High-level resistance to streptomycin inEscherichia coli with R1 plasmid and the analysis of genetic determinants

Some properties of streptomycin-resistant mutants of Escherichia coli were analyzed. In a R+ culture, the phenotype under study may be significantly selected at a frequency of 10(-5) on media with higher streptomycin level. The lrs mutation is present in the cells prior to the action of streptomycin and remains in the cells even after curing of the R1 plasmid. The mapping of the lrs gene by conjugation with a concomitant transfer of chromosome and the R1 plasmid in different Hfr strains of E. coli failed to establish the localization of this gene in the tested chromosome regions. The presence of a cryptic plasmid was detected in cells with the lrs mutation after curing of the R1 plasmid. This plasmid codes neither fertility functions nor chloramphenicol-acetyltransferase, streptomycin-adenyltransferase, or ampicillin-beta-lactamase.