Cluster of an erythromycin-resistant variant of the Spanish multiply resistant 23F clone of Streptococcus pneumoniae in South Africa

A multiply resistant serotype 23F pneumococcus was cultured from a 3-year-old child with otitis media and five carriers at the child's day-care centre in South Africa. Attempts to eradicate carriage with mupirocin failed. The strains were identical to the Spanish clone of multiply resistant pneumococci by serotype, antimicrobial susceptibility pattern, restriction analysis of the penicillin-binding protein 1A, 2B and 2X genes and by multilocus enzyme electrophoresis. The data suggest that this clone has acquired erythromycin resistance. A possible link between the index case and residence of a family member in Spain was found.

Interspecies recombination between the penA genes of Neisseria meningitidis and commensal Neisseria species during the emergence of penicillin resistance in N. meningitidis: natural events and laboratory simulation

The penicillin-binding protein 2 genes (penA) of penicillin-resistant Neisseria meningitidis have a mosaic structure that has arisen by the introduction of regions from the penA genes of Neisseria flavescens or Neisseria cinerea. Chromosomal DNA from both N. cinerea and N. flavescens could transform a penicillin-susceptible isolate of N. meningitidis to increased resistance to penicillin. With N. flavescens DNA, transformation to resistance was accompanied by the introduction of the N. flavescens penA gene, providing a laboratory demonstration of the interspecies recombinational events that we believe underlie the development of penicillin resistance in many meningococci in nature. Surprisingly, with N. cinerea DNA, the penicillin-resistant transformants did not obtain the N. cinerea penA gene. However, the region of the penA gene derived from N. cinerea in N. meningitidis K196 contained an extra codon (Asp-345A) which was not found in any of the four N. cinerea isolates that we examined and which is known to result in a decrease in the affinity of PBP 2 in gonococci.

Molecular epidemiology of penicillin-resistant pneumococci isolated in Nairobi, Kenya

A total of 26% of the pneumococci isolated from an outpatient clinic in Nairobi, Kenya, during 1991 to 1992 had intermediate levels of penicillin resistance. Gene fingerprinting and DNA sequencing were used to distinguish the penicillin-binding protein (PBP) 1A, 2B, and 2X genes in 23 resistant isolates. Isolates were grouped into those that had identical forms of each of the three PBP genes (fingerprint groups) and those that had identical rRNA gene restriction patterns (ribotypes). Both methods divided the isolates into 11 groups. In a few cases, horizontal gene transfer appeared to have distributed an identical altered PBP gene into different pneumococcal lineages. Eight isolates were indistinguishable by ribotyping or multilocus enzyme electrophoresis and contained identical PBP 1A genes. Although these isolates were therefore members of the same clone, they were divided into two fingerprint groups which contained different PBP 2X and 2B genes. Presumably, members of this clone have acquired different altered PBP 2X and 2B genes on two separate occasions. One of these fingerprint groups contained isolates of serotype 14, whereas the other contained isolates of both serotypes 14 and 7. The identification of isolates in the latter group that are identical by all criteria, except serotype, implies the occurrence of a change in serotype. The predominant serotypes of the penicillin-resistant pneumococci from Nairobi were serotypes 14 and 19. In both cases, isolates of the same serotype which required the same MIC of penicillin were not members of a single clone, indicating that identity of serotype and MIC are not sufficient criteria for defining clones of resistant pneumococci even when the bacteria are isolated from a single clinic.

Genetic diversity of penicillin-binding protein 2B and 2X genes from Streptococcus pneumoniae in South Africa

Streptococcus pneumoniae (the pneumococcus) is believed to have developed resistance to penicillin by the production of altered forms of penicillin-binding proteins (PBPs) that have decreased affinity for penicillin. Sixty-eight clinical isolates of serogroup 6 and 19 pneumococci (MICs, < 0.015 to 8 micrograms/ml) were randomly selected from hospitals across South Africa which are at substantial geographic distance from each other. The polymerase chain reaction was used to isolate the penicillin-binding domain of PBPs 2B and 2X from the chromosomal DNAs of the bacteria; the purified PBP DNA was digested with restriction enzymes, the fragments were end-labelled and separated on polyacrylamide gels, and the DNA fingerprints were visualized following autoradiography. Fingerprint analysis revealed that at least 19 PBP 2B gene variants occur in the serogroup 6 and 19 pneumococci. The PBP 2B gene revealed a uniform profile among penicillin-susceptible isolates, with variation from this profile occurring only in isolates for which MICs were > or = 0.06 micrograms/ml. Analysis of the PBP 2X gene revealed a greater diversity in the population with 26 variant genes, including some diversity among susceptible isolates. Discrete profiles of both genes were found only within narrow bands of the penicillin MIC, so that the gene pattern predicted the MIC. PBP 2X gene variation and the lack of variability among PBP 2B genes in pneumococci inhibited at low MICs confirm that PBP 2X alteration may be responsible for low-level penicillin resistance, while alterations in both PBP 2B and PBP 2X are required for high-level resistance. The extensive diversity of PBP genes in South African serogroup 6 and 19 strains suggests that altered PBP genes have arisen frequently in this population.

Horizontal spread of an altered penicillin-binding protein 2B gene between Streptococcus pneumoniae and Streptococcus oralis

The region encoding the transpeptidase domain of the penicillin-binding protein 2B (PBP 2B) gene of two penicillin-resistant clinical isolates of Streptococcus oralis was > 99.6% identical in nucleotide sequence to that of a penicillin-resistant serotype 6 isolate of Streptococcus pneumoniae. The downstream 849 base pairs of these genes were identical. Analysis of the data indicates that the PBP gene has probably been transferred from S. pneumoniae into S. oralis, rather than vice versa, and shows that one region of this resistance gene has been distributed horizontally both within S. pneumoniae and into two different viridans group streptococci.

How clonal are bacteria?

Data from multilocus enzyme electrophoresis of bacterial populations were analyzed using a statistical test designed to detect associations between genes at different loci. Some species (e.g., Salmonella) were found to be clonal at all levels of analysis. At the other extreme, Neisseria gonorrhoeae is panmictic, with random association between loci. Two intermediate types of population structure were also found. Neisseria meningitidis displays what we have called an "epidemic" structure. There is significant association between loci, but this arises only because of the recent, explosive, increase in particular electrophoretic types; when this effect is eliminated the population is found to be effectively panmictic. In contrast, linkage disequilibrium in a population of Rhizobium meliloti exists because the sample consisted of two genetically isolated divisions, often fixed for different alleles: within each division association between loci was almost random. The method of analysis is appropriate whenever there is doubt about the extent of genetic recombination between members of a population. To illustrate this we analyzed data on protozoan parasites and again found panmictic, epidemic, and clonal population structures.

