Genetic Analysis of Streptomycin Resistance in ESCHERICHIA COLI

TLDR: StrA mutants may be classified by their different patterns of phenotypic suppression, which is evident in mutants, either missense or nonsense, whose phenotype is positive or leaky in strA+ cells, but which becomes negative upon the introduction of a strA mutation.

Abstract: H E strA locus in Escherichia coli determines the primary structure of a protein of the 30s ribosomal subunit (TRAUB and NOMURA 1968; OZAKI, MIZUSHIMA and NOMURA 1969). The phenotype classically used for selecting strA mutants is survival to the bactericidal action of streptomycin. These survivors either are indifferent to streptomycin (“resistant”) or they require the presence of the aminoglycoside for growth (“dependent”). In addition to its bactericidal action, streptomycin has other effects on the wild-type strA+ sensitive cell and on the ribosomes extracted from it, including the ability to promote phenotypic suppression in uiuo (GORINI and KATAJA 1964) and to induce misreading in uitro (DAVIES, GILBERT and GORINI 1964). These effects are similarly eliminated or reduced in strA mutants. Presumably more fundamentally related to the role of the strA protein in ribosomal function is an impairment in translation efficiency brought about by the strA mutation. Such an impairment is evident in mutants, either missense or nonsense, whose phenotype is positive or leaky in strA+ cells, but which becomes negative upon the introduction of a strA mutation. Since this impairment in translation efficiency might be lethal if not contained within limits compatible with cell growth, it is evident only in mutant strains whose growth is limited by the availability of a tRNA species able to translate properly the defective codon. The limiting tRNA may be either a suppressor that translates the mutated codon by conventional base-pairing or a normal tRNA that translates it by misreading ( GORINI 1969). In addition to the classification as indifferent or dependent, strA mutants may be classified by their different patterns of phenotypic suppression. Certain strA alleles are competent for permitting phenotypic suppression by streptomycin of argF40, an amber mutation in the structural gene for ornithine transcarbamylase, while others are incompetent (GORINI and KATAJA 1964). A third class of strA alleles is incompetent for nrgF40 suppression but competent for the suppression of certain other mutations (ANDERSON, GORINI and BRECKENRIDGE 1965). The existence of these three classes was confirmed when the strA alleles were intro-