseria gonorrhoeae and Neisseria meningitidis

The ponA gene encoding penicillin-binding protein 1 (PBP 1) from Neisseria gonorrhoeae was cloned by a reverse genetic approach. PBP 1 was purified from solubilized membranes of penicillin-susceptible strain FA19 by covalent ampicillin affinity chromatography and used to obtain an NH2-terminal amino acid sequence. A degenerate oligonucleotide based on this protein sequence and a highly degenerate oligonucleotide based on a conserved amino acid motif found in all class A high-molecular-mass PBPs were used to isolate the PBP 1 gene (ponA). The ponA gene encodes a protein containing all of the conserved sequence motifs found in class A PBPs, and expression of the gene in Escherichia coli resulted in the appearance of a new PBP that comigrated with PBP 1 purified from N. gonorrhoeae. A comparison of the gonococcal ponA gene to its homolog isolated from Neisseria meningitidis revealed a high degree of identity between the two gene products, with the greatest variability found at the carboxy terminus of the two deduced PBP 1 protein sequences.

Bacteriophage lysis: mechanism and regulation

Bacteriophage lysis involves at least two fundamentally different strategies. Most phages elaborate at least two proteins, one of which is a murein hydrolase, or lysin, and the other is a membrane protein, which is given the designation holin in this review. The function of the holin is to create a lesion in the cytoplasmic membrane through which the murein hydrolase passes to gain access to the murein layer. This is necessary because phage-encoded lysins never have secretory signal sequences and are thus incapable of unassisted escape from the cytoplasm. The holins, whose prototype is the lambda S protein, share a common organization in terms of the arrangement of charged and hydrophobic residues, and they may all contain at least two transmembrane helical domains. The available evidence suggests that holins oligomerize to form nonspecific holes and that this hole-forming step is the regulated step in phage lysis. The correct scheduling of the lysis event is as much an essential feature of holin function as is the hole formation itself. In the second strategy of lysis, used by the small single-stranded DNA phage phi X174 and the single-stranded RNA phage MS2, no murein hydrolase activity is synthesized. Instead, there is a single species of small membrane protein, unlike the holins in primary structure, which somehow causes disruption of the envelope. These lysis proteins function by activation of cellular autolysins. A host locus is required for the lytic function of the phi X174 lysis gene E.

Gonococcal penicillin-binding protein 3 and the surface-exposed 44kDa peptidoglycan-binding protein appear to be the same molecule

The outer membrane of Neisseria gonorrhoeae contains a 44,000 dalton (44kDa) surface-exposed protein which has the reported ability to form covalent interactions with peptidoglycan (PG). This PG-binding outer-membrane protein (OMP) appears to be highly conserved since it has been detected in all isolates examined. It also appears to be invariant since its primary structure among strains gives evidence of being identical (Judd et al., 1991). While studying the interaction of gonococcal penicillin-binding proteins (PBPs) with human lysosomal cathepsin G, we noticed that the 44kDa PG-binding OMP exhibited certain properties similar to PBP3. In this study we sought to obtain biochemical evidence to ascertain whether these proteins were the same. We found that both proteins fractionated with other sarkosyl-insoluble OMPs and that they exhibited similar susceptibility to cleavage in situ by enzymatically active cathepsin G. Moreover, a purified preparation of the 44kDa OMP was found to covalently bind radiolabelled benzylpenicillin in vitro. Thus, the data presented herein suggest that the 44kDa PG-binding OMP and PBP3 are the same OMP.

Preparation and characterization of monoclonal antibodies against native membrane-bound penicillin-binding protein 1B of Escherichia coli

We prepared monoclonal antibodies against penicillin-binding protein 1B (PBP 1B) of Escherichia coli to study the membrane topology, spatial organization, and enzyme activities of this protein. The majority of the antibodies derived with PBP 1B as the immunogen reacted against the carboxy terminus. To obtain monoclonal antibodies recognizing other epitopes, we used PBP 1B lacking the immunodominant carboxy-terminal 65 amino acids as the immunogen. Eighteen monoclonal antibodies directed against membrane-bound PBP 1B were isolated and characterized. The epitopes recognized by those monoclonal antibodies were located with various truncated forms of PBP 1B. We could distinguish four different epitope areas located on different parts of the molecule. Interestingly, we could not isolate monoclonal antibodies against the amino terminus, although they were specifically selected for. This is attributed to its predicted extreme hydrophilicity and flexibility, which could make the amino terminus very sensitive to proteolytic degradation. All antibodies reacted against native PBP 1B in a dot-blot immunobinding assay. One monoclonal antibody also recognized PBP 1B in a completely sodium dodecyl sulfate-denatured form. This suggests that all the other monoclonal antibodies recognize conformational epitopes. These properties make the monoclonal antibodies suitable tools for further studies.

