Comparison of the sequences of class A beta-lactamases and of the secondary structure elements of penicillin-recognizing proteins

Primary structure of OXA-3 and phylogeny of oxacillin-hydrolyzing class D beta-lactamases

We determined the nucleotide sequence of the blaOXA-3(pMG25) gene from Pseudomonas aeruginosa. The bla structural gene encoded a protein of 275 amino acids representing one monomer of 31,879 Da for the OXA-3 enzyme. Comparisons between the OXA-3 nucleotide and amino acid sequences and those of class A, B, C, and D beta-lactamases were performed. An alignment of the eight known class D beta-lactamases including OXA-3 demonstrated the presence of conserved amino acids. In addition, conserved motifs composed of identical amino acids typical of penicillin-recognizing proteins and specific class D motifs were identified. These conserved motifs were considered for possible roles in the structure and function of oxacillinases. On the basis of the alignment and identity scores, a dendrogram was constructed. The phylogenetic data obtained revealed five groups of class D beta-lactamases with large evolutionary distances between each group.

DD-peptidases and beta-lactamases: catalytic mechanisms and specificities

DD-peptidases and beta-lactamases share several common properties, including the formation of an acylenzyme intermediate in their catalytic pathways. In their interactions with beta-lactam antibiotics, the stability of this intermediate is much higher with the peptidases than with the beta-lactamases. The structural factors responsible for this difference have not been identified. The evolution of beta-lactamases is taking place before our eyes, since mutants are constantly selected which can hydrolyze the molecules newly introduced as "beta-lactamase resistant" in the chemotherapeutic arsenal.

Contribution of mutant analysis to the understanding of enzyme catalysis: the case of class A beta-lactamases

Class A beta-lactamases represent a family of well studied enzymes. They are responsible for many antibiotic resistance phenomena and thus for numerous failures in clinical chemotherapy. Despite the facts that five structures are known at high resolution and that detailed analyses of enzymes modified by site-directed mutagenesis have been performed, their exact catalytic mechanism remains controversial. This review attempts to summarize and to discuss the many available data.

A common system controls the induction of very different genes. The class-A beta-lactamase of Proteus vulgaris and the enterobacterial class-C beta-lactamase

Among the Enterobacteriaceae, Proteus vulgaris is exceptional in the inducible production of a 29-kDa beta-lactamase (cefuroximase) with an unusually high activity towards the beta-lactamase-stable oximino-cephalosporins (e.g. cefuroxime and cefotaxime). Sequencing of the corresponding gene, cumA, showed that the derived CumA beta-lactamase belonged to the molecular class A. The structural gene was under the direct control of gene cumR, which was transcribed backwards and whose initiation codon was 165 bp away from that of the beta-lactamase gene. This resembled the arrangement of structural and regulator genes ampC and ampR of the 39-kDa molecular-class-C beta-lactamase AmpC present in many enterobacteria. Moreover, cloned genes ampD and ampG for negative modulation and signal transduction of AmpC beta-lactamase induction, respectively, were also able to restore constitutively CumA overproducing and non-inducible P. vulgaris mutants to the inducible, wild-type phenotype. The results indicate that controls of the induction phenomena are equivalent for the CumA and AmpC beta-lactamase. Very different structural genes can thus be under the control of identical systems.

Site-directed mutagenesis of proposed active-site residues of penicillin-binding protein 5 from Escherichia coli