Deletion analysis of the essentiality of penicillin-binding proteins 1A, 2B and 2X of Streptococcus pneumoniae

An internal fragment from each of the penicillin-binding protein (PBP) 1A, 2B and 2X genes of Streptococcus pneumoniae, which included the region encoding the active-site serine residue, was replaced by a fragment encoding spectinomycin resistance. The resulting constructs were tested for their ability to transform S. pneumoniae strain R6 to spectinomycin resistance. Spectinomycin-resistant transformants could not be obtained using either the inactivated PBP 2X or 2B genes, suggesting that deletion of either of these genes was a lethal event, but they were readily obtained using the inactivated PBP 1A gene. Analysis using the polymerase chain reaction confirmed that the latter transformants had replaced their chromosomal copy of the PBP 1A gene with the inactivated copy of the gene. Deletion of the PBP 1A gene was therefore tolerated under laboratory conditions and appeared to have little effect on growth or susceptibility to benzylpenicillin.

Genetics of resistance to third-generation cephalosporins in clinical isolates of Streptococcus pneumoniae

Resistance to third-generation cephalosporins in a clinical isolate of Streptococcus pneumoniae was shown to be due to the production of altered forms of penicillin-binding proteins (PBPs) 2X and 1A. The cloned PBP2X gene from the resistant strain was able to transform a susceptible strain to an intermediate level of resistance. The resulting transformant could be transformed to the full level of resistance of the clinical isolate using the cloned PBP1A gene from the latter strain. Chromosomal DNA from the resistant strain (and from other resistant strains) could readily transform a susceptible strain to the full level of resistance to third-generation cephalosporins (greater than 250-fold for cefotaxime; greater than 100-fold for ceftriaxone) in a single step (transformation frequency of about 10(-5)). The resistant transformants obtained with chromosomal DNA were shown by gene fingerprinting to have gained both the PBP1A and PBP2X genes from the DNA donor.

Sequence diversity within the argF, fbp and recA genes of natural isolates of Neisseria meningitidis: interspecies recombination within the argF gene

Studies of natural populations of Neisseria meningitidis using multilocus enzyme electrophoresis have shown extensive genetic variation within this species, which, it has been proposed, implies a level of sequence diversity within meningococci that is greater than that normally considered as the criterion for species limits in bacteria. To obtain a direct measure of the sequence diversity among meningococci, we obtained the nucleotide sequences of most of the argF, recA and fbp genes of eight meningococci of widely differing electrophoretic type (from the reference collection of Caugant). Sequence variation between the meningococcal strains ranged from 0-0.6% for fbp, 0-1.3% for argF, and 0-3.3% for recA. These levels of diversity are no greater than those found within Escherichia coli 'housekeeping' genes and suggest that multilocus enzyme electrophoresis may overestimate the extent of nucleotide sequence diversity within meningococci. The average sequence divergence between the Neisseria meningitidis strains and N. gonorrhoeae strain FA19 was 1.0% for fbp and 1.6% for recA. The argF gene, although very uniform among the eight meningococcal isolates, had a striking mosaic structure when compared with the gonococcal argF gene: two regions of the gene differed by greater than 13% in nucleotide sequence between meningococci and gonococci, whereas the rest of the gene differed by less than 1.7%. One of the diverged regions was shown to have been introduced from the argF gene of a commensal Neisseria species that is closely related to Neisseria cinerea. The source of the other region was unclear.

Genetic diversity of penicillin-resistant Neisseria meningitidis

The genetic relatedness of 42 penicillin-resistant Neisseria meningitidis isolates obtained during a 2-year period from a single hospital was studied by multilocus enzyme electrophoresis and by restriction fragment length polymorphism (RFLP) analysis of penicillin-binding protein (PBP) 2 genes. The PBP 2 genes of 7 susceptible strains gave identical RFLP profiles. Sixteen different PBP 2 RFLP profiles were found among the 42 resistant strains, but 4 were found in greater than 1 resistant isolate. Multilocus enzyme electrophoresis revealed a high level of genetic diversity. Four clusters of resistant strains could be distinguished at a genetic distance of 0.75. Resistant strains with the most common PBP 2 RFLP profile were restricted to one of these clusters and may be derived from a common ancestral strain. However, resistant strains with the 3 other common RFLP profiles were distributed in two or more of the clusters.

Role of interspecies transfer of chromosomal genes in the evolution of penicillin resistance in pathogenic and commensal Neisseria species

The two pathogenic species of Neisseria, N. meningitidis and N. gonorrhoeae, have evolved resistance to penicillin by alterations in chromosomal genes encoding the high molecular weight penicillin-binding proteins, or PBPs. The PBP 2 gene (penA) has been sequenced from over 20 Neisseria isolates, including susceptible and resistant strains of the two pathogenic species, and five human commensal species. The genes from penicillin-susceptible strains of N. meningitidis and N. gonorrhoeae are very uniform, whereas those from penicillin-resistant strains consist of a mosaic of regions resembling those in susceptible strains of the same species, interspersed with regions resembling those in one, or in some cases, two of the commensal species. The mosaic structure is interpreted as having arisen from the horizontal transfer, by genetic transformation, of blocks of DNA, usually of a few hundred base pairs. The commensal species identified as donors in these interspecies recombinational events (N. flavescens and N. cinerea) are intrinsically more resistant to penicillin than typical isolates of the pathogenic species. Transformation has apparently provided N. meningitidis and N. gonorrhoeae with a mechanism by which they can obtain increased resistance to penicillin by replacing their penA genes (or the relevant parts of them) with the penA genes of related species that fortuitously produce forms of PBP 2 that are less susceptible to inhibition by the antibiotic. The ends of the diverged blocks of DNA in the penA genes of different penicillin-resistant strains are located at the same position more often than would be the case if they represent independent crossovers at random points along the gene. Some of these common crossover points may represent common ancestry, but reasons are given for thinking that some may represent independent events occurring at recombinational hotspots.

