Physical characterization of plasmids determining synthesis of a microcin which inhibits methionine synthesis in Escherichia coli

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Abstract
Plasmid DNA isolated from each of 3 antibiotic-resistant clinical strains of E. coli producing the same microcin showed multiple bands upon agarose gel electrophoresis. Transformants selected either for microcin resistance or ampicillin resistance yielded plasmid DNA corresponding in size to only 1 of the multiple bands. Plasmids, isolated from all 3 hosts, which determined microcin resistance and microcin production measured about 4 megadaltons by sucrose density, restriction enzyme and contour length analyses; cleavage of the DNA by each of 8 restriction enzymes showed the same response, and DNA-DNA hybridization indicated complete homology. The antibiotic resistance plasmids of the 3 host strains were uniformly larger, were of different sizes and showed different restriction enzyme cleavage patterns. One of these R plasmids (pCP106) also determined the synthesis of the same microcin, and DNA-DNA hybridization studies indicated an approximate 2.4-megadalton homology with the 4-megadalton microcin plasmid pCP101. The microcin plasmids were present at approximately 20 copies/genome equivalent and were nonconjugative, whereas the R plasmids had a copy number of about 1, were conjugative and could mobilize the microcin plasmid. Microcin plasmid pCP101 showed replication properties similar to those of a number of small multicopy plasmids such as ColE1.

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