Alignment of the amino acid sequence of penicillin-binding protein 5 (PBP5) with the sequences of other members of the family of active-site-serine penicillin-interacting enzymes predicted the residues playing a role in the catalytic mechanism of PBP5. Apart from the active-site (Ser44), Lys47, Ser110-Gly-Asn, Asp175 and Lys213-Thr-Gly were identified as the residues making up the conserved boxes of this protein family. To determine the role of these residues, they were replaced using site-directed mutagenesis. The mutant proteins were assayed for their penicillin-binding capacity and DD-carboxypeptidase activity. The Ser44Cys and the Ser44Gly mutants showed a complete loss of both penicillin-binding capacity and DD-carboxypeptidase activity. The Lys47Arg mutant also lost its DD-carboxypeptidase activity but was able to bind and hydrolyse penicillin, albeit at a considerably reduced rate. Mutants in the Ser110-Gly-Asn fingerprint were affected in both acylation and deacylation upon reaction with penicillin and lost their DD-carboxypeptidase activity with the exception of Asn112Ser and Asn112Thr. The Asp175Asn mutant showed wild-type penicillin-binding but a complete loss of DD-carboxypeptidase activity. Mutants of Lys213 lost both penicillin-binding and DD-carboxypeptidase activity except for Lys213His, which still bound penicillin with a k+2/K' of 0.2% of the wild-type value. Mutation of His216 and Thr217 also had a strong effect on DD-carboxypeptidase activity. Thr217Ser and Thr217Ala showed augmented hydrolysis rates for the penicillin acyl-enzyme. This study reveals the residues in the conserved fingerprints to be very important for both DD-carboxypeptidase activity and penicillin-binding, and confirms them to play crucial roles in catalysis.

Catalytic mechanism of active-site serine beta-lactamases: role of the conserved hydroxy group of the Lys-Thr(Ser)-Gly triad

The role of the conserved hydroxy group of the Lys-Thr(Ser)-Gly [KT(S)G] triad has been studied for a class A and a class C beta-lactamase by site-directed mutagenesis. Surprisingly, the disappearance of this functional group had little impact on the penicillinase activity of both enzymes. The cephalosporinase activity was much more affected for the class A S235A (Ser235-->Ala) and the class C T316V (Thr315-->Val) mutants, but the class C T316A mutant was less impaired. Studies were extended to beta-lactams, where the carboxy group on C-3 of penicillins or C-4 of cephalosporins had been modified. The effects of the mutations were the same on these compounds as on the unmodified regular penicillins and cephalosporins. The results are compared with those obtained with a similar mutant (T299V) of the Streptomyces R61 DD-peptidase. With this enzyme the mutation also affected the interactions with penicillins and severely decreased the peptidase activity. The strict conservation of the hydroxy group on the second residue of the KT(S)G triad is thus much more easy to understand for the DD-peptidase and the penicillin-binding proteins than for beta-lactamases, especially those of class C.

The mechanism of action of DD-peptidases: the role of Threonine-299 and -301 in the Streptomyces R61 DD-peptidase

The side chains of residues Thr299 and Thr301 in the Streptomyces R61 DD-peptidase have been modified by site-directed mutagenesis. These amino acids are part of a beta-strand which forms a wall of the active-site cavity. Thr299 corresponds to the second residue of the Lys-Thr(Ser)-Gly triad, highly conserved in active-site beta-lactamases and penicillin-binding proteins (PBPs). Modification of Thr301 resulted only in minor alterations of the catalytic and penicillin-binding properties of the enzyme. No selective decrease of the rate of acylation was observed for any particular class of compounds. By contrast, the loss of the hydroxy group of the residue in position 299 yielded a seriously impaired enzyme. The rates of inactivation by penicillins were decreased 30-50-fold, whereas the reactions with cephalosporins were even more affected. The efficiency of hydrolysis against the peptide substrate was also seriously decreased. More surprisingly, the mutant was completely unable to catalyse transpeptidation reactions. The conservation of an hydroxylated residue in this position in PBPs is thus easily explained by these results.

