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Inducible expression of chromosomal AmpC β-lactamase is a major cause of β-lactam antibiotic resistance in the Gram-negative bacteria Pseudomonas aeruginosa and Enterobacteriaceae. AmpC expression is induced by the LysR-type transcriptional regulator (LTTR) AmpR, which activates ampC expression in response to changes in peptidoglycan (PG) metabolite levels that occur during exposure to β-lactams. Under normal conditions, AmpR represses ampC transcription by binding the PG precursor UDP-N-acetylmuramic acid (MurNAc)-pentapeptide. When exposed to β-lactams, however, PG catabolites (1,6-anhydroMurNAc-peptides) accumulate in the cytosol, which have been proposed to competitively displace UDP-MurNAc-pentapeptide from AmpR and convert it into an activator of ampC transcription. Here we describe the molecular interactions between AmpR (from Citrobacter freundii), its DNA operator, and repressor UDP-MurNAc-pentapeptide. Non-denaturing mass spectrometry revealed AmpR to be a homotetramer that is stabilized by DNA containing the T-N11-A LTTR binding motif and revealed that it can bind four repressor molecules in an apparently stepwise manner. A crystal structure of the AmpR effector-binding domain bound to UDP-MurNAc-pentapeptide revealed that the terminal D-Ala-D-Ala motif of the repressor forms the primary contacts with the protein. This observation suggests that 1,6-anhydroMurNAc-pentapeptide may convert AmpR into an activator of ampC transcription more effectively than 1,6-anhydroMurNAc-tripeptide (which lacks the D-Ala-D-Ala motif). Finally, small angle x-ray scattering demonstrates that the AmpR·DNA complex adopts a flat conformation similar to the LTTR protein AphB and undergoes only a slight conformational change when binding UDP-MurNAc-pentapeptide. Modeling the AmpR·DNA tetramer bound to UDP-MurNAc-pentapeptide predicts that the UDP-MurNAc moiety of the repressor participates in modulating AmpR function.

Constitutive AmpC hyperproduction is the most frequent mechanism of resistance to the weak AmpC inducers antipseudomonal penicillins and cephalosporins. Previously, we demonstrated that inhibition of the β- N -acetylglucosaminidase NagZ prevents and reverts this mechanism of resistance, which is caused by ampD and/or dacB (PBP4) mutations in Pseudomonas aeruginosa . In this work, we compared NagZ with a second candidate target, the AmpG permease for GlcNAc-1,6-anhydromuropeptides, for their ability to block AmpC expression pathways. Inactivation of nagZ or ampG fully restored the susceptibility and basal ampC expression of ampD or dacB laboratory mutants and impaired the emergence of one-step ceftazidime-resistant mutants in population analysis experiments. Nevertheless, only ampG inactivation fully blocked ampC induction, reducing the MICs of the potent AmpC inducer imipenem from 2 to 0.38 μg/ml. Moreover, through population analysis and characterization of laboratory mutants, we showed that ampG inactivation minimized the impact on resistance of the carbapenem porin OprD, reducing the MIC of imipenem for a PAO1 OprD mutant from >32 to 0.5 μg/ml. AmpG and NagZ targets were additionally evaluated in three clinical isolates that are pan-β-lactam resistant due to AmpC hyperproduction, OprD inactivation, and overexpression of several efflux pumps. A marked increase in susceptibility to ceftazidime and piperacillin-tazobactam was observed in both cases, while only ampG inactivation fully restored wild-type imipenem susceptibility. Susceptibility to meropenem, cefepime, and aztreonam was also enhanced, although to a lower extent due to the high impact of efflux pumps on the activity of these antibiotics. Thus, our results suggest that development of small-molecule inhibitors of AmpG could provide an excellent strategy to overcome the relevant mechanisms of resistance (OprD inactivation plus AmpC induction) to imipenem, the only currently available β-lactam not significantly affected by P. aeruginosa major efflux pumps.

AmpC hyperproduction is the most frequent mechanism of resistance to penicillins and cephalosporins in Pseudomonas aeruginosa and is driven by ampD mutations or the recently described inactivation of dacB , which encodes the nonessential penicillin-binding protein (PBP) PBP 4. Recent work showed that nagZ inactivation attenuates β-lactam resistance in ampD mutants. Here we explored whether the same could be true for the dacB mutants with dacB mutations alone or in combination with ampD mutations. The inactivation of nagZ restored the wild-type β-lactam MICs and ampC expression of PAO1 dacB and ampD mutants and dramatically reduced the MICs (for example, the MIC for ceftazidime dropped from 96 to 4 μg/ml) and the level of ampC expression (from ca. 1,000-fold to ca. 50-fold higher than that for PAO1) in the dacB - ampD double mutant. On the other hand, nagZ inactivation had little effect on the inducibility of AmpC. The NagZ inhibitor O -(2-acetamido-2-deoxy- d -glucopyranosylidene)amino- N -phenylcarbamate attenuated the β-lactam resistance of the AmpC-hyperproducing strains, showing a greater effect on the dacB mutant (reducing the ceftazidime MICs from 24 to 6 μg/ml) than the ampD mutant (reducing the MICs from 8 to 4 μg/ml). Additionally, nagZ inactivation in the dacB mutant blocked the overexpression of creD ( blrD ), which is a marker of the activation of the CreBC (BlrAB) regulator involved in the resistance phenotype. Finally, through population analysis, we show that the inactivation of nagZ dramatically reduces the capacity of P. aeruginosa to develop ceftazidime resistance, since spontaneous mutants were not obtained at concentrations ≥8 μg/ml (the susceptibility breakpoint) for the nagZ mutant but were obtained with wild-type PAO1. Therefore, NagZ is envisaged to be a candidate target for preventing and reverting β-lactam resistance in P. aeruginosa .

Hyperproduction of AmpC beta-lactamase (AmpC) is a formidable mechanism of resistance to penicillins and cephalosporins in Gram-negative bacteria such as Pseudomonas aeruginosa and Enterobacteriaceae. AmpC expression is regulated by the LysR-type transcriptional regulator AmpR. ampR and ampC genes form a divergent operon with overlapping promoters to which AmpR binds and regulates the transcription of both genes. AmpR induces ampC by binding to one member of the family of 1,6-anhydro-N-acetylmuramyl peptides, which are cytosolic catabolites of peptidoglycan that accumulate during beta-lactam challenge. To gain structural insights into AmpR regulation, we determined the crystal structure of the effector binding domain (EBD) of AmpR from Citrobacter freundii up to 1.83 A resolution. The AmpR EBD is dimeric and each monomer comprises two subdomains that adopt alpha/beta Rossmann-like folds. Located between the monomer subdomains is a pocket that was found to bind the crystallization buffer molecule 2-(N-morpholino)ethanesulfonic acid. The pocket, together with a groove along the surface of subdomain I, forms a putative effector binding site into which a molecule of 1,6-anhydro-N-acetylmuramyl pentapeptide could be modeled. Amino acid substitutions at the base of the interdomain pocket either were found to render AmpR incapable of inducing ampC (Thr103Val, Ser221Ala and Tyr264Phe) or resulted in constitutive ampC expression (Gly102Glu). While the substitutions that prevented ampC induction did not alter the overall AmpR EBD structure, circular dichroism spectroscopy revealed that the nonconservative Gly102Glu mutation affected EBD secondary structure, confirming previous work suggesting that Gly102Glu induces a conformational change to result in constitutive AmpC production.