ampG is essential for high-level expression of AmpC beta-lactamase in Enterobacter cloacae.
Mutants of Enterobacter cloacae 55 were studied to delineate more completely the genetics of inducible expression of AmpC beta-lactamase. E. cloacae 55M-L, derived by mutagenesis from a mutant with high-level cefotaxime resistance (MIC, greater than 64 micrograms/ml), E. cloacae 55M, demonstrated a novel phenotype by producing only low levels of AmpC constitutively. Neither the parental phenotype of E. cloacae 55M nor the wild-type phenotype of E. cloacae 55 could be restored in E. cloacae 55M-L by the introduction of functional ampR, ampC, or ampD genes. Cloning each of these genes from E. cloacae 55M-L confirmed the same genotype for this mutant as for its parental strain. Mutation of E. cloacae 55M-L to the E. cloacae 55M phenotype was found to occur spontaneously at a frequency of 10(-8). All such revertants demonstrated an inducible wild-type phenotype after introduction of a functional ampD. These results suggested that the E. cloacae 55M-L phenotype was due to a mutation in an as yet unrecognized gene, designated ampG. Verification of this gene was obtained by the restoration of the E. cloacae 55M phenotype in E. cloacae 55M-L by introduction of a cloned 2.9-kilobase BamHI fragment from the E. cloacae 55 chromosome. Transformation of both ampG and ampD into E. cloacae 55M-L reconstituted the inducible wild-type phenotype. These results indicate that ampG is required for the activation of ampC by AmpR. Without ampG, neither induction nor high-level expression of AmpC is possible. It is likely that the ampG gene product and AmpD together modulate the ability of AmpR to activate ampC expression.
Korfmann
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Department of Medical Microbiology, Creighton University School of Medicine, Omaha, Nebraska 68178.
Sanders
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D000667:Ampicillin / Q000494:pharmacology*; D003001:Cloning, Molecular; D004269:DNA, Bacterial / Q000235:genetics; D004754:Enterobacter / Q000201:enzymology / Q000235:genetics*; D004755:Enterobacteriaceae / Q000235:genetics*; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics; D015964:Gene Expression Regulation, Bacterial; D015971:Gene Expression Regulation, Enzymologic; D005798:Genes, Bacterial; D005838:Genotype; D009693:Nucleic Acid Hybridization; D010403:Penicillin Resistance; D010641:Phenotype; D010957:Plasmids; D001618:beta-Lactamases / Q000096:biosynthesis / Q000235:genetics*
D004269:DNA, Bacterial; D000667:Ampicillin; D001618:beta-Lactamases