Cloning and sequence analysis of the gene for a carbapenem-hydrolyzing class A beta-lactamase, Sme-1, from Serratia marcescens S6

Serratia marcescens S6 produces a pI 9.7 carbapenem-hydrolyzing beta-lactamase that is probably encoded by the chromosome (Y. Yang, P. Wu, and D. M. Livermore, Antimicrob. Agents Chemother. 34:755-758, 1990). A total of 11.3 kb of genomic DNA from this strain was cloned into plasmid pACYC184 in Escherichia coli. After further subclonings, the carbapenem-hydrolyzing beta-lactamase gene (blaSme-1) was sequenced (EMBL accession number Z28968). The gene corresponded to an 882-bp open reading frame which encoded a 294-amino-acid polypeptide. This open reading frame was preceded by a -10 and a -35 region consistent with a putative promoter sequence of members of the family Enterobacteriaceae. This promoter was active in E. coli and S. marcescens, as demonstrated by primer extension analysis. N-terminal sequencing showed that the Sme-1 enzyme had a 27-amino-acid leader peptide and enabled calculation of the molecular mass of the mature protein (29.3 kDa). Sequence alignment revealed that Sme-1 is a class A serine beta-lactamase and not a class B metalloenzyme. The earlier view that the enzyme was zinc dependent was discounted. Among class A beta-lactamases, Sme-1 had the greatest amino acid identity (70%) with the pI 6.9 carbapenem-hydrolyzing beta-lactamase, NMC-A, from Enterobacter cloacae NOR-1. Comparison of these two protein sequences suggested a role for specific residues in carbapenem hydrolysis. The relatedness of Sme-1 to other class A beta-lactamases such as the TEM and SHV types was remote. This work details the sequence of the second carbapenem-hydrolyzing class A beta-lactamase from an enterobacterial species and the first in the genus Serratia.

Role of residue Lys315 in the mechanism of action of the Enterobacter cloacae 908R beta-lactamase

The role of the highly conserved Lys315 residue in the catalytic mechanism of a class C beta-lactamase has been probed by site-directed mutagenesis. Lys315 has been replaced by a histidine in the Enterobacter cloacae 908R beta-lactamase, thus introducing a tritratable group to probe the role of the positive charge, and by a glutamine. The effects of these mutations have been studied on the kinetics of penicillin G and cephalothin turnover and on the pre-steady-state kinetics with carbenicillin at different pH. Results showed that substrate binding was not impaired by the mutations, so that an interaction with the substrate-free carboxylate in the Henri-Michaelis complex could be ruled out. Lys315 must have a catalytic role as shown by the decreased acylation and deacylation rates observed with the mutant enzymes. The mutants exhibited a lower activity at acidic pH, and this observation could be correlated with a decreased affinity for (3-aminophenyl)boronate, a compound devoid of free carboxylate which binds to the active site and forms an adduct mimicking the tetrahedral intermediate. This suggested that Lys315 was somehow involved in accelerating the nucleophilic substitutions along the reaction pathway. The study was extended to modified substrates where the free carboxylate had been esterified. Neither acylation nor deacylation seemed severely impaired with these compounds, showing that the interaction between the enzyme and the substrate-free carboxylate did not play a major role in catalysis.

Transcription and expression analysis, using lacZ and phoA gene fusions, of Mycobacterium fortuitum beta-lactamase genes cloned from a natural isolate and a high-level beta-lactamase producer

The gene encoding a class A beta-lactamase was cloned from a natural isolate of Mycobacterium fortuitum (blaF) and from a high-level amoxicillin-resistant mutant that produces large amounts of beta-lactamase (blaF*). The nucleotide sequences of the two genes differ at 11 positions, including two in the region upstream from the coding sequence. Gene fusions to Escherichia coli lacZ and transcription and expression analysis of the cloned genes in Mycobacterium smegmatis indicated that high-level production of the beta-lactamase in the mutant is mainly or wholly due to a single base pair difference in the promoter. These analyses also showed that transcription and translation start at the same position. A comparison of the amino acid sequence of BlaF, as predicted from the nucleotide sequence, with the determined N-terminal amino acid sequence indicated the presence of a typical signal peptide. The fusion of blaF (or blaF*) to the E. coli gene phoA resulted in the production of BlaF-PhoA hybrid proteins that had alkaline phosphatase activity. These results demonstrate that phoA can be used as a reporter gene for studying protein export in mycobacteria.

