AmpD, essential for both beta-lactamase regulation and cell wall recycling, is a novel cytosolic N-acetylmuramyl-L-alanine amidase

In enterobacteria, the ampD gene encodes a cytosolic protein which acts as a negative regulator of beta-lactamase expression. It is shown here that the AmpD protein is a novel N-acetylmuramyl-L-alanine amidase (E.C.3.5.1.28) participating in the intracellular recycling of peptidoglycan fragments. Surprisingly, AmpD exhibits an exclusive specificity for substrates containing anhydro muramic acid. This anhydro bond is mainly found in the peptidoglycan degradation products formed by the periplasmic lytic transglycosylases and thus might behave as a 'recycling tag' allowing the enzyme to distinguish these fragments from the newly synthesized peptidoglycan precursors. The AmpD substrate (or substrates) which accumulates in the absence of the corresponding enzymatic activity acts as an intracellular positive effector for beta-lactamase expression and might represent an element of a communication network between the chromosome and the cell wall peptidoglycan.

Copurification of glucosamine-1-phosphate acetyltransferase and N-acetylglucosamine-1-phosphate uridyltransferase activities of Escherichia coli: characterization of the glmU gene product as a bifunctional enzyme catalyzing two subsequent steps in the pathway for UDP-N-acetylglucosamine synthesis

The glmU gene product of Escherichia coli was recently identified as the N-acetylglucosamine-1-phosphate uridyltransferase activity which catalyzes the formation of UDP-N-acetylglucosamine, an essential precursor for cell wall peptidoglycan and lipopolysaccharide biosyntheses (D. Mengin-Lecreulx and J. van Heijenoort, J. Bacteriol. 175:6150-6157, 1993). Evidence that the purified GlmU protein is in fact a bifunctional enzyme which also catalyzes acetylation of glucosamine-1-phosphate, the preceding step in the same pathway, is now provided. Kinetic parameters of both reactions were investigated, indicating in particular that the acetyltransferase activity of the enzyme is fivefold higher than its uridyltransferase activity. In contrast to the uridyltransferase activity, which is quite stable and insensitive to thiol reagents, the acetyltransferase activity was rapidly lost when the enzyme was stored in the absence of reducing thiols or acetyl coenzyme A or was treated with thiol-alkylating agents, suggesting the presence of at least one essential cysteine residue in or near the active site. The acetyltransferase activity is greatly inhibited by its reaction product N-acetylglucosamine-1-phosphate and, interestingly, also by UDP-N-acetylmuramic acid, which is one of the first precursors specific for the peptidoglycan pathway. The detection in crude cell extracts of a phosphoglucosamine mutase activity finally confirms that the route from glucosamine-6-phosphate to UDP-N-acetylglucosamine occurs via glucosamine-1-phosphate in bacteria.

Replacement of diaminopimelic acid by cystathionine or lanthionine in the peptidoglycan of Escherichia coli

In Escherichia coli, auxotrophy for diaminopimelic acid (A2pm) can be suppressed by growth with exogenous cystathionine or lanthionine. The incorporation of cystathionine into peptidoglycan metabolism was examined with a dapA metC mutant, whereas for lanthionine, a dapA metA mutant strain was used. Analysis of peptidoglycan precursors and sacculi isolated from cells grown with epimeric cystathionine or lanthionine showed that meso-A2pm was totally replaced in the same position by either sulfur-containing amino acid. Moreover, mainly L-allo-cystathionine (95%) or meso-lanthionine (93%) was incorporated into the precursors and sacculi. For this purpose, a new, efficient high-pressure liquid chromatography (HPLC) technique for analysis of the cystathionine isomers was developed. The formation of the UDP-MurNAc tripeptide appeared to be a critical step, since the MurE synthetase accepted meso-lanthionine or D-allo- or L-allo-cystathionine in vitro as good substrates, although with higher Km values. Presumably, the 10-fold-higher UDP-MurNAc-L-Ala-D-Glu pool of cells grown with cystathionine or lanthionine ensured a normal rate of synthesis. The kinetic parameters of the MurF synthetase catalyzing the addition of D-alanyl-D-alanine were very similar for the meso-A2pm-,L-allo-cystathionine-, and meso-lanthionine-containing UDP-MurNAc tripeptides. HPLC analysis of the soluble fragments resulting from 95% digestion by Chalaropsis N-acetylmuramidase of the peptidoglycan material in isolated sacculi revealed that the proportion of the main dimer was far lower in cystathionine and lanthionine sacculi.

