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Microcins are low-molecular-weight compounds produced and excreted by Enterobacteriaceae. They inhibit the growth of a wide spectrnm of microorganisms. Microcin-synthesizing transconjugants were obtained in seven out of eight experiments of conjugational transfer between wild-type microcinogenic strains of Escherichia coli and E. coli strain BM21. The physical analysis of one of the transconjugant strains that has acquired the ability to produce microcin 17 showed the presence of extrachromosomal DNA as a plasmid (pRYC17) of molecular weight 36 x 106 (18.3-,um length), which is absent in the "microcincured" derivative strain. pRYC17 was incompatible with plasmids of the IncFII group. Other suspected plasmids containing the information for the synthesis of microcins have not been clearly classified. Strains producing microcins 93, 136, and 140 show a partial incompatibility with IncFIM group of plasmids.

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