Characterization of a plasmid-borne and constitutively expressed blaMOX-1 gene encoding AmpC-type beta-lactamase

A 1954-bp DNA fragment containing the blaMOX-1 gene, identified on a large resident plasmid (pRMOX-1) of Klebsiella pneumoniae NU2936, was sequenced and an open reading frame (ORF) coding for a 390-amino-acid (aa) MOX-1 was found. The total deduced aa sequence of MOX-1 shared considerable homology with that of AmpC-type class C beta-lactamases of Gram- bacteria, especially of Pseudomonas aeruginosa PAO1 [51.3%; 63.8% at the nucleotide (nt) level]. However, the regulatory gene ampR and a 38-bp AmpR-binding region were not present upstream from blaMOX-1, although the expression of P. aeruginosa ampC is directly regulated by AmpR. Possible -35 and -10 regions, a Shine-Dalgarno (SD) sequence and terminators were identified which are peculiar to blaMOX-1. On the other hand, a sequence highly homologous (91.6%) to the region upstream from dhfrX in the In7 integron carried by plasmid pDGO100 was found upstream from blaMOX-1 at nt 1 to 488. No significant difference was detected between the promoter activities of blaMOX-1 in ampD- and ampD+ strains of Enterobacter cloacae, as measured by the chloramphenicol acetyltransferase (CAT) assay. These results clearly show that blaMOX-1 belongs to the group of ampC-related bla genes and that it is expressed constitutively, independently of transcriptional regulators such as AmpR, AmpG and AmpD. Homology analysis among AmpC enzymes or ampC genes implied that integration of the chromosomal ampC gene into a large resident plasmid, followed by transconjugation, was involved in the evolution of blaMOX-1.

Sequence analysis of PER-1 extended-spectrum beta-lactamase from Pseudomonas aeruginosa and comparison with class A beta-lactamases

We have determined the nucleotide sequence (EMBL accession number, Z 21957) of the cloned chromosomal PER-1 extended-spectrum beta-lactamase gene from a Pseudomonas aeruginosa RNL-1 clinical isolate, blaPER-1 corresponds to a 924-bp open reading frame which encodes a polypeptide of 308 amino acids. This open reading frame is preceded by a -10 and a -35 region consistent with a putative P. aeruginosa promoter. Primer extension analysis of the PER-1 mRNA start revealed that this promoter was active in P. aeruginosa but not in Escherichia coli, in which PER-1 expression was driven by vector promoter sequences. N-terminal sequencing identified the PER-1 26-amino-acid leader peptide and enabled us to calculate the molecular mass (30.8 kDa) of the PER-1 mature form. Analysis of the percent GC content of blaPER-1 and of its 5' upstream sequences, as well as the codon usage for blaPER-1, indicated that blaPER-1 may have been inserted into P. aeruginosa genomic DNA from a nonpseudomonad bacterium. The PER-1 gene showed very low homology with other beta-lactamase genes at the DNA level. By using computer methods, assessment of the extent of identity between PER-1 and 10 beta-lactamase amino acid sequences indicated that PER-1 is a class A beta-lactamase. PER-1 shares around 27% amino acid identity with the sequenced extended-spectrum beta-lactamases of the TEM-SHV series and MEN-1 from Enterobacteriaceae species. The use of parsimony methods showed that PER-1 is not more closely related to gram-negative than to gram-positive bacterial class A beta-lactamases. Surprisingly, among class A beta-lactamases, PER-1 was most closely related to the recently reported CFXA from Bacteroides vulgatus, with which it shared 40% amino acid identity. This work indicates that non-Enterobacteriaceae species such as P. aeruginosa may possess class A extended-spectrum beta-lactamase genes possibly resulting from intergeneric DNA transfer.

