An Alternative Route for Recycling of N -Acetylglucosamine from Peptidoglycan Involves the N -Acetylglucosamine Phosphotransferase System in Escherichia coli

Abstract

A set of enzymes dedicated to recycling of the amino sugar components of peptidoglycan has previously been identified in Escherichia coli. The complete pathway includes the nagA-encoded enzyme, N-acetylglucosamine-6-phosphate (GlcNAc6P) deacetylase, of the catabolic pathway for use of N-acetylglucosamine (GlcNAc). Mutations in nagA result in accumulation of millimolar concentrations of GlcNAc6P, presumably by preventing peptidoglycan recycling. Mutations in the genes encoding the key enzymes upstream of nagA in the dedicated recycling pathway (ampG, nagZ, nagK, murQ, and anmK), which were expected to interrupt the recycling process, reduced but did not eliminate accumulation of GlcNAc6P. A mutation in the nagE gene of the GlcNAc phosphotransferase system (PTS) was found to reduce by 50% the amount of GlcNAc6P which accumulated in a nagA strain and, together with mutations in the dedicated recycling pathway, eliminated all the GlcNAc6P accumulation. This shows that the nagE-encoded PTS transporter makes an important contribution to the recycling of peptidoglycan. The manXYZ-encoded PTS transporter makes a minor contribution to the formation of cytoplasmic GlcNAc6P but appears to have a more important role in secretion of GlcNAc and/or GlcNAc6P from the cytoplasm. Peptidoglycan (PG) or murein, the rigid shape-forming layer of the bacterial cell envelope, undergoes extensive degradation and resynthesis during normal bacterial growth. It is estimated that 40 to 50% of the PG is broken down and reused each generation (for a review, see reference 22). PG is a matrix of chains of alternating N-acetylglucosamine (GlcNAc) and Nacetylmuramic acid (MurNAc) sugars cross-linked by peptide bridges. Over the last 20 years the pathways for recycling both the peptide and amino sugar portions of the PG have been elucidated, and a number of genes involved in this process have been identified. Most of the genes involved encode dedicated enzymes whose only function seems to be to recover the material produced during PG turnover and to reuse it to synthesize more PG or as a source of energy. However, some of the enzymes shown to be involved have apparently been recruited from another metabolic pathway (e.g., murQ-and nagA-encoded enzymes [see below]), while other specialized PG-recycling enzymes have a subsidiary function (e.g., ampG-and ampD-encoded enzymes in ␤-lactamase induction [20]).The pathway for recycling the amino sugar part of PG in Escherichia coli is shown in Fig.

Cell-wall recycling and synthesis in Escherichia coli and Pseudomonas aeruginosa – their role in the development of resistance

“…The NagE phosphotransferase phosphorylates and imports GlcNAc into the cytoplasm, adding to the cytoplasmic pool of GlcNAc-6-P [154]. In E. coli, the absence of nagE resulted in a decrease of 50 % of GlcNAc-6-P in the cytoplasm [154].…”

“…An alternative route of transfer of GlcNAc into the cytoplasm using NagE exists for both E. coli and P. aeruginosa [154]. The NagE phosphotransferase phosphorylates and imports GlcNAc into the cytoplasm, adding to the cytoplasmic pool of GlcNAc-6-P [154].…”

Peptidoglycan Hydrolases of Escherichia coli

“…The pathways of this recycling have been worked out and reviewed in detail (196,204). Most enzymes of the process were characterized, and the effect of their deletion was studied.…”

“…The tripeptide is ligated into UDP-MurNAc by the Mpl ligase, and the resulting tripeptide nucleotide precursor returns to the biosynthesis pathway (96,176). GlcNAc and 1,6-anhydro-MurNAc are both converted into GlcNAc-6-phosphate, which is available for reuse (119,196,204). The kinase NagK produces GlcNAc-6-P directly from GlcNAc, whereas the conversion of 1,6-anhydro-MurNAc is a two-step process.…”

Allosteric Regulation of Glucosamine-6-Phosphate Deaminase (NagB) and Growth ofEscherichia colion Glucosamine

“…A mutation in the mlc gene, which controls the manXYZ-encoded transporter, increases transport of GlcN two-to threefold (1,19,22) and was found to improve the growth rate for all the strains carrying the different plasmid constructs. Mutations which eliminate the nagA-encoded GlcNAc6P deacetylase produce high levels of the allosteric activator GlcNAc6P (1,25,33) derived from the recycling of PG (15,17) but had no effect on the growth rate. These experiments show that it is the supply of substrate that is limiting the growth rate on GlcN and not the absence of the allosteric activator.…”