AmpN-AmpG Operon Is Essential for Expression of L1 and L2

AmpG is an inner membrane permease which transports products of murein sacculus degradation from the periplasm into the cytosol in Gram-negative bacteria. This process is linked to induction of the chromosomal ampC beta-lactamase gene in some members of the Enterobacteriaceae and in Pseudomonas aeruginosa. In this study, the ampG homologue of Stenotrophomonas maltophilia KJ was analyzed. The ampG homologue and its upstream ampN gene form an operon and are cotranscribed under the control of the promoter PampN. Expression from PampN was found to be independent of -lactam exposure and ampN and ampG products. A ampN allele exerted a polar effect on the expression of ampG and resulted in a phenotype of null -lactamase inducibility. Complementation assays elucidated that an intact ampN-ampG operon is essential for -lacta-mase induction. Consistent with ampG of Escherichia coli, the ampN-ampG operon of S. maltophilia did not exhibit a gene dosage effect on -lactamase expression. The AmpG permease of E. coli could complement the -lactamase inducibility of ampN or ampGmutants of S. maltophilia, indicating that both species have the same precursor of activator ligand(s) for -lactamase induction. Many Gram-negative bacteria have a chromosomally en-coded ampR--lactamase module and display an inducible -lactamase activity, including Citrobacter freundii (20), En