title: Escherichia coli K-12 mutants hyperproducing chromosomal beta-lactamase by gene repetitions.
abstract: Escherichia coli K-12 ampicillin-resistant mutants hyperproducing chromosomal beta-lactamase arose spontaneously from strains carrying ampA1 ampC(+). Such mutants were found even in a recA background. Two Amp(r)-100 strains were analyzed genetically. The Amp(r)-100 resistance level of both strains could be transduced by direct selection for ampicillin resistance. Several classes of ampicillin-resistant transductants were found that differed from one another in the beta-lactamase activity and the ampicillin resistance mediated by an ampA1 ampC(+)-carrying strain. The data suggested that beta-lactamase hyperproduction was due to repetitions of the chromosomal amp genes. The size of the repeated region was calculated from cotransduction estimates, using the formula of Wu (Genetics 54:405-410, 1966), and was found to be about 1 min in one strain and 1.5 min in the other. Second-step Amp(r)-400 mutants were isolated from an Amp(r)-100 strain. The resistance of these mutants was apparently also due to repetitions, each mediating a resistance to about 10 mug/ml. Mutants of wild-type strains that were moderately resistant to ampicillin also gave rise to intermediate-resistance classes, suggesting repetitions of the wild-type amp alleles. F' factors hyperproducing chromosomal beta-lactamase by gene repetitions were constructed. They mediated levels of ampicillin resistance comparable to that of naturally occurring resistance plasmids. The expression of beta-lactamase hyperproduction was not affected by the presence of ampA and ampC alleles in trans and did not act in trans on the other alleles.
mesh_terms: D000483:Alleles; D000667:Ampicillin / Q000494:pharmacology; D002874:Chromosome Mapping; D002876:Chromosomes, Bacterial; D003227:Conjugation, Genetic; D004926:Escherichia coli / Q000187:drug effects / Q000201:enzymology / Q000235:genetics*; D005144:F Factor; D005796:Genes; D010403:Penicillin Resistance; D010405:Penicillinase / Q000096:biosynthesis*; D014161:Transduction, Genetic
chemical_list: D000667:Ampicillin; D010405:Penicillinase