The signal transducer encoded by ampG is essential for induction of chromosomal AmpC -lactamase in Escherichia coli by -lactam antibiotics and unspecific inducers
Chemical mutagenesis of the AmpC -lactamase-hyperinducible Escherichia coli strain SN0301/pNu305 carrying the cloned ampC and ampR genes from Citrobacter freundii 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 ampC, ampR, ampD and ampE, 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 E. coli chromosome. Complementation with wild-type ampG restored inducibility of -lactamase to wild-type levels. The nucleotide sequence of all four mutant ampG alleles (ampG1, ampG3, ampG4 and ampG5) 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 (ampG4), with low-level inducibility, the nucleotide sequence was identical to wild-type ampG. Spontaneous back-mutation of the chromosomal ampG1 mutant resulted in restoration of wild-type inducibility and a return to the wild-type ampG sequence. Unspecific induction by components of the growth medium was also dependent on intact ampG function.
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The signal transducer encoded by ampG is essential for induction of chromosomal AmpC -lactamase in Escherichia coli by -lactam antibiotics and unspecific inducers