Evidence for myosin motors on organelles in squid axoplasm

Squid axoplasm has proved a rich source for the identification of motors involved in organelle transport. Recently, squid axoplasmic organelles have been shown to move on invisible tracks that are sensitive to cytochalasin, suggesting that these tracks are actin filaments. Here, an assay is described that permits observation of organelles moving on unipolar actin bundles. This assay is used to demonstrate that axoplasmic organelles move on actin filaments in the barbed-end direction, suggesting the presence of a myosin motor on axoplasmic organelles. Indeed, axoplasm contains actin-dependent ATPase activity, and a pan-myosin antibody recognized at least four bands in Western blots of axoplasm. An approximately 235-kDa band copurified in sucrose gradients with KI-extracted axoplasmic organelles, and the myosin antibody stained the organelle surfaces by immunogold electron microscopy. The myosin is present on the surface of at least some axoplasmic organelles and thus may be involved in their transport through the axoplasm, their movement through the cortical actin in the synapse, or some other aspect of axonal function.

Evolution of an enzyme activity: crystallographic structure at 2-A resolution of cephalosporinase from the ampC gene of Enterobacter cloacae P99 and comparison with a class A penicillinase

The structure of the class C ampC beta-lactamase (cephalosporinase) from Enterobacter cloacae strain P99 has been established by x-ray crystallography to 2-A resolution and compared to a class A beta-lactamase (penicillinase) structure. The binding site for beta-lactam (penicillinase) structure. The binding site for beta-lactam antibiotics is generally more open than that in penicillinases, in agreement with the ability of the class C beta-lactamases to better bind third-generation cephalosporins. Four corresponding catalytic residues (Ser-64/70, Lys-67/73, Lys-315/234, and Tyr-150/Ser-130 in class C/A) lie in equivalent positions within 0.4 A. Significant differences in positions and accessibilities of Arg-349/244 may explain the inability of clavulanate-type inhibitors to effectively inactivate the class C beta-lactamases. Glu-166, required for deacylation of the beta-lactamoyl intermediate in class A penicillinases, has no counterpart in this cephalosporinase; the nearest candidate, Asp-217, is 10 A from the reactive Ser-64. A comparison of overall tertiary folding shows that the cephalosporinase, more than the penicillinase, is broadly similar to the ancestral beta-lactam-inhibited enzymes of bacterial cell wall synthesis. On this basis, it is proposed that the cephalosporinase is the older of the two beta-lactamases, and, therefore, that a local refolding in the active site, rather than a simple point mutation, was required for the primordial class C beta-lactamase to evolve to the class A beta-lactamase having an improved ability to catalyze the deacylation step of beta-lactam hydrolysis.

Interactions between active-site-serine beta-lactamases and mechanism-based inactivators: a kinetic study and an overview

The interactions between three class A beta-lactamases and three beta-lactamase inactivators (clavulanic acid, sulbactam and olivanic acid MM13902) were studied. Interestingly, the interaction between the Streptomyces cacaoi beta-lactamase and clavulanate indicated little irreversible inactivation. With sulbactam, irreversible inactivation was found to occur with the three studied enzymes, but no evidence for transiently inactivated adducts was found. Irreversible inactivation of the S. albus G and S. cacaoi enzymes was particularly slow. With olivanate, irreversible inactivation was also observed with the three enzymes, but with the S. cacaoi enzyme, no hydrolysis could be detected. A tentative summary of the results found in the literature is also presented (including 6 beta-halogenopenicillanates), and the general conclusions underline the diversity of the mechanisms and the wide variations of the rate constants observed when class A beta-lactamases interact with beta-lactamase inactivators, in agreement with the behaviours of the same enzymes towards their good and poor substrates.