The glutamate racemase activity from Escherichia coli is regulated by peptidoglycan precursor UDP-N-acetylmuramoyl-L-alanine

The murI gene product of Escherichia coli was recently identified as the glutamate racemase activity which catalyzes the formation of D-glutamic acid, one of the essential components of bacterial cell-wall peptidoglycan [Doublet et al. (1993) J. Bacteriol. 175, 2970-2979]. We here describe the purification to homogeneity and the kinetic properties of this enzyme. In vitro, the glutamate racemase activity shows an absolute requirement for UDP-N-acetylmuramoyl-L-alanine (UDP-MurNAc-L-Ala), the substrate of the D-glutamic acid-adding enzyme which catalyzes the subsequent step in the pathway for peptidoglycan synthesis. The affinity of the enzyme for this activator is particularly high (KD = 4 microM) and specific, since no other peptidoglycan precursor from UDP-GlcNAc to UDP-MurNAc-pentapeptide is an effector. Minor chemical modifications of the UDP-MurNAc-L-Ala molecule, such as the reduction of the uracyl moiety, suppress its activating effect. This specific in vitro requirement most likely represents the physiological mechanism which regulates the activity of the glutamate racemase in vivo. It adjusts the formation of D-glutamic acid to the requirements of peptidoglycan synthesis and avoids an excessive racemization of the intracellular pool of L-glutamic acid.

Directed evolution of biosynthetic pathways. Recruitment of cysteine thioethers for constructing the cell wall of Escherichia coli

We report that expansion of thioether biosynthesis in Escherichia coli generates sulfur-containing amino acids that can replace meso-diaminopimelate, the essential amino acid used for cross-linking the cell wall. This was accomplished by jointly overexpressing the metB gene coding for L-cystathionine gamma-synthase and disrupting the metC gene, whose product, L-cystathionine beta-lyase, is responsible for the destruction of L-cystathionine and other L-cysteine thioethers. As a result, meso-lanthionine and L-allo-cystathionine were produced endogenously and incorporated in the peptidoglycan, thereby enabling E. coli strains auxotrophic for diaminopimelate to grow in its absence. Thus, current techniques of metabolic engineering can be applied to evolving the chemical constitution of living cells beyond its present state.

Identification of the glmU gene encoding N-acetylglucosamine-1-phosphate uridyltransferase in Escherichia coli

The physiological properties of the EcoURF-1 open reading frame, which precedes the glmS gene at 84 min on the Escherichia coli chromosome (J. E. Walker, N. J. Gay, M. Saraste, and A. N. Eberle, Biochem. J. 224:799-815, 1984), were investigated. A thermosensitive conditional mutant in which the synthesis of the gene product was impaired at 43 degrees C was constructed. The inactivation of the gene in exponentially growing cells rapidly inhibited peptidoglycan synthesis. As a result, various alterations of cell shape were observed, and cell lysis finally occurred when the peptidoglycan content was 37% lower than that of normally growing cells. Analysis of the pools of peptidoglycan precursors revealed a large accumulation of N-acetylglucosamine-1-phosphate and the concomitant depletion of the pools of the seven peptidoglycan nucleotide precursors located downstream in the pathway, a result indicating that the mutational block was in the step leading from N-acetylglucosamine-1-phosphate and UTP to the formation of UDP-N-acetylglucosamine. In vitro assays showed that the overexpression of this gene in E. coli cells, directed by appropriate plasmids, led to a high overproduction (from 25- to 410-fold) of N-acetylglucosamine-1-phosphate uridyltransferase activity. This allowed us to purify this enzyme to homogeneity in only two chromatographic steps. The gene for this enzyme, which is essential for peptidoglycan and lipopolysaccharide biosyntheses, was designated glmU.