Genes encoding two lipoproteins in the leuS-dacA region of the Escherichia coli chromosome

The coding of two rare lipoproteins by two genes, rlpA and rlpB, located in the leuS-dacA region (15 min) on the Escherichia coli chromosome was demonstrated by expression of subcloned genes in a maxicell system. The formation of these two proteins was inhibited by globomycin, which is an inhibitor of the signal peptidase for the known lipoproteins of E. coli. In each case, this inhibition was accompanied by formation of a new protein, which showed a slightly lower mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and which we suppose to be a prolipoprotein with an N-terminal signal peptide sequence similar to those of the bacterial major lipoproteins and lysis proteins of some bacteriocins. The incorporation of 3H-labeled palmitate and glycerol into the two lipoproteins was also observed. Sequencing of DNA showed that the two lipoprotein genes contained sequences that could code for signal peptide sequences of 17 amino acids (rlpA lipoprotein) and 18 amino acids (rlpB lipoprotein). The deduced sequences of the mature peptides consisted of 345 amino acids (Mr 35,614, rlpA lipoprotein) and 175 amino acids (Mr 19,445, rlpB lipoprotein), with an N-terminal cysteine to which thioglyceride and N-fatty acyl residues may be attached. These two lipoproteins may be important in duplication of the cells.

Sensitivity of Escherichia coli to various beta-lactams is determined by the interplay of outer membrane permeability and degradation by periplasmic beta-lactamases: a quantitative predictive treatment

In Gram-negative bacteria, beta-lactam antibiotics must overcome two barriers, the outer membrane and the periplasmic beta-lactamase, before they reach the targets of their action, penicillin-binding proteins. Although the barrier property of the outer membrane and catalytic property of the beta-lactamases have been studied and their significance in creating beta-lactam resistance emphasized, the interaction between these two barriers has not been treated quantitatively. Such treatment shows that the sensitivity, to a variety of beta-lactams, of the Escherichia coli K-12 cells containing very different levels of chromosomally coded AmpC beta-lactamase, or a plasmid-coded TEM-type beta-lactamase, can be predicted rather accurately from the penetration rate through the outer membrane and the hydrolysis rate in the periplasm. We further propose a new parameter, 'target access index', which is a quantitative expression of the result of interaction between the two barriers, and reflects the probability of success for the antibiotic to reach the targets.

Nucleotide sequence of the pbpA gene and characteristics of the deduced amino acid sequence of penicillin-binding protein 2 of Escherichia coli K12

We have determined the nucleotide sequence of the pbpA gene encoding penicillin-binding protein (PBP) 2 of Escherichia coli. The coding region for PBP 2 was 1899 base pairs in length and was preceded by a possible promoter sequence and two open reading frames. The primary structure of PBP 2, deduced from the nucleotide sequence, comprised 633 amino acid residues. The relative molecular mass was calculated to be 70867. The deduced sequence agreed with the NH2-terminal sequence of PBP 2 purified from membranes, suggesting that PBP 2 has no signal peptide. The hydropathy profile suggested that the NH2-terminal hydrophobic region (a stretch of 25 non-ionic amino acids) may anchor PBP 2 in the cytoplasmic membrane as an ectoprotein. There were nine homologous segments in the amino acid sequence of PBP 2 when compared with PBP 3 of E. coli. The active-site serine residue of PBP 2 was predicted to be Ser-330. Around this putative active-site serine residue was found the conserved sequence of Ser-Xaa-Xaa-Lys, which has been identified in all of the other E. coli PBPs so far studied (PBPs 1A, 1B, 3, 5 and 6) and class A and class C beta-lactamases. In the higher-molecular-mass PBPs 1A, 1B, 2 and 3, Ser-Xaa-Xaa-Lys-Pro was conserved. In the putative peptidoglycan transpeptidase domain there were six amino acid residues, which are common only in the PBPs of higher molecular mass.