Cloning and characterization of the endogenous cephalosporinase gene, cepA, from Bacteroides fragilis reveals a new subgroup of Ambler class A beta-lactamases

Bacteroides fragilis CS30 is a clinical isolate resistant to high concentrations of benzylpenicillin and cephaloridine but not to cephamycin or penem antibiotics. beta-Lactam resistance is mediated by a chromosomally encoded cephalosporinase produced at a high level. The gene encoding this beta-lactamase was cloned from genomic libraries constructed in Escherichia coli and then mated with B. fragilis 638 for identification of ampicillin-resistant (Apr) strains. Apr transconjugants contained a nitrocefin-reactive protein with the physical and enzymatic properties of the original CS30 isolate. The beta-lactamase gene (cepA) was localized by deletion analysis and subcloned, and its nucleotide sequence was determined. The 903-bp cepA open reading frame encoded a 300-amino-acid precursor protein (predicted molecular mass, 34,070 Da). A beta-lactamase-deficient mutant strain of B. fragilis 638 was constructed by insertional inactivation with the cepA gene of CS30, demonstrating strict functional homology between these chromosomal beta-lactamase genes. An extensive comparison of the CepA protein sequence by alignment with other beta-lactamases revealed the strict conservation of at least four elements common to Ambler class A. A further comparison of the CepA protein sequence with protein sequences of beta-lactamases from two other Bacteroides species indicated that they constitute their own distinct subgroup of class A beta-lactamases.

Common components in the assembly of type 4 fimbriae, DNA transfer systems, filamentous phage and protein-secretion apparatus: a general system for the formation of surface-associated protein complexes

The Pseudomonas aeruginosa genes pilB-D and pilQ are necessary for the assembly of type 4 fimbriae. Homologues of these genes and of the subunit (pilin) gene have been described in various different bacterial species, but not always in association with type 4 fimbrial biosynthesis and function. Pil-like proteins are also involved in protein secretion, DNA transfer by conjugation and transformation, and morphogensis of filamentous bacteriophages. It seems likely that the Pil homologues function in the processing and export of proteins resembling type 4 fimbrial subunits, and in their organization into fimbrial-like structures. These may either be true type 4 fimbriae, or components of protein complexes which act in the transport of macromolecules (DNA or protein) into or out of the cell. Some PilB-like and PilQ-like proteins are apparently also involved in the assembly of non-type 4 polymeric structures (filamentous phage virions and conjugative pili). The diverse studies summarized in this review are providing insight into an extensive infrastructural system which appears to be utilized in the formation of a variety of cell surface-associated complexes.

Characterization of pilQ, a new gene required for the biogenesis of type 4 fimbriae in Pseudomonas aeruginosa

Type 4 fimbriae are produced by a variety of pathogens, in which they appear to function in adhesion to epithelial cells, and in a form of surface translocation called twitching motility. Using transposon mutagenesis of Pseudomonas aeruginosa, we have identified a new locus required for fimbrial assembly. This locus contains the gene pilQ which encodes a 77 kDa protein with an N-terminal hydrophobic signal sequence characteristic of secretory proteins. pilQ mutants lack the spreading colony morphology characteristic of twitching motility, are devoid of fimbriae, and are resistant to the fimbrial-specific bacteriophage PO4. The pilQ gene was mapped to Spel fragment 2, which is located at 0-5 minutes on the P. aeruginosa PAO1 chromosome, and thus it is not closely linked to the previously characterized pilA-D, pilS,R or pilT genes. The pilQ region also contains ponA, aroK and aroB-like genes in an organization very similar to that of corresponding genes in Escherichia coli and Haemophilus influenzae. The predicted amino acid sequence of PilQ shows homology to the PulD protein of Klebsiella oxytoca and related outer membrane proteins which have been found in association with diverse functions in other species including protein secretion, DNA uptake and assembly of filamentous phage. PilQ had the highest overall homology to an outer membrane antigen from Neisseria gonorrhoeae, encoded by omc, that may fulfil the same role in type 4 fimbrial assembly in this species.

