[Genetic study of plasmid-associated high-frequency mutations to streptomycin resistance in Escherichia coli].

Escherichia coli CTR1(RT1)RHfm1) carrying two H-factors and having unusually high frequency of mutation to high level streptomycin resistance is studied. The high frequency of mutation (about 10(-6) to streptomycin resistance is connected with the presence of R factor RHfm1, controlling the resistance to chloramphenicol and low level streptomacin resistance, but not with RT1, controlling the resistance to tetracycline. Spontaneous or ethidium bromide-induced loss of RHfm1 is accompanied by a decrease of the mutation frequency to 10(-9). RHfm1 is efficiently transmissible to other strains at 28 degrees C. The acquisition of RHfm1 by strains of E. coli K-12 ans S. typhimurium LT2 was followed by a 1000--10000-fold increase of the frequejcy of mutation to streptomycin resistance. Some streptomycin resistant mutants were isolated, and chromosome location of the mutations was demonstrated. The streptomycin resistant mutants were unable to transmit high level of resistance to streptomycin with R factor, but only low level one. The loss of RHfm1 by streptomycin resistant mutants was accompanied by the return to the streptomycin sensitivity of the initial R- strans (E. coli K-12 mutants) or by a decrease of the streptomycin resistance to the level, only 2-fold higher than that of R- wild type (E. coli CTR1 mutant). Thus, the mutantions had practically no effect on streptomycin resistance of R- strains, but could lead to high resistance phenotypes in the presence of RHfm1. The mutant loci in all three studied strains were found to be closely linked to the locus "fus" on the genetic map of E. coli.