Acyltransferase activities of the high-molecular-mass essential penicillin-binding proteins

The high-molecular-mass penicillin-binding proteins (HMM-PBPs), present in the cytoplasmic membranes of all eubacteria, are involved in important physiological events such as cell elongation, septation or shape determination. Up to now it has, however, been very difficult or impossible to study the catalytic properties of the HMM-PBPs in vitro. With simple substrates, we could demonstrate that several of these proteins could catalyse the hydrolysis of some thioesters or the transfer of their acyl moiety on the amino group of a suitable acceptor nucleophile. Many of the acyl-donor substrates were hippuric acid or benzoyl-D-alanine derivatives, and their spectroscopic properties enabled a direct monitoring of the enzymic reaction. In their presence, the binding of radioactive penicillin to the PBPs was also inhibited.

Expression and purification of a soluble form of penicillin-binding protein 2 from both penicillin-susceptible and penicillin-resistant Neisseria gonorrhoeae

Resistance to penicillin in non-beta-lactamase-producing strains of Neisseria gonorrhoeae (CMRNG strains) is mediated in part by the production of altered forms of penicillin-binding protein 2 (PBP 2) that have a decreased affinity for penicillin. The reduction in the affinity of PBP 2 is largely due to the insertion of an aspartic acid residue (Asp-345a) into the amino acid sequence of PBP 2. Truncated forms of N. gonorrhoeae PBP 2, which differed only by the insertion of Asp-345a, were constructed by placing the region of the penA genes encoding the periplasmic domain of PBP 2 (amino acids 42-581) into an ATG expression vector. When the recombinant PBP 2 molecules were overexpressed in Escherichia coli, insoluble PBP 2 inclusion bodies, which could be isolated by low-speed centrifugation of cell lysates, were formed. These insoluble aggregates were solubilized and the truncated PBP 2 polypeptides were partially purified by cation-exchange chromatography and gel filtration in the presence of denaturant prior to the refolding of the enzyme in vitro. After renaturation, gel filtration was used to separate monomeric soluble PBP 2 from improperly folded protein aggregates and other protein contaminants. A 4-liter culture of induced E. coli cells yielded 1.4 mg of soluble PBP 2 or PBP 2' (PBP 2 containing the Asp-345a insertion), both of which were estimated to be 99% pure. The affinity of soluble PBP 2' for [3H]penicillin G was decreased fourfold relative to that of soluble PBP 2, and their affinities were found to be identical to the affinities of the full-length PBP 2 enzymes that were previously determined in N. gonorrhoeae membranes. Furthermore, soluble PBP 2 displayed a rank order of affinity for several other beta-lactam antibiotics that was consistent with the rank order of affinities previously reported for the native molecules. On the basis of these results, both of these soluble PBPs should be suitable for crystallization and X-ray crystallographic analysis.

The mutation Lys234His yields a class A beta-lactamase with a novel pH-dependence

The lysine-234 residue is highly conserved in beta-lactamases and in nearly all active-site-serine penicillin-recognizing enzymes. Its replacement by a histidine residue in the Streptomyces albus G class A beta-lactamase yielded an enzyme the pH-dependence of which was characterized by the appearance of a novel pK, which could be attributed to the newly introduced residue. At low pH, the kcat, value for benzylpenicillin was as high as 50% of that of the wild-type enzyme, demonstrating that an efficient active site was maintained. Both kcat. and kcat/Km dramatically decreased above pH 6 but the decrease in kcat./Km could not be attributed to larger Km values. Thus a positive charge on the side chain of residue 234 appears to be more essential for transition-state stabilization than for initial recognition of the substrate ground state.

Horizontal transfer of multiple penicillin-binding protein genes, and capsular biosynthetic genes, in natural populations of Streptococcus pneumoniae

Multiply antibiotic-resistant serotype 23F isolates of Streptococcus pneumoniae are prevalent in Spain and have also been recovered recently in the United Kingdom and the United States. Analysis of populations of these isolates by multilocus enzyme electrophoresis, and restriction endonuclease cleavage electrophoretic profiling of penicillin-binding protein (PBP) genes, has demonstrated that these isolates are a single clone (Muñoz et al., 1991). Here we report studies of non-serotype 23F penicillin-resistant pneumococci isolated in Spain and the United Kingdom. One of the isolates expressed serotype 19 capsule but was otherwise indistinguishable from the serotype 23F clone on the basis of multilocus enzyme electrophoresis, antibiotic resistance profiling, and restriction endonuclease patterns of genes encoding PBP1A, PBP2B and PBP2X, a result which suggests that horizontal transfer of capsular biosynthesis genes had occurred. These same techniques revealed that six other resistant isolates, all expressing serotype 9 polysaccharide capsule, represent a clone. Interestingly, the chromosomal lineage of this clone is not closely related to the 23F clone; however, the serotype 9 and 23F clones harbour apparently identical PBP1A, -2B and -2X genes. To explain these data, we favour the interpretation that horizontal gene transfer in natural populations has distributed genes encoding altered forms of PBP1A, -2B and -2X to distinct evolutionary lineages of S. pneumoniae.

Interspecies recombinational events during the evolution of altered PBP 2x genes in penicillin-resistant clinical isolates of Streptococcus pneumoniae

Penicillin resistance in pneumococci is due to the appearance of high molecular-weight penicillin-binding proteins (PBPs) that have reduced affinity for the antibiotic. We have compared the PBX 2x genes (pbpX) of one penicillin-susceptible and five penicillin-resistant clinical isolates of Streptococcus pneumoniae isolated from various parts of the world. All of the resistant isolates contained a low-affinity form of PBP 2x. The 2 kb region of the two penicillin-susceptible isolates differed at only eight nucleotide sites (0.4%) and resulted in one single amino acid difference in PBP 2x. In contrast, the sequences of the PBP 2x genes from the resistant isolates differed overall from those of the susceptible isolates at between 7 and 18% of nucleotide sites and resulted in between 27 and 86 amino acid substitutions in PBP 2x. The altered PBP 2x genes consisted of regions that were similar to those of susceptible strains (less than 3% diverged), alternating with regions that were very different (18-23% diverged). The presence of highly diverged regions within the PBP 2x genes of the resistant isolates contrasts with the uniformity of the sequences of the amylomaltase genes from the same isolates, and with the uniformity of the PBP 2x genes in the two susceptible isolates. It suggests that the altered PBP 2x genes have arisen by localized interspecies recombinational events involving the PBP 2x genes of closely related streptococci, as has been suggested to occur for altered PBP 2b genes (Dowson et al., 1989b). The PBP 2x genes from the resistant isolates could transform the susceptible strain R6 to increased levels of resistance to beta-lactam antibiotics, indicating that the altered forms of PBP 2x in the resistant isolates contribute to their resistance to penicillin.

