Genetics of Nonsense Suppressor tRNAs in Escherichia coli

Abstract

The genetic approach to tRNA originated in the study of nonsense suppressors in Escherichia coli that could specifically restore nonsense mutations in bacteria or bacteriophages. Three codons, UAG (amber), UAA (ocher), and UGA (opal), do not code for an amino acid due to the lack of tRNAs with anticodons corresponding to these codons, called nonsense codons. They are not normally present within the coding regions of genes, but can be generated by nonsense mutations, resulting in the termination of peptide chain elongation at the site. Accordingly, if a mutation occurs in a tRNA gene that alters the coding specificity to recognize a nonsense codon, the altered tRNA may suppress the nonsense mutations in other genes by inserting a specific amino acid during the process of translation. Although the suppression may take place in various ways (for reviews, see Gorini 1971; Steege and Söll 1979), nonsense suppression has been generally attributed to the tRNA suppression. Missense or frameshift mutations are also suppressed by altered tRNAs specific to each mutation (for a review, see Steege and Söll 1979).

In the case of suppressor gene mutations, a mutant normally shows positive character (su⁺) differing from ordinary genes and is dominant over its allelic wild-type gene (su⁻). Accordingly, the detection of strains carrying nonsense suppressors or the isolation of new suppressor mutants from su⁻ strains is relatively easy in genetic operation, and, in fact, many different nonsense suppressors (e.g., su⁺1, su⁺2, su⁺B, su⁺C, etc.) have been detected before they were fully understood. They can...