AmpD is required for regulation of expression of NmcA, a carbapenem-hydrolyzing beta-lactamase of Enterobacter cloacae

To further elucidate the induction process of the carbapenem-hydrolyzing be ta -lactamase of Ambler class A, NmcA, ampD genes of the wild-type (WT) str ain and of ceftazidime-resistant mutants of Enterobacter cloacae NOR-1 were cloned and tested in transcomplementation experiments. Ceftazidime-resista nt E. cloacae NOR-1 mutants exhibited derepressed expression of the AmpC-ty pe cephalosporinase and of the carbapenem-hydrolyzing beta -lactamase NmcA. The ampD genes of Escherichia coli and E. cloacae WT NOR-1 transcomplement ed the ceftazidime-resistant E. cloacae NOR-1 mutants to the WT level of be ta -lactamase expression, while the mutated ampD alleles of E. cloacae NOR- 1 failed to do so. The deduced E. cloacae NOR-1 WT AmpD protein exhibited 9 5 and 91% amino acid identity with the E. cloacae O29 and E. cloacae 14 WT AmpD proteins, respectively. Of the 12 ceftazidime-resistant E. cloacae NOR -1 strains, 3 had AmpD proteins with amino acid changes, while the others h ad truncated AmpD proteins. Most of these mutations were located outside th e conserved regions that link the AmpD proteins to the cell wall hydrolases . AmpD from E. cloacae NOR-1 is involved in the regulation of expression of both beta -lactamases (NmcA and AmpC), suggesting that structurally unrela ted genes may be under the control of an identical genetic system.