The murI gene of Escherichia coli is an essential gene that encodes a glutamate racemase activity

The murI gene of Escherichia coli was recently identified on the basis of its ability to complement the only mutant requiring D-glutamic acid for growth that had been described to date: strain WM335 of E. coli B/r (P. Doublet, J. van Heijenoort, and D. Mengin-Lecreulx, J. Bacteriol. 174:5772-5779, 1992). We report experiments of insertional mutagenesis of the murI gene which demonstrate that this gene is essential for the biosynthesis of D-glutamic acid, one of the specific components of cell wall peptidoglycan. A special strategy was used for the construction of strains with a disrupted copy of murI, because of a limited capability of E. coli strains grown in rich medium to internalize D-glutamic acid. The murI gene product was overproduced and identified as a glutamate racemase activity. UDP-N-acetylmuramoyl-L-alanine (UDP-MurNAc-L-Ala), which is the nucleotide substrate of the D-glutamic-acid-adding enzyme (the murD gene product) catalyzing the subsequent step in the pathway for peptidoglycan synthesis, appears to be an effector of the racemase activity.

Identification of the Escherichia coli murI gene, which is required for the biosynthesis of D-glutamic acid, a specific component of bacterial peptidoglycan

The murI gene of Escherichia coli, whose inactivation results in the inability to form colonies in the absence of D-glutamic acid, was identified in the 90-min region of the chromosome. The complementation of an auxotrophic E. coli B/r strain by various DNA sources allowed us to clone a 2.5-kbp EcoRI chromosomal fragment carrying the murI gene into multicopy plasmids. The murI gene corresponds to a previously sequenced open reading frame, ORF1 (J. Brosius, T. J. Dull, D. D. Sleeter, and H. F. Noller. J. Bacteriol. 148:107-127, 1987), located between the btuB gene, encoding the vitamin B12 outer membrane receptor protein, and the rrnB operon, which contains the genes for 16S, 23S, and 5S rRNAs. The murI gene product is predicted to be a protein of 289 amino acids with a molecular weight of 31,500. Attempts to identify its enzymatic activity were unsuccessful. Cells altered in the murI gene accumulate UDP-N-acetylmuramyl-L-alanine to a high level when depleted of D-glutamic acid. Pools of precursors located downstream in the pathway are consequently depleted, and cell lysis finally occurs when the peptidoglycan content is 25% lower than that of normally growing cells.

Peptidoglycan biosynthesis in Escherichia coli: variations in the metabolism of alanine and D-alanyl-D-alanine

The in vivo functioning of the alanine/D-alanyl-D-alanine pathway of Escherichia coli was investigated by determining precursor pool levels and specific enzyme activities under various growth conditions. Cells grown on D- or L-alanine showed several remarkable features compared with cells grown on other carbon sources: 10-fold higher values of the D-alanyl-D-alanine and the UDP-MurNAc-pentapeptide pools, a 240-fold increase of the alanine racemase activity, and the absence of bacteriolysis after treatment with D-cycloserine at high concentrations (50 micrograms ml-1). In cells grown on glucose, D-cycloserine (1 micrograms ml-1) led to depletion of the D-alanyl-D-alanine pool and to lysis, which was efficiently antagonized by chloramphenicol. A threefold increase of the dipeptide pool was observed when cells were treated with chloramphenicol alone. The alanine racemase activity was lowest in glucose-grown cells and the D-alanine:D-alanine ligase and D-alanyl-D-alanine-adding activities were the same whatever the carbon source. Molecular masses of 53-56 kDa and 56-60 kDa were estimated for the partially purified inducible alanine racemase and D-alanine:D-alanine ligase respectively.

Over-production, purification and properties of the uridine diphosphate N-acetylmuramoyl-L-alanine:D-glutamate ligase from Escherichia coli

The UDP-N-acetylmuramoyl-L-alanine:D-glutamate ligase of Escherichia coli was over-produced in strains that harbour recombinant plasmids bearing the murD gene under the control of the lac or PR promoter. Purification to homogeneity was achieved by a two-step procedure from a 181-fold over-producing strain. The N-terminal sequence of the purified protein was determined and correlated with the nucleotide sequence of the murD gene. The purified activity was highly dependent on the concentration of potassium phosphate and Mg2+. The enzyme also catalysed the reverse reaction. The Km values for UDP-N-acetylmuramoyl-L-alanine; D-glutamate and ATP/Mg2+ were estimated at 7.5, 55 and 138 microM, respectively. Under the most optimal in vitro conditions determined, a turnover number of 931 min-1 was estimated. When considering the plasmid-free parental strain, the copy number of the murD gene product was not more than 1000.cell-1.