Intercontinental spread of a multiresistant clone of serotype 23F Streptococcus pneumoniae

Isolates of serotype 23F Streptococcus pneumoniae with high levels of resistance of penicillin have been commonly recovered in Spain for more than a decade. Recently penicillin-resistant serotype 23F S. pneumoniae strains were also isolated from children attending a day-care center in Cleveland. A number of Spanish and Cleveland isolates were compared by electrophoretic analysis of penicillin-binding protein (PBP) profiles and DNA restriction endonuclease cleavage profiles of the PBP 2X and 2B genes amplified with the polymerase chain reaction and by multilocus enzyme electrophoresis. All strains were identical by these criteria. The findings demonstrate that the Spanish and Cleveland isolates are clonally related and suggest that this antibiotic resistant clone of serotype 23F S. pneumoniae has spread intercontinentally from Spain to the United States.

Penicillin-resistant isolates of Neisseria lactamica produce altered forms of penicillin-binding protein 2 that arose by interspecies horizontal gene transfer

Isolates of Neisseria lactamica that have increased resistance to penicillin have emerged in recent years. Resistance to penicillin was shown to be due to the production of altered forms of penicillin-binding protein 2 (PBP 2) that have reduced affinity for the antibiotic. The sequences of the PBP 2 genes (penA) from two penicillin-resistant isolates were almost identical (less than or equal to 1% sequence divergence) to that of a penicillin-susceptible isolate, except in a 175-bp region where the resistant and susceptible isolates differed by 27%. The nucleotide sequences of these divergent regions were identical (or almost identical) to the sequence of the corresponding region of the penA gene of N. flavescens NCTC 8263. Altered forms of PBP 2 with decreased affinity for penicillin in the two penicillin-resistant isolates of N. lactamica appear, therefore, to have arisen by the replacement of part of the N. lactamica penA gene with the corresponding region from the penA gene of N. flavescens.

Localized sex in bacteria

Electrophoretic studies suggest that bacterial populations consist of a number of independent clones and that genetic recombination is rare in nature. But DNA sequencing reveals that individual bacterial genes have a mosaic structure that could have arisen only by recombination. How can these observations be reconciled and what is their relevance to prokaryotic evolution?

Mapping of conformational epitopes of monoclonal antibodies against Escherichia coli penicillin-binding protein 1B (PBP 1B) by means of hybrid protein analysis: implications for the tertiary structure of PBP 1B

We have analyzed the location of the epitope areas of the four monoclonal antibody groups against penicillin-binding protein 1B (PBP 1B; T. den Blaauwen, F. B. Wientjes, A. H. J. Kolk, B. G. Spratt, and N. Nanninga, J. Bacteriol. 171:1393-1401). They could be specified by studying monoclonal antibody binding patterns to amino- and carboxy-terminal truncated PBP 1B molecules. Monoclonal antibodies against conformational epitopes, with the exception of one epitope area, did not recognize PBP 1B molecules that had not been translocated across the membrane. Apparently, translocation is required for PBP 1B to fully obtain its native conformation.

The balance between different peptidoglycan precursors determines whether Escherichia coli cells will elongate or divide

The rodA(Sui) mutation allows cell division to take place at 42 degrees C in ftsI23 mutant cells, which produce a thermolabile penicillin-binding protein 3 (PBP3, the septation-specific peptidoglycan transpeptidase). We show here that the mutation in rodA is a single-base change from a glutamine to a chain termination (amber) codon, and that an amber suppressor (supE) present in the strain restores the ability to produce a reduced level of normal RodA protein. The reduced level of RodA is accompanied by an increase in the levels of two other proteins (PBP2 and PBP5) encoded by genes in the rodA operon. We show that an increased level of PBP5 is by itself sufficient to restore cell division to ftsI23 cells at 42 degrees C. Two other treatments were found to restore division capacity to the mutant: an increase in PBP6 (which is a D-alanine carboxypeptidase like PBP5) or suitable concentrations of D-cycloserine. All of the above treatments have the effect of reducing the number of pentapeptide side chains in peptidoglycan and increasing the number of tripeptides. We conclude that the effect of the rodA(Sui) mutation is to indirectly increase the availability of tripeptide side chains, which are used preferentially by PBP3 as acceptors in transpeptidation. A change in the proportions of different kinds of peptide side chain in the peptidoglycan can therefore determine whether cells will divide.

Penicillin-resistant viridans streptococci have obtained altered penicillin-binding protein genes from penicillin-resistant strains of Streptococcus pneumoniae

Penicillin-resistant strains of Streptococcus pneumoniae possess altered forms of penicillin-binding proteins (PBPs) with decreased affinity for penicillin. The PBP2B genes of these strains have a mosaic structure, consisting of regions that are very similar to those in penicillin-sensitive strains, alternating with regions that are highly diverged. Penicillin-resistant strains of viridans groups streptococci (e.g., S. sanguis and S. oralis) that produce altered PBPs have also been reported. The PBP2B genes of two penicillin-resistant clinical isolates of S. sanguis were identical in sequence to the mosaic class B PBP2B genes found in penicillin-resistant serotype 23 strains of S. pneumoniae. Emergence of penicillin resistance appears to have occurred by the horizontal transfer of an altered PBP2B gene from penicillin-resistant S. pneumoniae into S. sanguis. The PBP2B genes of three penicillin-resistant S. oralis strains were similar to the mosaic class B PBP2B gene of penicillin-resistant strains of S. pneumoniae but possessed an additional block of diverged sequence. Penicillin resistance in S. oralis has also probably arisen by horizontal transfer of this variant form of the class B mosaic PBP2B gene from a penicillin-resistant strain of S. pneumoniae.

Genetic diversity of penicillin-binding protein 2 genes of penicillin-resistant strains of Neisseria meningitidis revealed by fingerprinting of amplified DNA

A 2-kilobase fragment containing the penicillin-binding protein 2 gene (penA) was amplified by using the polymerase chain reaction with DNA prepared from 35 penicillin-resistant strains of Neisseria meningitidis isolated in England, Ireland, and Spain (MICs, 0.16 to 1.28 micrograms of benzylpenicillin per ml) and from 10 penicillin-susceptible strains (MICs, less than or equal to 0.04 micrograms of benzylpenicillin per ml). The penA genes were digested with HpaII or TaqYI; and the resulting fragments were end-labeled, fractionated on a polyacrylamide gel, and autoradiographed, to produce gene fingerprints. The fingerprints of the penA genes of the 10 penicillin-susceptible strains were identical. In contrast, the fingerprints of the penA genes of all but two of the penicillin-resistant strains differed markedly from those of the penicillin-susceptible strains. The altered penA genes of group B penicillin-resistant strains were very diverse, and 14 different fingerprints were found among the 15 strains that we examined. The penA genes of the 19 resistant group C strains were less diverse, and two major fingerprint patterns predominated.

