Genetic characterization of streptomycin-resistant and-dependent mutants of Escherichia coli K12

Summary

Streptomycin high resistant and dependent mutants of E. coli K12 were isolated and characterized with regard to a) permissiveness for phenotypic suppression by streptomycin of the conditional streptomycin-dependent mutation thr-1, and b) restriction of the activity of the sup-59 amber suppressor. The majority of the streptomycin-resistant the strA mutation. Streptomycin-dependent (Sm^HD) mutants were grouped into three main classes, Sm^D Et⁻ Pm^S, Sm^D Et^D Pm^S and Sm^D Et^D Pm^D or Drug^D, according to their ability to allow paromomycin or ethanol to act as substitutes for streptomycin for growth. In addition, streptomycin-dependent mutants were classified as either permissive (Per⁺) or nonpermissive (Per⁻) with regard to the streptomycin-mediated phenotypic suppression of the thr-1 mutation and of two T4 amber mutations. This behavior was shown to be due to different allelic states of another gene, strM. Mutations at strM were responsible for the Per⁻ phenotype in Sm^HD strains. In streptomycin-sensitive or-resistant bacteria, strM remained largely unexpressed. strM was located at approximately min 68 of the standard E. coli map. In strM heterozygous merodiploids, the wild-type Per⁺ phenotype was dominant over Per⁻. The nature of the strM product remains unknown. Restriction of sup-59 activity by streptomycin resistant and dependent mutations was infrequent, and was not correlated with the mutant's behavior in phenotypic suppression. Although strM-mediated phenotypic suppression counteracted the restriction imposed by strA mutations, apparently it did not act by restoring the activity of genetic suppressors.