The murG gene of Escherichia coli codes for the UDP-N-acetylglucosamine: N-acetylmuramyl-(pentapeptide) pyrophosphoryl-undecaprenol N-acetylglucosamine transferase involved in the membrane steps of peptidoglycan synthesis

Physiological properties of the murG gene product of Escherichia coli were investigated. The inactivation of the murG gene rapidly inhibits peptidoglycan synthesis in exponentially growing cells. As a result, various alterations of cell shape are observed, and cell lysis finally occurs when the peptidoglycan content is 40% lower than that of normally growing cells. Analysis of the pools of peptidoglycan precursors reveals the concomitant accumulation of UDP-N-acetylglucosamine (UDP-GlcNAc) and UDP-N-acetylmuramyl-pentapeptide (UDP-MurNAc-pentapeptide) and, to a lesser extent, that of undecaprenyl-pyrophosphoryl-MurNAc-pentapeptide (lipid intermediate I), indicating that inhibition of peptidoglycan synthesis occurs after formation of the cytoplasmic precursors. The relative depletion of the second lipid intermediate, undecaprenyl-pyrophosphoryl-MurNAc-(pentapeptide)GlcNAc, shows that inactivation of the murG gene product does not prevent the formation of lipid intermediate I but inhibits the next reaction in which GlcNAc is transferred to lipid intermediate I. In vitro assays for phospho-MurNAc-pentapeptide translocase and N-acetylglucosaminyl transferase activities finally confirm the identification of the murG gene product as the transferase that catalyzes the conversion of lipid intermediate I to lipid intermediate II in the peptidoglycan synthesis pathway. Plasmids allowing for a high overproduction of the transferase and the determination of its N-terminal amino acid sequence were constructed. In cell fractionation experiments, the transferase is essentially associated with membranes when it is recovered.

Over-production, purification and properties of the uridine-diphosphate-N-acetylmuramoyl-L-alanyl-D-glutamate: meso-2,6-diaminopimelate ligase from Escherichia coli

The UDP-N-acetylmuramoyl-L-alanyl-D-glutamate:meso-2,6-diaminopimelate ligase was over-produced and purified from two plasmid-harbouring strains of Escherichia coli. The first strain, E. coli JM83(pHE5), gave a 15-fold over-production relative to parental strain. The enzyme could be partially purified (8.8-fold) by ion-exchange chromatography. With the second strain, E. coli JM83(pMLD25), a very strong over-production was obtained, since the enzyme represented about 20% of the cytoplasmic proteins. Purification yielded 77% protein homogeneity. However, the enzymatic activity, which was very unstable, was lost during the purification procedure. Several properties of the enzyme were studied. The enzyme gave maximal activity around pH 8. The isoelectric point was 5.2. The activity was increased by potassium phosphate. Reverse and exchange reactions could be catalysed. The N-terminal sequence of the protein was determined and correlated with the nucleotide sequence of the murE gene. The actual initiation codon was assigned.

Nucleotide sequence and organization of the upstream region of the Corynebacterium glutamicum lysA gene

Maximum expression of the Corynebacterium glutamicum lysA gene is dependent upon the presence of a 2.3 kb region immediately 5' of the lysA reading frame. Subcloning and functional analysis of the upstream region implied that this region contained the lysA promoter. Sequence determination of the upstream region revealed a single open reading frame, orfX, in the same orientation as lysA. The orfX coding sequence exhibited all the sequence characteristics of a gene with the potential for a 550-amino-acid polypeptide product. Expression of lysA is coupled to that of orfX via a common promoter located immediately 5' of orfX. The RNA start site has been determined by S1 nuclease mapping. Both the orfX and the lysA gene are expressed as a single 3.0 kb RNA transcript. These data indicate that orfX and lysA are genes within a two-gene operon. Expression of the lysA gene is not subject to regulation by lysine. The orfX gene product was shown not to be directly linked to the lysine biosynthetic pathway, nor is it the enzyme incorporating DAP into the peptidoglycan precursor.

Inhibition of peptidoglycan biosynthesis in Bacillus megaterium by daptomycin

The effects of daptomycin on exponential phase cells of Bacillus megaterium were investigated. Bacteriostasis was observed for concentrations between 1 and 3 micrograms/ml and maximal rate of cell lysis at 10 micrograms/ml. At sublytic concentrations (1.5-3 micrograms/ml), the variations of the pools of UDP-N-acetylglucosamine and UDP-N-acetylmuramyl-pentapeptide, as well as the incorporation of (14C)-N-acetylglucosamine into peptidoglycan were studied. From the results it was concluded that the lethal target of daptomycin could be a metabolic step between glucosamine 6-phosphate and UDP-N-acetylglucosamine.