Insertion of an extra amino acid is the main cause of the low affinity of penicillin-binding protein 2 in penicillin-resistant strains of Neisseria gonorrhoeae

Non-beta-lactamase-producing, penicillin-resistant strains of Neisseria gonorrhoeae (CMRNG strains) produce altered forms of penicillin-binding protein 2 (PBP2) that have decreased affinity for penicillin. A feature of PBP2 from all CMRNG strains is the presence of an additional residue (Asp-345A) that is absent from PBP2 of penicillin-sensitive strains. The role of the additional aspartic acid residue in the decreased affinity of PBP2 is unclear as PBP2 of all previously examined CMRNG strains possess several other amino acid sequence alterations, in addition to the insertion of Asp-345A, compared to PBP2 of penicillin-sensitive strains. Site-directed mutagenesis has been used to insert the Asp-345A codon into the penA gene from a penicillin-sensitive gonococcus. The resulting penA gene expressed an altered form of PBP2 that had a decreased affinity for benzylpenicillin and was able to transform a penicillin-sensitive strain of N. gonorrhoeae to an increased level of resistance to benzylpenicillin. Insertion of amino acids other than aspartic acid did not produce forms of PBP2 that provided increased resistance to penicillin. Removal of the Asp-345A codon from the penA gene of a CMRNG strain reduced its ability to transform a penicillin-sensitive strain to an increased level of penicillin resistance. The reduction in the affinity of PBP2 in CMRNG strains is therefore largely, although not exclusively, due to the insertion of Asp-345A. Clinical isolates that produce altered forms of PBP2 that differ from that of penicillin-sensitive strains only in the insertion of Asp-345A have been identified.

A simple method for maximizing the yields of membrane and exported proteins expressed in Escherichia coli

The feasibility of using a beta-lactamase fusion approach for maximizing the levels of periplasmic or membrane-bound proteins expressed in Escherichia coli was investigated. The coding region for mature TEM beta-lactamase was fused after the signal peptide and aminoterminal portion of the coding region of a weakly expressed periplasmic protein, PBP3*. The resultant plasmid was mutagenized and transformants expressing increased levels of ampicillin resistance were selected. The PBP3* gene of the unmutagenized beta-lactamase fusion plasmid, and of two mutant derivatives encoding increased ampicillin resistance, were then reassembled and the latter constructs were found to express increased levels of PBP3*. The applications of a beta-lactamase fusion approach in monitoring and optimizing levels of extracytoplasmic gene products expressed in E. coli are considered.

Horizontal transfer of penicillin-binding protein genes in penicillin-resistant clinical isolates of Streptococcus pneumoniae

Resistance to penicillin in clinical isolates of Streptococcus pneumoniae has occurred by the development of altered penicillin-binding proteins (PBPs) that have greatly decreased affinity for the antibiotic. We have investigated the origins of penicillin-resistant strains by comparing the sequences of the transpeptidase domain of PBP2B from 6 penicillin-sensitive and 14 penicillin-resistant strains. In addition we have sequenced part of the amylomaltase gene from 2 of the sensitive and 6 of the resistant strains. The sequences of the amylomaltase gene of all of the strains and of the PBP2B gene of the penicillin-sensitive strain show that S. pneumoniae is genetically very uniform. In contrast the PBP2B genes of the penicillin-resistant strains show approximately equal to 14% sequence divergence from those of the penicillin-sensitive strains and the development of penicillin resistance has involved the replacement, presumably by transformation, of the original PBP2B gene by a homologous gene from an unknown source. This genetic event has occurred on at least two occasions, involving different sources, to produce the two classes of altered PBP2B genes found in penicillin-resistant strains of S. pneumoniae. There is considerable variation among the PBP2B genes of the resistant strains that may have arisen by secondary transformation events accompanied by mismatch repair subsequent to their original introductions into S. pneumoniae.

Recruitment of a penicillin-binding protein gene from Neisseria flavescens during the emergence of penicillin resistance in Neisseria meningitidis

Non-beta-lactamase-producing, penicillin-resistant strains of Neisseria meningitidis produce altered forms of penicillin-binding protein 2 that have decreased affinity for penicillin. The sequence of the penicillin-binding protein 2 gene (penA) from a penicillin-resistant strain of N. meningitidis was compared to the sequence of the same gene from penicillin-sensitive strains and from penicillin-sensitive and penicillin-resistant strains of Neisseria gonorrhoeae. The penA genes from penicillin-sensitive strains of N. gonorrhoeae and N. meningitidis were 98% identical. The gene from the penicillin-resistant strain of N. meningitidis consisted of regions that were almost identical to the corresponding regions in the penicillin-sensitive strains (less than 0.2% divergence) and two regions that were very different from them (approximately 22% divergence). The two blocks of altered sequence have arisen by the replacement of meningococcal sequences with the corresponding regions from the penA gene of Neisseria flavescens and result in an altered form of penicillin-binding protein 2 that contains 44 amino acid substitutions and 1 amino acid insertion compared to penicillin-binding protein 2 of penicillin-sensitive strains of N. meningitidis. A similar introduction of part of the penA gene of N. flavescens, or a very similar commensal Neisseria species, appears to have occurred independently during the development of altered penA genes in non-beta-lactamase-producing penicillin-resistant strains of N. gonorrhoeae.

Membrane topology of penicillin-binding protein 3 of Escherichia coli

The beta-lactamase fusion vector, pJBS633, has been used to analyse the organization of penicillin-binding protein 3 (PBP3) in the cytoplasmic membrane of Escherichia coli. The fusion junctions in 84 in-frame fusions of the coding region of mature TEM beta-lactamase to random positions within the PBP3 gene were determined. Fusions of beta-lactamase to 61 different positions in PBP3 were obtained. Fusions to positions within the first 31 residues of PBP3 resulted in enzymatically active fusion proteins which could not protect single cells of E. coli from killing by ampicillin, indicating that the beta-lactamase moieties of these fusion proteins were not translocated to the periplasm. However, all fusions that contained greater than or equal to 36 residues of PBP3 provided single cells of E. coli with substantial levels of resistance to ampicillin, indicating that the beta-lactamase moieties of these fusion proteins were translocated to the periplasm. PBP3 therefore appeared to have a simple membrane topology with residues 36 to the carboxy-terminus exposed on the periplasmic side of the cytoplasmic membrane. This topology was confirmed by showing that PBP3 was protected from proteolytic digestion at the cytoplasmic side of the inner membrane but was completely digested by proteolytic attack from the periplasmic side. PBP3 was only inserted in the cytoplasmic membrane at its amino terminus since replacement of its putative lipoprotein signal peptide with a normal signal peptide resulted in a water-soluble, periplasmic form of the enzyme. The periplasmic form of PBP3 retained its penicillin-binding activity and appeared to be truly water-soluble since it fractionated, in the absence of detergents, with the expected molecular weight on Sephadex G-100 and was not retarded by hydrophobic interaction chromatography on Phenyl-Superose.