Identification of the active site in penicillin-binding protein 3 of Escherichia coli

We report the sequence of the active site tryptic peptide of penicillin-binding protein 3 from Escherichia coli. Purified penicillin-binding protein 3 was labeled with [14C]penicillin G and digested with trypsin, and the resulting radioactive peptides were isolated by a combination of gel filtration and high-pressure liquid chromatography. The major radioactive peak from high-pressure liquid chromatography was sequenced, and the peptide Thr-Ile-Thr-Asp-Val-Phe-Glu-Pro-Gly-Ser-Thr-Val-Lys, which comprises residues 298 to 310 in the amino acid sequence, was identified. This sequence is compared with the active site sequences from other penicillin-binding proteins and beta-lactamases.

Mutation in Pseudomonas aeruginosa causing simultaneous defects in penicillin-binding protein 5 and in enzyme activities of penicillin release and D-alanine carboxypeptidase

Penicillin-binding protein 5 in Pseudomonas aeruginosa had moderately penicillin-sensitive D-alanine carboxypeptidase activity. As in Escherichia coli, a defect in this enzyme activity was not lethal.

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.

Peptidoglycan biosynthesis in Neisseria gonorrhoeae strains sensitive and intrinsically resistant to beta-lactam antibiotics

Treatment of penicillin-sensitive and intrinsically resistant Neisseria gonorrhoeae strains with their respective inhibitory concentrations of penicillin caused rapid cell death. When the peptidoglycan syntheses of these two strains were examined in the presence of penicillin, the sensitive strain continued to make this cell wall polymer for an extended time, whereas the resistant strain underwent a rapid and marked depression in synthesis. Examination of the labeled sodium dodecyl sulfate-insoluble peptidoglycan made in the presence of inhibitory concentrations of penicillin revealed further differences. The primary effect on the penicillin-sensitive gonococcus was a slight change in peptide cross-linking and a sharp decline in the degree of O-acetylation. In contrast, the resistant strain exhibited a substantial decline in cross-linking, with a very moderate change in O-acetylation. The degree of saturation of the individual penicillin-binding proteins (PBPs) was assessed under these conditions. PBP 2, which exhibits a reduced affinity for penicillin in the resistant strain, appeared to be related to O-acetylation, whereas PBP 1 was implicated in the transpeptidation reaction.

Deletion of the penicillin-binding protein 6 gene of Escherichia coli

A strain of Escherichia coli with a deletion of the penicillin-binding protein 6 gene (dacC) has been constructed. The properties of this strain establish that the complete lack of penicillin-binding protein 6 has no marked effect on the growth of E. coli.

Induction and control of the autolytic system of Escherichia coli

Various methods of inducing autolysis of Escherichia coli cells were investigated, some being described here for the first time. For the autolysis of growing cells only induction methods interfering with the biosynthesis of peptidoglycan were taken into consideration, whereas with harvested cells autolysis was induced by rapid osmotic or EDTA shock treatments. The highest rates of autolysis were observed after induction by moenomycin, EDTA, or cephaloridine. The different autolyses examined shared certain common properties. In particular, regardless of the induction method used, more or less extensive peptidoglycan degradation was observed, and 10(-2) M Mg2+ efficiently inhibited the autolytic process. However, for other properties a distinction was made between methods used for growing cells and those used for harvested cells. Autolysis of growing cells required RNA, protein, and fatty acid synthesis. No such requirements were observed with shock-induced autolysis performed with harvested cells. Thus, the effects of Mg2+, rifampicin, chloramphenicol, and cerulenin clearly suggest that distinct factors are involved in the control of the autolytic system of E. Coli. Uncoupling agents such as sodium azide, 2,4-dinitrophenol, and carbonyl-cyanide-m-chlorophenyl hydrazone used at their usual inhibiting concentration had no effect on the cephaloridine or shock-induced autolysis.

Interaction of beta-lactam antibiotics with penicillin-binding proteins from Bacillus megaterium

The binding properties of 25 beta-lactam antibiotics to Bacillus megaterium membranes have been studied. The affinities of the antibiotics for the penicillin-binding proteins (PBPs) are also reported. We found that PBP 4 has the highest affinity for nearly all the antibiotics studied whereas PBP 5 has the lowest affinity. Both PBP 4 and PBP 5 appear to be dispensable for the maintenance of bacterial growth and survival and appear to be DD-carboxypeptidases. Only the beta-lactam cefmetazol bound preferentially to PBP 5 and has been used to study the inhibition of DD-carboxypeptidase. Comparative studies with beta-lactam that simultaneously result in (a) binding to PBPs 1 and 3, (b) inhibition of cell growth and (c) lysis, stressed the importance of PBPs 1 and 3 for cell growth and survival.