Correlation between the effects of fosfomycin and chloramphenicol on Escherichia coli

It was speculated that the increase of the UDP-GlcNAc pool observed with chloramphenicol can modulate the residual PEP:UDP-GlcNAc-enolpyruvate activity of fosfomycin-treated cells. This provided an explanation on how chloramphenicol can insure the formation of enough UDP-MurNAc-pentapeptide to sustain peptidoglycan synthesis at a rate that will antagonize fosfomycin-induced lysis.

Organization of the murE-murG region of Escherichia coli: identification of the murD gene encoding the D-glutamic-acid-adding enzyme

The 2-min region of the Escherichia coli genome contains a large cluster of genes from pbpB to envA that code for proteins involved in peptidoglycan biosynthesis and cell division. From pLC26-6 of the collection of Clarke and Carbon (L. Clarke and J. Carbon, Cell 9:91-99, 1976) plasmids carrying different fragments from the 8-kilobase-pair region downstream of pbpB were constructed and analyzed for their ability to direct protein synthesis in maxicells, to complement various thermosensitive mutations, and to overproduce enzymatic activities. We report the localization of the previously unidentified murD gene coding for the D-glutamic acid-adding enzyme within this region. Our data show that the genes are in the order pbpB-murE-murF-X-murD-Y-murG, where X and Y represent chromosomal fragments from 1 to 1.5 kilobase pairs, possibly coding for unknown proteins. Furthermore, the murE and murF genes, encoding the meso-diaminopimelic acid and D-alanyl-D-alanine-adding enzymes, respectively, may be translationally coupled when transcription is initiated upstream of murE, within the preceding structural gene pbpB coding for penicillin-binding protein 3.

Variations in UDP-N-acetylglucosamine and UDP-N-acetylmuramyl-pentapeptide pools in Escherichia coli after inhibition of protein synthesis

The pool levels of the nucleotide precursors of peptidoglycan were analyzed after inhibition of protein synthesis in various Escherichia coli strains. In all cases UDP-N-acetylglucosamine (UDP-GlcNAc) and UDP-N-acetylmuramyl-pentapeptide (UDP-MurNAc-pentapeptide) cell pools increased upon treatment with chloramphenicol or tetracycline. Similar results were observed after the treatment of K-12 strains with valine. Since the intermediate nucleotide precursors did not accumulate after the arrest of protein synthesis and since a feedback mechanism was unlikely, the increases of the UDP-MurNAc-pentapeptide pool appeared as a consequence of that of the UDP-GlcNAc pool by the unrestricted functioning of the intermediate steps of the pathway. The highest increase (sixfold) of UDP-GlcNAc was observed with strain K-12 HfrH growing in minimal medium and treated with chloramphenicol. When a pair of isogenic Rel+ and Rel- strains were considered, both the UDP-GlcNAc and UDP-MurNAc-pentapeptide pools increased upon treatment with chloramphenicol or valine. However, the UDP-GlcNAc pool of the Rel+ strain was at a high natural level, which increased only moderately (20%) after the addition of valine. The increase of the UDP-GlcNAc pool after the various treatments could be due to an effect on some upstream step by an unknown mechanism. The possible correlations of the variations of the precursor pools with the rate of synthesis and extent of cross-linking of peptidoglycan were also considered.

Incorporation of LL-diaminopimelic acid into peptidoglycan of Escherichia coli mutants lacking diaminopimelate epimerase encoded by dapF

