Microcin C51 is an antibiotic with a wide application range produced by Escherichia coli cells. Using insertions of transposon Tn5 a set of mutations was induced in the recombinant plasmid pAST, which determines synthesis of microcin C51. The mutations were physically mapped. Complementation analysis was performed for insertions and deletions in the Mic+ plasmids pAST and pUHAB that lead to the absence of microcin or immunity to it. The analysis showed that at least three plasmid genes take part in microcin production and two genes determine immunity of producer cells to microcin. Functioning of the ompR gene product was necessary for synthesis of microcin C51.