Chemical mutagenesis of the AmpC -lactamase-hyperinducible strain SN0301/pNu305 carrying the cloned and genes from OS60 gave four independent mutants in which -lactamase was no longer inducible, or was inducible only to a low level, by -lactam antibiotics. The genes and , which were essential for -lactamase induction, were functional in these mutants. In all four mutants, the sites of mutation were mapped to 9.9 min on the chromosome. Complementation with wild-type restored inducibility of -lactamase to wild-type levels. The nucleotide sequence of all four mutant alleles (ampG1, ampG3, ampG4 and ) was determined. In three of the mutants, a single base exchange led to an amino acid change from glycine to aspartate at different sites in the deduced amino acid sequence. In the fourth mutant (), with low-level inducibility, the nucleotide sequence was identical to wild-type . Spontaneous back-mutation of the chromosomal mutant resulted in restoration of wild-type inducibility and a return to the wild-type sequence. Unspecific induction by components of the growth medium was also dependent on intact function.


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