Sequences of wild-type and mutant ampD genes of Citrobacter freundii and Enterobacter cloacae

The ampD gene product regulates the expression of AmpC beta-lactamase in gram-negative bacteria and is proposed to be involved in peptidoglycan metabolism. In this study, we sequenced the ampD wild type and three mutant genes of Enterobacter cloacae and Citrobacter freundii. They exhibited a high degree of homology with the corresponding gene of Escherichia coli except in the carboxy termini, where, in the wild-type genes of E. cloacae and C. freundii, four additional amino acids yielding the Ser-X-X-Lys motif were found. Evidence that this C-terminal region of the ampD gene product is necessary for activity was shown by constructing a deletion of the last 16 amino acids. The spontaneous mutation of ampD02 is an out-of-frame insertion and yields an inactive AmpD protein. The single-base-pair substitution of Gly for Asp-121 in ampD05 is responsible for a hyperinducible phenotype. These results demonstrate regions of the ampD gene and the corresponding protein which have functional importance for the induction of AmpC beta-lactamase in E. cloacae.