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

Abstract

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.

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

“…The first indication that this gene may play a critical role in cell-wall recycling stemmed from observations of its influence on blactamase induction [165]. Its specific role as an amidase in recycling was confirmed when it was noticed that the loss of ampD resulted in the accumulation of GlcNAc-anhMurNAc with peptide chains [163,164]. Subsequently, a major increase of anhydromuramyl tripeptides in ampD-deficient E. coli cells led to the identification of an LD-carboxypeptidase LdcA [166].…”

“…AmpD and LdcA E. coli AmpD cleaves the peptide chain exclusively attached to the 1,6-anhydromuramyl moieties [163,164]. The first indication that this gene may play a critical role in cell-wall recycling stemmed from observations of its influence on blactamase induction [165].…”

“…The absence of ampD leads to the accumulation of anhMurNAc-tripeptide in the cytosol and constitutive overproduction of b-lactamase even in the absence of induction, resulting in a high resistance to b-lactams [163]. The most common mechanism for constitutive ampC overexpression in clinical strains leading to b-lactam resistance in Enterobacteriaceae as well as P. aeruginosa is to mutate ampD [200][201][202][203] (Table 3).…”

The β-Lactamase Gene Regulator AmpR Is a Tetramer That Recognizes and Binds the d-Ala-d-Ala Motif of Its Repressor UDP-N-acetylmuramic Acid (MurNAc)-pentapeptide

“…In the cytosol, the nonreducing GlcNAc residue is removed from the fragments by the glycosidase NagZ (11)(12)(13) to yield a series of 1,6-anhydroMurNAc-tripeptides, -tetrapeptides, and -pentapeptides. The amidase AmpD cleaves the amide bond linking the remaining sugar, 1,6-anhydroMurNAc, to the peptide stems (14,15), and the resulting pool of monosaccharide and peptide catabolites are subsequently used to build UDP-MurNAc-pentapeptide, an essential anabolite of cell wall synthesis (5). In the absence of ␤-lactams, UDP-MurNAc-pentapeptide binds to AmpR and causes the repression of ampC transcription (16).…”

DHA-1 plasmid-mediated AmpC β-lactamase expression and regulation of Klebsiella pnuemoniae isolates

“…Although the mechanism of AmpC enzyme synthesis remains unclear, this enzyme is possibly regulated by an Amp composite operon, which is present in numerous Gram-negative bacteria containing AmpC as a structural gene, as well as AmpC, AmpR, AmpG, AmpD and AmpE as regulatory genes. The AmpC gene codes for the AmpC enzyme, which is then regulated by AmpR, AmpG, AmpD and AmpE (6)(7)(8)(9). Although the AmpC enzyme may be translated from the AmpC gene, this enzyme has numerous mutations (10).…”