High-molecular-weight penicillin-binding proteins from membranes of bacilli

Mixtures of high-molecular-weight, cephalosporin-sensitive penicillin-binding proteins (PBPs) can be purified from Bacillus subtilis membranes by cephalosporin affinity chromatography (G. Kleppe and J. L. Strominger, J. Biol. Chem. 254:4856-4862, 1979). By appropriate modification of this technique, B. subtilis PBP 1 was purified to homogeneity, and a mixture of Bacillus stearothermophilus PBPs 1, 2, and 4 was isolated. [14C]penicillin-PBP complexes of high-molecular-weight PBPs purified from membranes of these two bacilli, after denaturation, were found to have chemical reactivities typical of the penicilloyl-serine derivative formed by D-alanine carboxypeptidase from B. stearothermophilus. Although enzymatic activity catalyzed by these and several other high-molecular-weight PBPs from gram-positive organisms has not been detected with cell wall-related substrates, a slow, enzymatic acylation of B. subtilis PBPs 1, 2ab, and 4 by [14C]-diacetyl-L-lysyl-D-alanyl-D-lactate was demonstrated. Further study is necessary to clarify the physiological relevance of the slow acylation by this analog of a natural cell wall biosynthetic intermediate.

A role in vivo for penicillin-binding protein-4 of Staphylococcus aureus

The degree of cross-linking of the peptidoglycan of Staphylococcus aureus H and mutants lacking penicillin-binding proteins 1 and 4 was studied. No major changes were observed in organisms lacking protein 1 whereas loss of protein 4 was accompanied by a marked reduction in the degree of cross-linking and the absence of a membrane-bound 'model' transpeptidase activity. A similar effect was achieved when cultures of the staphylococci were treated with the beta-lactam antibiotic cefoxitin. At low concentrations (0.05 microgram ml-1) cefoxitin shows highest affinity for protein 4 to which it appears to bind irreversibly. Treatment of the mutant lacking protein 4 with this concentration of the antibiotic did not affect the degree of cross-linkage. The possibility that the decrease in cross-linkage was a consequence of DD-carboxypeptidase activity on peptidoglycan precursors was investigated. Although both S. aureus H and the mutants possessed such activity it was insensitive to benzylpenicillin and cefoxitin and the role of this enzyme(s) in peptidoglycan biosynthesis remains unknown. We conclude that in vivo protein 4 acts as a transpeptidase involved in the secondary cross-linking of peptidoglycan and this activity is necessary to achieve the high degree of cross-linkage observed in the peptidoglycan of staphylococci.

Competition of beta-lactam antibiotics for the penicillin-binding proteins of Neisseria gonorrhoeae

The affinities of nine structurally different beta-lactam antibiotics for the three major gonococcal penicillin-binding proteins (PBPs) were determined by using a competition assay with tritium-labeled penicillin and live, growing bacteria. Each determination was carried out in parallel in isogenic pairs of penicillin-susceptible (minimal inhibitory concentration of penicillin, 0.0075 microgram/ml) and intrinsically penicillin-resistant (minimal inhibitory concentration of penicillin, 0.5 microgram/ml) cells. Evidence is presented indicating that (i) PBP 3 may be a dispensable function; (ii) acquisition of resistance is accompanied by change in the beta-lactam antibiotic affinities of PBP 2 but not of PBP 1; (iii) PBP 2 appears to be the most important physiological target in the penicillin-susceptible strain; in the penicillin-resistant strain, PBP 1 seems to assume this role. The relative affinities of various beta-lactam antibiotics for the individual PBPs showed substantial variation with the antibiotic structure.

On the DD-carboxypeptidase enzyme system of Streptomyces strain K15

Streptomyces K15 possesses a set of exocellular and cell-bound D-alanyl-D-alanine carboxypeptidases. Four of them have been isolated to the stage where each enzyme preparation contains on single penicillin-binding protein. The exocellular 54000-Mr enzyme is extremely sensitive to benzylpenicillin and performs low transpeptidase activity on the carbonyl-donor/amino-acceptor tetrapeptide ACLLys(Gly)-DAla-DAla. The exocellular 40 000-Mr enzyme and the two lysozyme-releasable 40 000-Mr and 38 000-Mr enzymes are moderately sensitive to benzylpenicillin and have a high propensity to catalyse dimer formation from the aforementioned tetrapeptide monomer.