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.

Extensive re-modelling of the transpeptidase domain of penicillin-binding protein 2B of a penicillin-resistant South African isolate of Streptococcus pneumoniae

Clinical isolates of Streptococcus pneumoniae that have greatly increased levels of resistance to penicillin (greater than 1000-fold) have been reported from South Africa during the last ten years. Penicillin resistance in these strains is entirely due to the development of penicillin-binding proteins (PBPs) with decreased affinity for penicillin. We have cloned and sequenced the coding region for the transpeptidase domain of penicillin-binding protein 2B from three penicillin-sensitive strains of S. pneumoniae and from a penicillin-resistant South African strain. The amino acid sequences of the transpeptidase domains of PBP2B of the three penicillin-sensitive strains were identical and there were only between one and four differences in the nucleotide sequences of their coding regions. The corresponding region of the PBP2B gene from the penicillin-resistant strain differed by 74 nucleotide substitutions which resulted in 17 alterations in the amino acid sequence of PBP2B. The most remarkable alteration that has occurred during the development of the 'penicillin-resistant' form of PBP2B is the substitution of seven consecutive residues in a region that is predicted to form a loop at the bottom of the penicillin-binding site.

Penicillin-binding protein 2 genes of non-beta-lactamase-producing, penicillin-resistant strains of Neisseria gonorrhoeae

Oligonucleotides that correspond to regions of the penicillin-binding protein 2 gene (penA) that differ between penicillin-sensitive and penicillin-resistant strains have been used as probes to classify the penA genes in a collection of penicillin-resistant gonococci isolated in Britain. 44/47 of those gonococcal strains that had minimal inhibitory concentrations of greater than or equal to 0.25 microgram benzylpenicillin per ml contained extensively altered penA genes which appeared to be very similar (or identical) to one or other of the two classes of altered penA genes that have been described previously. Since these two classes of altered penA genes are related, it appears that the great majority of the altered penA genes on non-beta-lactamase-producing penicillin-resistant gonococci have a clonal origin. The other three penicillin-resistant strains had altered penA genes that were different to those described previously. A crucial step in the development of the altered forms of PBP2 with decreased affinity for penicillin appears to have been the insertion of an extra codon within the transpeptidase domain of the penA gene. This insertion was found in the penA gene of all gonococci with minimal inhibitory concentrations of greater than 0.016 microgram benzylpenicillin per ml but was not found in any strains with minimal inhibitory concentrations of less than or equal to 0.016 microgram per ml.

An amino acid substitution in penicillin-binding protein 3 creates pointed polar caps in Escherichia coli

The pbpB gene product penicillin-binding protein 3 (PBP3) of Escherichia coli is one of the major targets of beta-lactam antibiotics. At the permissive temperature, the temperature-sensitive pbpBr1 mutant, which was obtained after selection for increased resistance to cephalexin, shows a dramatic change in shape which has never been observed before; the polar caps are pointed. We show that the substitution of amino acid Asn-361 by Ser, previously shown to be responsible for increased cephalexin resistance and for temperature sensitivity, causes the pointed polar caps. However, comparison of the morphological and physiological characteristics of the pbpBr1 mutant with those of other pbpB mutants suggests that the formation of pointed polar caps is not correlated with temperature sensitivity or cephalexin resistance. Partial inactivation of PBP3 by subinhibitory concentrations of cephalexin, furazlocillin, and piperacillin resulted in the formation of slightly pointed polar caps, suggesting that the shape of the polar caps is correlated with PBP3 activity. The large change in the shape of the polar caps was accompanied by a small change in the kinetics of peptidoglycan synthesis and in the local rate of surface synthesis activity along the cell envelope.

Penicillin-binding proteins of gram-negative bacteria

beta-Lactam antibiotics exert their antibacterial effects by inactivating the high-molecular-weight penicillin-binding proteins (PBPs) that are responsible for the final stages of peptidoglycan biosynthesis. The availability of the amino acid sequences of several low-molecular-weight PBPs, high-molecular-weight PBPs, and active-site serine beta-lactamases has provided evidence that these groups of enzymes have a common, but distant, evolutionary origin. This view is strongly supported by the recent finding of a similarity in the three-dimensional structures of a low-molecular-weight PBP and class A beta-lactamases. The high-molecular-weight PBPs of Escherichia coli are believed to possess an amino-terminal peptidoglycan transglycosylase domain and a carboxy-terminal penicillin-sensitive transpeptidase domain. These enzymes are inserted in the cytoplasmic membrane only at their amino termini, and water-soluble forms have been obtained that should be suitable for crystallization and X-ray analysis. Resistance to beta-lactam antibiotics mediated by alterations of PBPs has been reported in some gram-negative bacteria. In isolates of Neisseria gonorrhoeae with chromosomally mediated resistance, penicillin-resistant PBPs have arisen from the introduction of multiple amino acid substitutions within the transpeptidase domain of the enzymes.

Hybrid penicillin-binding proteins in penicillin-resistant strains of Neisseria gonorrhoeae

Benzylpenicillin has been used extensively for approximately 40 years in the treatment of gonorrhoea. The intense selective pressures resulting from the continual exposure of Neisseria gonorrhoeae to penicillin have resulted in the emergence of resistant strains that produce altered forms of penicillin-binding proteins (PBPs) with decreased affinity for the antibiotic. A comparison of the sequences of the PBP-2 genes from penicillin-sensitive and penicillin-resistant strains, suggests that penicillin-resistant forms of PBP 2 may have arisen both by amino-acid substitutions and insertions, and by the exchange of a region encoding part of the penicillin-sensitive transpeptidase domain with the homologous region from a closely related species.