Mechanism of action of DD-peptidases: role of asparagine-161 in the Streptomyces R61 DD-peptidase

The role of residue Asn-161 in the interaction between the Streptomyces R61 DD-peptidase and various substrates or beta-lactam inactivators was probed by site-directed mutagenesis. The residue was successively replaced by serine and alanine. In the first case, acylation rates were mainly affected with the peptide and ester substrates but not with the thiol-ester substrates and beta-lactams. However, the deacylation rates were decreased 10-30-fold with the substrates yielding benzoylglycyl and benzoylalanyl adducts. The Asn161Ala mutant was more generally affected, although the acylation rates with cefuroxime and cefotaxime remained similar to those observed with the wild-type enzyme. Surprisingly, the deacylation rates of the benzoylglycyl and benzoylalanyl adducts were very close to those observed with the wild-type enzyme. The results also indicate that the interaction with the peptide substrate and the transpeptidation reaction were more sensitive to the mutations than the other reactions studied. The results are discussed and compared with those obtained with the Asn-132 mutants of a class A beta-lactamase.

A class-A beta-lactamase from Pseudomonas stutzeri that is highly active against monobactams and cefotaxime

A beta-lactamase produced by Pseudomonas stutzeri was purified to protein homogeneity, and its physicochemical and catalytic properties were determined. Its profile was unusual since, in addition to penicillins, the enzyme hydrolysed second- and third-generation 'beta-lactamase-stable' cephalosporins and monobactams with similar efficiencies. On the basis of the characteristics of the interaction with beta-iodopenicillanic acid, the enzyme could be classified as a class-A beta-lactamase. However, when compared with most class-A beta-lactamases, it exhibited significantly lower kcat./Km values for the compounds usually considered to be the best substrates of these enzymes.

A comparative study of class-D beta-lactamases

Three class-D beta-lactamases (OXA2, OXA1 and PSE2) were produced and purified to protein homogeneity. 6 beta-Iodopenicillanate inactivated the OXA2 enzyme without detectable turnover. Labelling of the same beta-lactamase with 6 beta-iodo[3H]penicillanate allowed the identification of Ser-70 as the active-site serine residue. In agreement with previous reports, the apparent M(r) of the OXA2 enzyme as determined by molecular-sieve filtration, was significantly higher than that deduced from the gene sequence, but this was not due to an equilibrium between a monomer and a dimer. The heterogeneity of the OXA2 beta-lactamase on ion-exchange chromatography contrasted with the similarity of the catalytic properties of the various forms. A first overview of the enzymic properties of the three 'oxacillinases' is presented. With the OXA2 enzyme, 'burst' kinetics, implying branched pathways, seemed to prevail with many substrates.

blaI and blaR1 regulate beta-lactamase and PBP 2a production in methicillin-resistant Staphylococcus aureus

For Staphylococcus aureus, it is hypothesized that two genes located upstream of the beta-lactamase gene, blaZ, are required for the inducible expression of beta-lactamase. blaR1 is predicted to encode a signal-transducing membrane protein, and blaI is predicted to encode a repressor protein. These same two genes may also regulate the production of penicillin-binding protein 2a (PBP 2a), a protein essential for expression of methicillin resistance. To confirm that these two genes encode products that can control both beta-lactamase and PBP 2a production, blaI, blaR1, and blaZ with a 150-nucleotide deletion at the 3' end were subcloned from a 30-kb staphylococcal beta-lactamase plasmid and three beta-lactamase-negative strains of methicillin-resistant S. aureus were transformed with the recombinant plasmid containing that insert. The production of PBP 2a and a nonfunctional beta-lactamase was detected by fluorography and by immunoblots with polyclonal antisera directed against each of the proteins. Whereas the parent strains did not produce beta-lactamase and constitutively produced PBP 2a, PBP 2a and a truncated beta-lactamase were now inducible in the transformants. Therefore, two plasmid-derived genes regulate the production of both PBP 2a and beta-lactamase.