Properties of penicillin-binding proteins in Neisseria gonorrhoeae

The properties of penicillin-binding proteins (PBPs) of Neisseria gonorrhoeae were studied by comparing PBPs of clinical isolates of different penicillin susceptibility and by putting various beta-lactam antibiotics in competition with radioactive penicillin for PBP binding. Apparent molecular weights of the three major PBPs found were 87,000 (PBP 1), 59,000 (PBP 2), and 44,000 (PBP 3). Relative penicillin resistance was associated with decreased binding to PBP 2 and, to a lesser extent, to PBP 1. Cephaloridine and benzylpenicillin, which produced spheroplasts at minimal inhibitory concentrations, bound to all three PBPs. In contrast, antibiotics which produced a majority of enlarged but apparently intact cells bound only to PBP 2 (mecillinam) or to PBPs 2 and 3 (cephalexin) at their minimal inhibitory concentrations.

Purification of penicillin-binding protein 2 of Escherichia coli

Penicillin-binding protein 2 (PBP-2) of Escherichia coli K-12 was purified by covalent affinity chromatography using 6-aminopenicillanic acid covalently coupled to carboxymethyl-Sepharose (6-APA-CM-Sepharose). Purification of PBP-2 was accomplished by prebinding the methoxy cephalosporin, cefoxitin, to the Triton X-100-solubilized PBPs of E. coli and then incubating the PBPs with 6-APA-CM-Sepharose. Cefoxitin readily binds to all the E. coli PBPs except PBP-2 and, thus, in the presence of cefoxitin, only PBP-2 could bind to the 6-APA-CM-Sepharose. The purification of a mixture of all of the PBPs of E. coli by affinity chromatography is also described.

Effects of furazlocillin, a beta-lactam antibiotic which binds selectively to penicillin-binding protein 3, on Escherichia coli mutants deficient in other penicillin-binding proteins

Furazlocillin binds selectively to penicillin-binding protein 3 (PBP-3), prevents septation of Escherichia coli, and allows the cells to form long filaments without lysis. The effect of furazlocillin on the morphology, autolysis, and murein synthesis of E. coli mutants deficient in either PBP-1A, PBP-1Bs, or PBP-2 was studied. The results reveal that PBP-1A and PBP-1Bs functions are not equivalent since furazlocillin affects the morphology, autolysis, and murein synthesis of PBP1A- mutants quite differently from that of PBP-1Bs mutants. Different "PBP-2-" mutants were found to respond to furazlocillin in dramatically different ways: strain LS-1 cells formed elongated rods with a central bulge which eventually lysed, whereas SP6 cells formed stable "barbells" in which the two daughter cells were well separated but remained connected by a thick central region.

Deletion of the penicillin-binding protein 5 gene of Escherichia coli

A strain of Escherichia coli that has a deletion of the entire dacA gene has been constructed. The complete lack of penicillin-binding protein 5 in this strain establishes that the activity of this protein is not essential for the growth of E. coli.

In vivo interaction of beta-lactam antibiotics with the penicillin-binding proteins of Streptococcus pneumoniae

The interactions of several beta-lactam antibiotics with the penicillin-binding proteins (PBPs) of Streptococcus pneumoniae have been studied using whole organisms treated with such antibiotics and subsequently with [3H]benzylpenicillin. Differences in chemical structure were shown to cause major and selective changes in the affinities of the beta-lactams for the PBPs Only 4 of the 28 compounds tested induced a specific morphological effect (enlargement of the equatorial region) under the particular conditions tested. In 12 of the 18 beta-lactams studied, a close correlation was found between the minimal inhibitory concentrations and the concentrations required to half-saturate PBP2b. However, such a correlation was no longer apparent when the bacteria were treated with the antibiotics at their minimal inhibitory concentrations. These findings are discussed in the context of various approaches that have been used to identify the growth-inhibitory targets of beta-lactam antibiotics in bacteria.

