AmpD Is Required for Regulation of Expression of NmcA, a Carbapenem-Hydrolyzing β-Lactamase ofEnterobacter cloacae

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Abstract
To further elucidate the induction process of the carbapenem-hydrolyzing β-lactamase of Ambler class A, NmcA,ampDgenes of the wild-type (WT) strain and of ceftazidime-resistant mutants ofEnterobacter cloacaeNOR-1 were cloned and tested in transcomplementation experiments. Ceftazidime-resistantE.cloacaeNOR-1 mutants exhibited derepressed expression of the AmpC-type cephalosporinase and of the carbapenem-hydrolyzing β-lactamase NmcA. TheampDgenes ofEscherichia coliandE.cloacaeWT NOR-1 transcomplemented the ceftazidime-resistantE.cloacaeNOR-1 mutants to the WT level of β-lactamase expression, while the mutatedampDalleles ofE.cloacaeNOR-1 failed to do so. The deducedE.cloacaeNOR-1 WT AmpD protein exhibited 95 and 91% amino acid identity with theE.cloacaeO29 andE.cloacae14 WT AmpD proteins, respectively. Of the 12 ceftazidime-resistantE.cloacaeNOR-1 strains, 3 had AmpD proteins with amino acid changes, while the others had truncated AmpD proteins. Most of these mutations were located outside the conserved regions that link the AmpD proteins to the cell wall hydrolases. AmpD fromE.cloacaeNOR-1 is involved in the regulation of expression of both β-lactamases (NmcA and AmpC), suggesting that structurally unrelated genes may be under the control of an identical genetic system.

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