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Dual enzyme activities for the biosynthesis of peptidoglycan of the cell wall are located in major higher molecular weight penicillin-binding proteins (PBP) of Escherichia coli. Each of these proteins catalyzes the two successive final reactions in the synthesis of cross-linked peptidoglycan from the precursor N-acetylglucosaminyl-N-acetylmuramyl peptide linked to undecaprenol diphosphate; namely, the transglycosylation that extends the glycan chain and the penicillin-sensitive DD-transpeptidation that cross-links the glycan chains through two peptide side chains. Both transglycosylation and transpeptidation catalyzed by PBP-1Bs represent de novo synthesis of cross-linked peptidoglycan. Under appropriate conditions, about 25% cross-linkage was observed during the reaction, the main reaction product supposedly being a regularly cross-linked network of peptidoglycan. The two domains for the transglycosylase and transpeptidase activities were found to be located on a 50-kDa portion of the PBP-1Bs, which are about 90 kDa. Gene recombination experiments indicated that the transglycosylase domain is located upstream, i.e. on the N-terminal side of the transpeptidase domain, suggesting that the gene for these bifunctional peptides may have been formed by fusion of the genes for transglycosylase and transpeptidase that were previously located separately on the chromosome in this order.

The binding affinities of imipenem (N- formimidoylthienamycin ) to penicillin-binding proteins ( PBSs ) of Escherichia coli and Pseudomonas aeruginosa were determined by two different methods in which competition with [14C]benzylpenicillin for the binding sites was measured. By both methods imipenem was shown to have very high binding affinities to PBPs-2 and -4 in E. coli and P. aeruginosa, and appreciable affinities to most of their other major PBPs. But higher concentrations of imipenem were required for binding to the PBPs-3 in these bacteria. More direct information about the antibacterial activity of imipenem was obtained by measuring its inhibition of the peptidoglycan-synthetic enzyme activities of E. coli PBPs. The results of enzyme inhibitions were compatible with those obtained in binding experiments. The antibiotic inhibited the transpeptidase activities of PBPs-1A, -1B and -2, and the D-alanine carboxypeptidase activities of PBPs-4 and -5. The antibiotic also seemed to cause strong inhibition of the transglycosylase activity of PBP-1A by some unknown mechanism. It inhibited the transpeptidase activity of PBP-3 only weakly, which is consistent with the findings that it had low binding affinity to PBP-3 and did not inhibit septum formation by the cells.

A chromosomal region of Escherichia coli contiguous to the fabE gene at 71 min on the chromosomal map contains multiple genes that are responsible for determination of the rod shape and sensitivity to the amidinopenicillin mecillinam. The so-called mre region was cloned and analyzed by complementation of two closely related but distinct E. coli mutants characterized, respectively, by the mutations mre-129 and mre-678, that showed a rounded to irregular cell shape and altered sensitivities to mecillinam; the mre-129 mutant was supersensitive to mecillinam at 30 degrees C, but the mre-678 mutant was resistant. The mre-678 mutation also caused simultaneous overproduction of penicillin-binding proteins 1Bs and 3. A chromosomal region of the wild-type DNA containing the total mre region and the fabE gene was first cloned on a lambda phage; a 7-kilobase (kb) fragment containing the whole mre region, but not the fabE gene, was then recloned on a mini F plasmid, pLG339; and finally, a 2.8-kb fragment complementing only mre-129 was also cloned on this low-copy-number plasmid. The whole 7-kb fragment was required for complementing the mre-678 mutant phenotypes. Fragments containing fabE but not the mre-129 region could be cloned on a high-copy-number plasmid. Southern blot hybridization indicated that the mre-678 mutant had a large deletion of 5.25 kb in its DNA, covering at least part of the mre-129 gene.