Defective and plaque-forming lambda transducing bacteriophage carrying penicillin-binding protein-cell shape genes: genetic and physical mapping and identification of gene products from the lip-dacA-rodA-pbpA-leuS region of the Escherichia coli chromosome

A series of defective lambda transducing phage carrying genes from the lip-leuS region of the Escherichia coli chromosome (min 14 on the current linkage map) has been isolated. The phage defined the gene order as lac---lip-dacA-rodA-pbpA-leuS---gal. These included the structural genes for penicillin-binding protein 2 (pbpA) and penicillin-binding protein 5 (dacA) as well as a previously unidentified cell shape gene that we have called rodA. rodA mutants were spherical and very similar to pbpA mutants but were distinguishable from them in that they had no defects in the activity of penicillin-binding protein 2. The separation into two groups of spherical mutants with mutations that mapped close to lip was confirmed by complementation analysis. The genes dacA, rodA, and pbpA lie within a 12-kilobase region, and represent a cluster of genes involved in cell shape determination and peptidoglycan synthesis. A restriction map of the lip-leuS region was established, and restriction fragments were cloned from defective transducing phage into appropriate lambda vectors to generate plaque-forming phage that carried genes from this region. Analysis of the proteins synthesized from lambda transducing phage in ultraviolet light-irradiated cells of E. coli resulted in the identification of the leuS, pbpA, dacA, and lip gene products, but the product of the rodA gene was not identified. The nine proteins that were synthesized from the lip-leuS region accounted for 57% of its coding capacity. Phage derivatives were constructed that allowed about 50-fold amplification of the levels of penicillin-binding proteins 2 and 5 in the cytoplasmic membrane.

On the process of cellular division in Escherichia coli: isolation and characterization of penicillin-binding proteins 1a, 1b, and 3

Multiple mutants of Escherichia coli defective in penicillin-binding proteins (PBPs) were constructed, and into these strains Co1E1 plasmids carrying the genes for PBP-1a, -1b, or -3 were introduced. From these plasmid-carrying strains, PBP-1a and -1b were purified by ampicillin-Sepharose affinity chromatography and PBP-3 by cephalexin-Sepharose affinity chromatography. Improved purification was achieved by differential elution with NH2OH. Purified PBP-1b synthesized murein when added to the membrane fraction of a PBP-1b-defective mutant, which by itself failed to support murein synthesis in vitro. The PBP-1b preparation was able to synthesize murein from the lipid intermediate extracted with chloroform/methanol but was unable to utilize UDP-linked precursors for murein synthesis. Murein synthesis was inhibited by vancomysin, ristocetin, moenomycin, and enduracidin, but not by beta-lactam antibiotics. The synthesized murein was shown to contain crosslinked muropeptides. Their crosslinking was abolished by action of beta-lactam antibiotics. The PBP-1a and -3 preparations showed substantially no activity for murein synthesis in the same reaction system. None of the three PBPs showed D-alanine carboxypeptidase activity with UDP-N-acetylmuramoyl-pentapeptide as substrate or endopeptidase activity with bis(disaccharide-peptide) as substrate.

A mutant of Escherichia coli defective in penicillin-binding protein 5 and lacking D-alanine carboxypeptidase IA

A mutant of Escherichia coli defective in penicillin-binding protein 5 activity was isolated. The mutation (pfv) was shown to be located at 14.0 min on the E. coli chromosome map. Loss of penicillin-binding protein 5 in the pfv mutant was associated with the loss of D-alanine carboxypeptidase IA activity and increased sensitivity to beta-lactam antibiotics. We conclude that penicillin-binding protein 5 catalyzes the major D-alanine carboxypeptidase IA activity and that the enzyme activity, in vivo, protects E. coli cells from killing by low inhibitory concentrations of beta-lactam antibiotics.

Affinity of cefoperazone for penicillin-binding proteins

Cefoperazone (T-1551, CFP) a new semisynthetic cephalosporin, has a broad spectrum of antibacterial activity. We investigated the affinity of CFP to penicillin-binding proteins (PBPs) and the inhibition of peptidoglycan synthesis by CFP. CFP had high affinities for Escherichia coli PBP-3, -1Bs, -2, and -1A, in descending order, and low affinities for PBP-4, -5, and -6. Similarly, CFP showed high affinity for Pseudomonas aeruginosa PBP-3, -1A, -1B, -2, and -4, in descending order. It is known that E. coli PBP-3 and P. aeruginosa PBP-3 participate in cell division. These results are in good agreement with the formation of filamentous cells of E. coli and P. aeruginosa treated with CFP. CFP had lower inhibitory activities on D-alanine carboxypeptidase IA and IB of E. coli than that of penicillin G, but its inhibitory activities on the cross-link formation in peptidoglycan synthesis were the same as those of penicillin G and higher than those of ampicillin.