Recently a dapF mutant of Escherichia coli lacking the diaminopimelate epimerase was found to have an unusual large LL-diaminopimelic acid (LL-DAP) pool as compared with that of meso-DAP (C. Richaud, W. Higgins, D. Mengin-Lecreulx, and P. Stragier, J. Bacteriol. 169:1454-1459, 1987). In this report, the consequences of high cellular LL-DAP/meso-DAP ratios on the structure and metabolism of peptidoglycan were investigated. For this purpose new efficient high-pressure liquid chromatography techniques for the separation of the DAP isomers were developed. Sacculi from dapF mutants contained a high proportion of LL-DAP that varied greatly with growth conditions. The same was observed with the two DAP-containing precursors, UDP-N-acetylmuramyl-tripeptide and UDP-N-acetylmuramyl-pentapeptide. The limiting steps for the incorporation of LL-DAP into peptidoglycan were found to be its addition to UDP-N-acetylmuramyl-L-alanyl-D-glutamate and the formation of the D-alanyl-DAP cross-bridges. The Km value of the DAP-adding enzyme for LL-DAP was 3.6 x 10(-2) M as compared with 1.1 x 10(-5) M for meso-DAP. When isolated sacculi were treated with Chalaropsis N-acetylmuramidase and the resulting soluble products were analyzed by high-pressure liquid chromatography, the proportion of the main peptidoglycan dimer was lower in the dapF mutant than in the parental strain. Moreover, the proportion of LL-DAP was higher in the main monomer than in the main dimer, where it was almost exclusively located in the donor unit. There are thus very few D-alanyl-LL-DAP cross-bridges, if any. We also observed that large amounts of LL-DAP and N-succinyl-LL-DAP were excreted in the growth medium by the dapF mutant.

Molecular cloning, characterization, and chromosomal localization of dapF, the Escherichia coli gene for diaminopimelate epimerase

The Escherichia coli dapF gene was isolated from a cosmid library as a result of screening for clones overproducing diaminopimelate epimerase. Insertional mutagenesis was performed on the cloned dapF gene with a mini-Mu transposon, leading to chloramphenicol resistance. One of these insertions was transferred onto the chromosome by a double-recombination event, allowing us to obtain a dapF mutant. This mutant accumulated large amounts of LL-diaminopimelate, confirming the blockage in the step catalyzed by the dapF product, but did not require meso-diaminopimelate for growth. The dapF gene was localized in the 85-min region of the E. coli chromosome between cya and uvrD.

Effect of growth conditions on peptidoglycan content and cytoplasmic steps of its biosynthesis in Escherichia coli

In an attempt to bring some insight into how peptidoglycan synthesis is controlled in Escherichia coli, simple parameters, such as cell peptidoglycan content, the pool levels of its seven uridine nucleotide precursors, and the specific activities of five enzymes involved in their formation, were investigated under different growth conditions. When exponential-phase cells with generation times ranging from 25 to 190 min were examined, the peptidoglycan content apparently varied as the cell surface area changed, and no important variations in the pool levels of the nucleotide precursors or in the specific activities of the five enzymes considered were observed. The peptidoglycan of exponential-phase cells accounted for 0.7 to 0.8% of the dry cell weight, whereas that of stationary-phase cells accounted for 1.4 to 1.9%. Depending on the growth conditions, the number of peptidoglycan disaccharide peptide units per cell varied from 2.4 X 10(6) to 5.6 X 10(6). The levels of the nucleotide precursor pools as well as the specific activities of the D-glutamic acid- and D-alanyl-D-alanine-adding enzymes varied little with the growth phase. The specific activities of UDP-N-acetylglucosamine transferase, UDP-N-acetylglucosamine-enolpyruvate reductase, and the diaminopimelic acid-adding enzymes decreased by 20 to 50% at most in the late stationary phase. The results are discussed in terms of the possible importance for cell survival of the maintenance of a high capacity for peptidoglycan synthesis, whatever its rate under various growth conditions, and of a balance between the synthesis and breakdown of peptidoglycan during active growth.

Pool levels of UDP N-acetylglucosamine and UDP N-acetylglucosamine-enolpyruvate in Escherichia coli and correlation with peptidoglycan synthesis

A high-pressure liquid chromatography procedure was developed for the isolation and quantitation of UDP-N-acetylglucosamine, UDP-N-acetylglucosamine-enolpyruvate, and UDP-N-acetylmuramic acid, which are the early cytoplasmic precursors of bacterial peptidoglycan. In exponential-phase cells of Escherichia coli K-12, the intracellular concentration of UDP-N-acetylglucosamine was about 100 microM, whereas that of UDP-N-acetylglucosamine-enolpyruvate was only 2 microM. The phosphoenolpyruvate: UDP-N-acetylglucosamine transferase and UDP-N-acetylglucosamine-enolpyruvate reductase activities were investigated in extracts from E. coli. These activities appeared to be present in amounts sufficient for the ongoing rate of peptidoglycan synthesis. Certain uridine nucleotide peptidoglycan precursors were found to inhibit phosphoenolpyruvate: UDP-N-acetylglucosamine transferase activity.