Major peptidoglycan transglycosylase activities, which synthesize uncross-linked peptidoglycan from lipid-linked precursors, were solubilized from the membranes of Staphylococcus aureus and Micrococcus luteus and were partially purified. The transglycosylase activities were separated from penicillin-binding proteins by solubilization and by purification steps. Therefore, we concluded that these activities were not activities of the penicillin-binding proteins, which are the presumptive peptidoglycan transpeptidases in these gram-positive cocci. Unlike Escherichia coli, in which the network structure of peptidoglycan is synthesized by multiple two-headed penicillin-binding proteins with both transpeptidase and transglycosylase activities, these gram-positive cocci have cell wall peptidoglycan which seems to be synthesized by penicillin-binding protein transpeptidases and a separate transglycosylase.

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

The localization of the active site of penicillin-binding protein 5 from the dacA mutant of Escherichia coli strain TMRL 1222 has been determined. The protein was purified to homogeneity and labeled with [14C] penicillin G. The labeled protein was digested with trypsin, and the active site tryptic peptide was purified by a combination of gel filtration and high-pressure liquid chromatography. Sequencing of the purified [14C]penicilloyl peptide yielded the sequence Arg-Asp-Pro-Ala-Ser-Leu-Thr-Lys, which corresponds to residues 40-47 of the gene sequence (Broome-Smith, J., Edelman, A., and Spratt, B. G. (1983) in The Target of Penicillin (Hakenbeck, R., Holtje, J.-V., and Labischinski, H., eds) pp. 403-408, Walter de Gruyter, Berlin). The catalytic amino acid residue that forms a covalent bond with penicillin was identified by treating the purified [14C]penicilloyl peptide with a mixture of proteases and then separating the radioactive products using high-pressure liquid chromatography. Analysis of the radioactive peaks by amino acid analysis confirmed that it is the serine residue that reacts with the beta-lactam ring of penicillin.

Two cell shape-determining genes of Escherichia coli K-12, pbpA, the structural gene for penicillin-binding protein 2, and rodA, whose protein is unknown, were subcloned into plasmid vectors from the transducing phage lambda MAd lip24, which carries the lip-leuS region of the E. coli chromosome. Plasmids with restriction enzyme-created deletions or transposon Tn5 insertions were isolated, and studies of genetic complementation of these plasmids with chromosomal mutations were carried out. Thus, a physical and genetic map of the rodA-pbpA region was established. The genes rodA and pbpA lie side by side within a 4.4-kilobase-pair region. The size of the rodA gene has been shown to be between 0.86 and 1.6 kilobase pairs; such DNA would encode a protein with a molecular weight between 32,000 and 59,000. Since Tn5 mutagenesis of the rodA gene did not affect the expression of the pbpA gene and vice versa, the genes rodA and pbpA seem to have independent promoters. Analysis of the proteins synthesized from the constructed plasmids in maxicells revealed that the plasmid carrying the pbpA gene encoded penicillin-binding protein 2 and amplification of the protein occurred. The product of the rodA gene was not identified.

A beta-lactam-sensitive strain of Staphylococcus aureus could be converted to methicillin resistance by the introduction of a plasmid carrying the 4.3-kilobase HindIII chromosomal DNA fragment which encoded the mecA gene from a methicillin-resistant S. aureus. Transformant cells produced methicillin-resistant S. aureus-specific penicillin-binding protein constitutively, and additional insertion of an inducible penicillinase plasmid caused production of the pencillin-binding protein to become inducible.

We determined the active site of penicillin-binding protein (PBP) 2 of Escherichia coli. A water-soluble form of PBP 2, which was constructed by site-directed mutagenesis, was purified by affinity chromatography, labeled with dansyl-penicillin, and then digested with a combination of proteases. The amino acid composition of the labeled chymotryptic peptide purified by HPLC was identical with that of the amino acid sequence, Ala-Thr-Gln-Gly-Val-Tyr-Pro-Pro-Ala-Ser330-Thr-Val-Lys-Pro (residues 321-334) of PBP 2, which was deduced from the nucleotide sequence of the pbpA gene encoding PBP 2. This amino acid sequence was verified by sequencing the labeled tryptic peptide containing the labeled chymotryptic peptide region. A mutant PBP 2 (thiol-PBP 2), constructed by site-directed mutagenesis to replace Ser330 with Cys, lacked the penicillin-binding activity. These findings provided evidence that Ser330 near the middle of the primary structure of PBP 2 is the penicillin-binding active-site residue, as predicted previously on the basis of the sequence homology. Around this active site, the sequence Ser-Xaa-Xaa-Lys was observed, which is conserved in the active-site regions of all E. coli PBPs so far studied, class A and class C beta-lactamases, and D-Ala carboxypeptidases. The COOH-terminal amino acid of PBP 2 was identified as His633.