Use of a beta-lactamase fusion vector to investigate the organization of penicillin-binding protein 1B in the cytoplasmic membrane of Escherichia coli

The coding region for the mature form of TEM beta-lactamase was fused to random positions within the coding region of the penicillin-binding protein 1B (PBP 1B) gene and the nucleotide sequences across the fusion junctions of 100 in-frame fusions were determined. All fusion proteins that contained at least the NH2-terminal 94 residues of PBP 1B provided individual cells of E. coli with substantial levels of ampicillin resistance, suggesting that the beta-lactamase moiety had been translocated to the periplasm. Fusion proteins that contained less than or equal to 63 residues of PBP 1B possessed beta-lactamase activity, but could not protect single cells of E. coli from ampicillin, indicating that the beta-lactamase moiety of these fusion proteins remained in the cytoplasm. The beta-lactamase fusion approach suggested a model for the organization of PBP 1B in which the protein is embedded in the cytoplasmic membrane by a single hydrophobic transmembrane segment (residues 64-87), with a short NH2-terminal domain (residues 1-63), and the remainder of the polypeptide (residues 88-844) exposed on the periplasmic side of the cytoplasmic membrane. The proposed model for the organization of PBP 1B was supported by experiments which showed that the protein was completely digested by proteinase K added from the periplasmic side of the cytoplasmic membrane but was only slightly reduced in size by protease attack from the cytoplasmic side of the membrane.

Interaction between membrane proteins PBP3 and rodA is required for normal cell shape and division in Escherichia coli

In Escherichia coli, the products of several genes are required for septation, and the products of several others are required for the maintenance of the rod shape of the cells. We show here that the combination of certain mutations in a division gene (ftsI) with a specific mutation in one of the shape genes (rodA) could produce cells with normal shape and division, although separately these mutations led to a loss of the capacity to divide (ftsI) or to form normal rod-shaped cells (rodA). In contrast, combinations between other mutant alleles of these genes produced double mutants which had lost the capacity both to divide and to form rod-shaped cells. The mutual phenotypic correction observed within particular pairs of mutant genes suggests that the normal morphogenetic cycle of growth and division may require direct interaction between the two membrane proteins which are the products of these genes.

Peptidoglycan synthetic activities in membranes of Escherichia coli caused by overproduction of penicillin-binding protein 2 and rodA protein

Penicillin-binding protein (PBP)-2 and the RodA protein are known to function in determining the rod shape of Escherichia coli cells. Peptidoglycan biosynthetic reactions that required these two proteins were demonstrated in the membrane fraction prepared from an E. coli strain that overproduced both of these two proteins and which lacked PBP-1B activity (the major peptidoglycan synthetase activity in the normal E. coli membranes). The cross-linked peptidoglycan was synthesized from UDP-N-acetylmuramylpentapeptide and UDP-N-acetylglucosamine in the presence of a high concentration of cefmetazole that inhibited all of PBPs except PBP-2. The peptidoglycan was synthesized via a lipid intermediate and showed up to 30% cross-linking. The cross-linking reaction was strongly inhibited by the amidinopenicillin, mecillinam, and by other beta-lactam antibiotics that have a high affinity for PBP-2, but not by beta-lactams that had very low affinity for PBP-2. The formation of peptidoglycan required the presence of high levels of both PBP-2 and the RodA protein in the membranes, but it is unclear which of the two proteins was primarily responsible for the extension of the glycan chains (transglycosylation). However, the sensitivity of the cross-linking reaction to specific beta-lactam antibiotics strongly suggested that it was catalyzed by PBP-2. The transglycosylase activity of the membranes was sensitive to enramycin and vancomycin and was unusual in being stimulated greatly by a high concentration of a chelating agent.

A vector for the construction of translational fusions to TEM beta-lactamase and the analysis of protein export signals and membrane protein topology

A plasmid vector, pJBS633, that facilitates the construction of translational fusions of genes of interest to the coding region of the mature form of TEM beta-lactamase has been developed. Transformants containing in-frame fusions can be identified by their ability to grow when plated at high inocula on agar containing ampicillin (Ap). The cellular location of the beta-lactamase moiety of the fusion proteins can then be determined since only those that direct the translocation of the beta-lactamase across the cytoplasmic membrane to the periplasm result in the ability of individual cells of Escherichia coli to form isolated colonies in the presence of Ap. Conversely, those fusion proteins in which the beta-lactamase moiety remains cytoplasmic do not protect individual cells against Ap. Transformants expressing the latter class of fusion proteins can, however, be identified when plated at high inocula since, as cells start to lyse, the cytoplasmic beta-lactamase activity is released and provides Ap resistance to the surrounding cells. The vector contains the origin of replication of f1 phage so that single-stranded plasmid DNA can be obtained in the appropriate orientation to allow sequencing across the fusion junction using a universal primer complementary to the start of the coding region of mature TEM beta-lactamase. pJBS633 should be useful as a general vector for the construction of beta-lactamase fusions and, in particular, for the analysis of protein export signals and the determination of the organisation of proteins in the E. coli cytoplasmic membrane.

Kanamycin-resistant vectors that are analogues of plasmids pUC8, pUC9, pEMBL8 and pEMBL9

Analogues of the cloning vectors pUC8, pUC9, pEMBL8 +/- and pEMBL9 +/- that have kanamycin resistance (KmR) instead of ampicillin resistance (ApR) as the selectable marker have been developed. HindIII and SmaI sites within the KmR gene have been removed so that all of the cloning sites in the multi-linker region of these plasmids may be used except the AccI site.

Resistance to beta-lactam antibiotics by re-modelling the active site of an E. coli penicillin-binding protein

The beta-lactam antibiotics kill bacteria by inhibiting a set of penicillin-binding proteins (PBPs) that catalyse the final stages of peptidoglycan synthesis. In some bacteria the development of intrinsic resistance to beta-lactam antibiotics by the reduction in the affinity of PBPs causes serious clinical problems. The introduction of beta-lactam antibiotics that are resistant to hydrolysis by beta-lactamases may also result in the emergence of intrinsic resistance among the Enterobacteriaceae. The clinical problems that would arise from the emergence of resistant PBPs in enterobacteria have led us to examine the ease with which Escherichia coli can gain resistance to beta-lactams by the production of altered PBPs. The development of resistant PBPs also provides an interesting example of enzyme evolution, since it requires a subtle re-modeling of the enzyme active centre so that it retains affinity for its peptide substrate but excludes the structurally analogous beta-lactam antibiotics. We show here that only four amino-acid substitutions need to be introduced into PBP 3 of E. coli to produce a strain possessing substantial levels of resistance to a wide variety of cephalosporins. We also show that transfer of the gene encoding the resistant PBP 3 from the chromosome to a plasmid could result in the spread of intrinsic resistance not only to other strains of E. coli but also to other enterobacterial species.