The mechanism of action of DD-peptidases: the role of tyrosine-159 in the Streptomyces R61 DD-peptidase

Tyrosine-159 of the Streptomyces R61 penicillin-sensitive DD-peptidase was replaced by serine or phenylalanine. The second mutation yielded a very poorly active protein whose rate of penicillin binding was also drastically decreased, except for the reactions with nitrocefin and methicillin. The consequences of the first mutation were more surprising, since a large proportion of the thiolesterase activity was retained, together with the penicillin-binding capacity. Conversely, the peptidase properties was severely affected. In both cases, a drastic decrease in the transferase activity was observed. The results are compared with those obtained by mutation of the corresponding residue in the class A beta-lactamase of Streptomyces albus G.

An overview of the kinetic parameters of class B beta-lactamases

The catalytic properties of three class B beta-lactamases (from Pseudomonas maltophilia, Aeromonas hydrophila and Bacillus cereus) were studied and compared with those of the Bacteroides fragilis enzyme. The A. hydrophila beta-lactamase exhibited a unique specificity profile and could be considered as a rather specific 'carbapenemase'. No relationships were found between sequence similarities and catalytic properties. The problem of the repartition of class B beta-lactamases into sub-classes is discussed. Improved purification methods were devised for the P. maltophilia and A. hydrophila beta-lactamases including, for the latter enzyme, a very efficient affinity chromatography step on a Zn(2+)-chelate column.

Sequence of the vanY gene required for production of a vancomycin-inducible D,D-carboxypeptidase in Enterococcus faecium BM4147

Cloning and nucleotide sequencing identified the vanY gene as a member of the vancomycin-resistance van gene cluster of enterococcal plasmid, pIP816. The vanY gene was necessary for synthesis of the vancomycin-inducible D,D-carboxypeptidase activity previously proposed to be responsible for glycopeptide resistance. However, this activity was not required for peptidoglycan synthesis in the presence of glycopeptides. The deduced product of vanY did not display significant similarity with other D,D-carboxypeptidases.

Structure, function, and fate of the BlaR signal transducer involved in induction of beta-lactamase in Bacillus licheniformis

The membrane-spanning protein BlaR is essential for the induction of beta-lactamase in Bacillus licheniformis. Its nature and location were confirmed by the use of an antiserum specific for its carboxy-terminal penicillin sensor, its function was studied by genetic dissection, and the structure of the penicillin sensor was derived from hydrophobic cluster analysis of the amino acid sequence by using, as a reference, the class A beta-lactamases with known three-dimensional structures. During the first 2 h after the addition of the beta-lactam inducer, full-size BlaR, bound to the plasma membrane, is produced, and then beta-lactamase is produced. By 2 h after induction, BlaR is present in various (membrane-bound and cytosolic) forms, and there is a gradual decrease in beta-lactamase production. The penicillin sensors of BlaR and the class D beta-lactamases show strong similarities in primary structures. They appear to have the same basic spatial disposition of secondary structures as that of the class A beta-lactamases, except that they lack several alpha helices and, therefore, have a partially uncovered five-stranded beta sheet and a more readily accessible active site. Alterations of BlaR affecting conserved secondary structures of the penicillin sensor and specific sites of the transducer annihilate beta-lactamase inducibility.

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

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

Phylogeny of LCR-1 and OXA-5 with class A and class D beta-lactamases

The nucleotide sequences of blaLCR-1 and blaOXA-5 beta-lactamase genes have been determined. Polypeptide products of 260 and 267 amino acids with estimated molecular masses of 27 120 Da and 27,387 Da were obtained for the mature form of LCR-1 and OXA-5 proteins. A progressive alignment was used to evaluate the extent of identity between LCR-1 and OXA-5 with 29 other beta-lactamase amino acid sequences. The data showed that both belong to class D. We identified amino acids conserved in 24 positions for class A beta-lactamases and in 28 positions for five class D enzymes. The structural similarities between class A and class D beta-lactamases are more extensive than indicated by earlier biochemical studies with overall 16% identity between both classes. From the alignment, dendograms were constructed with a distance-matrix and parsimony methods which defined three major groups of proteins subdivided into clusters giving insight on beta-lactamase phylogeny and evolution.