A ribosomal ambiguity mutation

Abstract

The existence of a genetic determinant ram ‡ ‡ Abbreviations used: for the designation of genetic markers the proposals of Demerec, Adelberg Clark & Hartman (1966) have been followed and abbreviations correspond generally to those of Taylor & Trotter (1967) with the addition of apt for the speotinomycin gene. Other abbreviations are the following: CSD, conditionally streptomycin dependent (Gorini &Kataja, 1984u); Arg-CSD, streptomycin dependent under condition of absence of arginine; Sm. streptomycin; Sp, speotinomyoin; Phe, phenylalanine; De, isoleucine; Ser, serine; His, histidine; Pro, proline; aromix, mixture of growth factors required by aroE mutants which are blocked in the dehydrosbikimate reductase (Pittard & Wallace, 1966); poly-I?, polyuridine; poly-C, polycytidine. for a 30 s ribosomal component controlling translation ambiguity has been demonstrated. It is located in the strA (streptomycin sensitivity) region and is 99% cotransducible with the spc (spectinomycin sensitivity) gene. In vitro, ram mutant ribosomes misread extensively and by interchanging the subunits of ram and ram + ribosomes it is shown that misreading is a property of the 30 s subunit. In vivo, the ram mutation confers on the cell the ability to suppress all three nonsense codons. Streptomycin induces a similar generalized ambiguity phenotypically both in vitro and in vivo. The effects of ram and of streptomycin are additive. Ambiguity caused by ram, by streptomycin and by their combination is antagonized in vivo and in vitro by additional mutations at the strA locus. Thus the strA40 (resistant competent) and strA1 (resistant incompetent) alleles increasingly restrict ribosomal (as well as tRNA) suppression and return to normal the reduced growth rate produced by ram mutation in a wild-type strA + strain. Growth is inhibited completely in a ram1, strA + strain by sublethal doses of streptomycin and in ram1, strA40 strains by higher doses, while strain ram1, strA1 is indifferent to streptomycin. The bactericidal action of the drug is unrelated to these effects.