Volume 1

Gene expression / Benjamin Lewin.

  • Lewin, Benjamin.
Date:
1974-1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

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    the drug produces. Most, although not all, of the dependent colonies can be satisfied by addition of either streptomycin or paromycin or, indeed, by ethanol. All these reagents induce misreading in vitro. Such cells require a drug such as streptomycin even when grown on complete medium; its presence is not needed to suppress a mutation, as in conditional dependence, but rather to distort the ribosomes so as to ensure a proper reading of all genes. Without streptomycin, dependent cells cannot translate messengers properly. Mutations to give the dependent phenotype map at the same locus, rpxL, characterized by the wild type str-s (sensitive) and the mutant str-r (resistant) alleles which dictate whether cells are killed by or survive treatment with streptomycin. Luzzato, Schlessinger and Apirion (1969) have shown that, in fact, one allele, str-r, confers either resistance to streptomycin or dependence upon it; when genes of str-r E.coli K12 are transduced into a str-s strain of E.coli C, both streptomycin resistant and dependent transductants can be recovered. Momose and Gorini (1971) have since mapped a variety of dependent and resistant mutations; they are all sited in the same gene. The alternative expressions of this gene are determined by the genetic back ground of the cell in which it is located, for mutations at two other loci determine whether its phenotypic expression is displayed as str-r or str-d. These two loci are closely linked to the str (rpxL ) gene itself and both specify proteins of the 30S subunit. These proteins must interact with the SI2 protein coded by str (rpxL ) to influence the extent of misreading induced by streptomycin. In other words, cells which survive exposure to streptomycin by becoming dependent upon it differ from those which become resistant in the structure of one (or more) of the other ribosomal proteins such that their ribosomes require streptomycin if they are to translate mRNA into proteins with the correct amino acid sequence. The allele at the streptomycin locus, together with these two other genes, can therefore control the accuracy of the translation process. Streptomycin causes the ribosome to misread certain codons and this action may suppress muta tions; this explains why conditionally dependent cells require streptomycin. But ribosomes from resistant cells are not distorted into misreading by streptomycin, because the resistant mutations in the str (rpxL ) locus have imposed a restriction upon the efficiency of suppression; this restriction applies whether the suppression is mediated by streptomycin or by other means. Streptomycin itself, therefore, exerts a phenotypic suppression by causing the ribosome to misread codons; restrictive mutations in the streptomycin allele counter this effect and make reading more rigorous. The extent of the restriction varies with the individual mutation and this may play a role in deciding the response of the ribosome to streptomycin. Indeed, it was at first thought that slightly restrictive alleles might confer resistance to streptomycin, whereas alleles causing a greater degree of restriction would require the addi-