Lysis of Escherichia coli by beta-lactam antibiotics: deletion analysis of the role of penicillin-binding proteins 1A and 1B

Deletions of the ponA and ponB genes of Escherichia coli have been constructed in vitro and recombined into the chromosome to produce strains that completely lack penicillin-binding protein 1A or penicillin-binding protein 1B. In each case a DNA fragment internal to the gene was replaced by a fragment encoding an antibiotic resistance. The ponA and ponB deletions can therefore be readily introduced into other E. coli strains by P1 transduction of the antibiotic resistance. Although the complete absence of penicillin-binding protein 1A or penicillin-binding protein 1B was tolerated, the absence of both of these proteins was shown to result in bacterial lysis.

Amino acid substitutions that reduce the affinity of penicillin-binding protein 3 of Escherichia coli for cephalexin

The location of amino acid substitutions that allow an enzyme to discriminate between the binding of its normal substrate and a substrate analogue may be used to identify regions of the polypeptide that fold to form the substrate binding site. We have isolated a large number of cephalexin-resistant mutants of Escherichia coli in which the resistance is due to the production of altered forms of penicillin-binding protein 3 that have reduced affinity for the antibiotic. Using three mutagens, and a variety of selection procedures, we obtained only five classes of mutants which could be distinguished by their patterns of cross-resistance to other beta-lactam antibiotics. The three classes of mutants that showed the highest levels of resistance to cephalexin were cross-resistant to several other cephalosporins but not to penicillins or to the monobactam, aztreonam. The penicillin-binding protein 3 gene from 46 independent mutants was cloned and sequenced. Each member of the five classes of cephalexin-resistant mutants had the same amino acid substitution in penicillin-binding protein 3. The mutants that showed the highest levels of resistance to cephalexin had alterations of either Thr-308 to Pro, Val-344 to Gly, or Asn-361 to Ser. The Thr-308 to Pro substitution had occurred within the beta-lactam-binding site since the adjacent residue (Ser-307) has been shown to be acylated by benzylpenicillin. The Asn-361 to Ser change occurred in a region that showed substantial similarity to regions in both penicillin-binding protein 1A and 1B and may also define a residue that is located within the beta-lactam-binding site in the three-dimensional structure of the enzyme.

Sequences of the active-site peptides of three of the high-Mr penicillin-binding proteins of Escherichia coli K-12

The amino acid compositions of the radioactive peptides obtained from trypsin digestion of [14C]benzylpenicillin-labeled penicillin-binding proteins (PBPs) 1A, 1B, and 3 of Escherichia coli have been obtained. Complete digestion of these peptides with a combination of aminopeptidase M and carboxypeptidase Y showed that benzylpenicillin was bound to a serine residue in each of these proteins. Comparison of the compositions of the penicillin-labeled peptides with the complete amino acid sequences of PBPs 1A, 1B, and 3 showed that the acylated serine occurs near the middle of each of the proteins, within the conserved sequence Gly-Ser-Xaa-Xaa-Lys-Pro. The sequence around the acylated serine of these high Mr PBPs shows little similarity to that around the acylated serine of the low-Mr PBPs (D-alanine carboxypeptidases) or of the class A or class C beta-lactamases, except that in all of these enzymes which interact with penicillin the acylated serine residue occurs within the sequence Ser-Xaa-Xaa-Lys.

The nucleotide sequences of the ponA and ponB genes encoding penicillin-binding protein 1A and 1B of Escherichia coli K12

Penicillin-binding proteins 1A and 1B of Escherichia coli are the major peptidoglycan transglycosylase-transpeptidases that catalyse the polymerisation and insertion of peptidoglycan precursors into the bacterial cell wall during cell elongation. The nucleotide sequence of a 2764-base-pair fragment of DNA that contained the ponA gene, encoding penicillin-binding protein 1A, was determined. The sequence predicted that penicillin-binding protein 1A had a relative molecular mass of 93 500 (850 amino acids). The amino-terminus of the protein had the features of a signal peptide but it is not known if this peptide is removed during insertion of the protein into the cytoplasmic membrane. The nucleotide sequence of a 2758-base-pair fragment of DNA that contained the ponB gene, encoding penicillin-binding protein 1B, was also determined. Penicillin-binding protein 1B consists of two major components which were shown to result from the use of alternative sites for the initiation of translation. The large and small forms of penicillin-binding protein 1B were predicted to have relative molecular masses of 94 100 and 88 800 (844 and 799 amino acids). The amino acid sequences of penicillin-binding proteins 1A and 1B could be aligned if two large gaps were introduced into the latter sequence and the two proteins then showed about 30% identity. The amino acid sequences of the proteins showed no extensive similarity to the sequences of penicillin-binding proteins 3 or 5, or to the class A or class C beta-lactamases. Two short regions of amino acid similarity were, however, found between penicillin-binding proteins 1A and 1B and the other penicillin-binding proteins and beta-lactamases. One of these included the predicted active-site serine residue which was located towards the middle of the sequences of penicillin-binding proteins 1A, 1B and 3, within the conserved sequence Gly-Ser-Xaa-Xaa-Lys-Pro. The other region was 19-40 residues to the amino-terminal side of the active-site serine and may be part of a conserved penicillin-binding site in these proteins.

Production of thiol-penicillin-binding protein 3 of Escherichia coli using a two primer method of site-directed mutagenesis

The active site serine residue of penicillin-binding protein 3 of Escherichia coli that is acylated by penicillin (Ser-307) has been converted to a cysteine residue using a simple and efficient two primer method of site-directed mutagenesis. The resulting thiol-penicillin-binding protein 3 was expressed under the control of the lacUV5 promoter in a high copy number plasmid. Constitutive expression of the thiol-enzyme (but not of the wild-type enzyme) was lethal, and the plasmid could only be maintained in E. coli strains that carried the lacIq mutation. Induction of the expression of the thiol-enzyme resulted in inhibition of cell division and the growth of the bacteria into very long filamentous cells. The inhibition of septation was probably due to interference of the function of the wild-type penicillin-binding protein 3 in cell division by the enzymatically inactive thiol-enzyme, and this implies that penicillin-binding protein 3 acts as part of a complex in vivo. We were unable to detect any acylation of the thiol-enzyme by penicillin, but it is not yet clear if this was because the thioester was not formed at an appreciable rate, or if it was formed but was too unstable to be detected by a modified penicillin-binding protein assay.