title: Physical mapping and expression of hybrid plasmids carrying chromosomal beta-lactamase genes of Escherichia coli K-12.
abstract: Hybrid plasmids carrying the ampC gene of Escherichia coli K-12 that codes for the chromosomal beta-lactamase were physically studied. The ampC gene was mapped to a deoxyribonucleic acid segment encompassing 1,370 base pairs. The mapping was facilitated by the isolation of a plasmid carrying an insertion of the transposable element gamma delta (gamma delta) close to ampC. The ampA1 mutation, which increases the expression of ampC by a factor of about 20, was localized to a 370-base pair segment of the 1,370-base pair deoxyribonucleic acid segment that contains the ampC gene. Using a minicell protein labeling system, it was seen that plasmids carrying either ampA+, ampC, or ampA1 and ampC coded for a 36,000-dalton protein which comigrated with purified chromosomal beta-lactamase. In cells carrying plasmids that bore the ampA1 allele, the production of this protein was greater. In addition, a protein with a slightly higher molecular weight (38,000) was expressed by both ampA+ ampC and  ampA1 ampC plasmids in this protein labeling system. This protein might represent a precursor form of chromosomal beta-lactamasee. From E. coli K-12 strains carrying the ampA1 allele, second-step mutants were isolated that hyperproduced chromosomal beta-lactamase. By reciprocal recombination, plasmid derivatives were isolated that carried these mutations. Two second-step regulatory mutations mapped within the same 370-base pair region as ampA1. This piece of deoxyribonucleic acid therefore contains ampA, a control sequence region for ampC.
mesh_terms: D003001:Cloning, Molecular; D004251:DNA Transposable Elements; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics*; D005786:Gene Expression Regulation; D005796:Genes; D005809:Genes, Regulator; D009154:Mutation; D010405:Penicillinase / Q000096:biosynthesis / Q000235:genetics*; D010957:Plasmids; D001618:beta-Lactamases / Q000235:genetics*
chemical_list: D004251:DNA Transposable Elements; D010405:Penicillinase; D001618:beta-Lactamases