The defect in D-alanine carboxypeptidase IA activity in the dacA11191 mutant of Escherichia coli was correlated with a defect in the release of penicillin G from penicillin-binding protein 5. The results suggest that penicillin-binding protein 5 catalyzes the major D-alanine carboxypeptidase IA activity of the wild type and that the mutation results in a defect in the deacylation step catalyzed by this enzyme.

A new beta-lactam-inducible penicillin-binding protein (PBP) that has extremely low affinity to penicillin and most other beta-lactam antibiotics has been widely found in highly beta-lactam(methicillin)-resistant Staphylococcus aureus (MRSA). The gene for this protein was sequenced and the nucleotide sequence in its promoter and close upstream area was found to show close similarity with that of staphylococcal penicillinase, while the amino acid sequence over a wide range of the molecule was found to be similar to those of two PBPs of Escherichia coli, the shape-determining protein (PBP 2) and septum-forming one (PBP 3). Probably the MRSA PBP (Mr 76462) evolved by recombination of two genes: an inducible type I penicillinase gene and a PBP gene of a bacterium, causing the formation of a beta-lactam-inducible MRSA PBP.

Penicillin-binding proteins 1, 2 and 3 in Staphylococcus aureus were found to possess common properties. All have very strong affinities for both benzylpenicillin and imipenem (N-formimidoylthienamycin), and all have an activity which releases bound imipenem, but not bound benzylpenicillin. Lower molecular weight penicillin-binding protein 4, which has a rather weak affinity for benzylpenicillin and also weak penicillinase activity showed an extraordinarily high affinity for imipenem but no antibiotic-releasing activity.

A novel penicillin-binding protein, PBP-2' (Mr about 75,000), is known to be induced in excessively large amount by most beta-lactam compounds in cells of a clinically isolated strain of Staphylococcus aureus, TK784, that is highly resistant to beta-lactams and also most other antibiotics. This protein has very low affinities to most beta-lactam compounds and has been supposed to be the cause of the resistance of the cells to beta-lactams. A 14-kilobase DNA fragment was isolated from the cells that carried the gene encoding this penicillin-binding protein and also a genetically linked marker that is responsible for the resistance to tobramycin. This DNA was cloned on plasmid pACYC184 and was shown to cause both production of PBP-2' and resistance to tobramycin in Escherichia coli cells. However, the formation of PBP-2' in E. coli was only moderate and was independent of normal inducer beta-lactams. The PBP-2' formed in the E. coli cells showed slow kinetics of binding to beta-lactams similar to that of PBP-2' formed in the original S. aureus cells and gave a similar pattern of peptides to the latter when digested with the proteolytic V8 enzyme of S. aureus.

A mutant of Escherichia coli that is deficient in D-alanine carboxypeptidase IA has been isolated. The enzyme is membrane bound and moderately sensitive to penicillin. It catalyzes in vitro both D-alanine carboxypeptidase and transpeptidase reactions. Being able to synthesize crosslinked peptidoglycan both in vivo and in vitro despite the absence of enzyme activity, the newly isolated mutant grew normally under a wide range of growth conditions. Therefore, this enzyme, like D-alanine carboxypeptidase IB, is not required for normal peptidoglycan synthesis in E. coli. The defect in the activity of D-alanine carboxypeptidase IA in the mutant however was not associated with disappearance of penicillin-binding proteins 5 and 6 (which have been shown to be D-alanine carboxypeptidase IA) or any of the other protein bands that bind [14C]penicillin G. Genetic mapping studies showed that the mutation (dacA) is located close to leuS(13.7 min) on the E. coli chromosome map. Double mutants (dacA dacB) that are deficient in both D-alanine carboxypeptidases IA and IB were obtained. These double mutants also were found to grow normally and to catalyze normal formation of crosslinked peptidoglycan.

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

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