Genetic analysis of a streptomycin-resistant Escherichia coli mutant temperature-sensitive for nonsense suppression

Continuing the genetic and biochemical characterization of the streptomycin-resistant Escherichia coli mutant LD1, we confirmed that LD1 is temperature-sensitive for suppression of nonsense codons, and that this phenotype of the mutant and its streptomycin-resistance are genetically linked and are probably caused by a single mutation, strA (LD1). We also isolated a spontaneous revertant, called LD1-R, which partially relieves the restriction of nonsense suppression caused by the strA (LD1) mutation. LD1-R is derived by an additional mutation (revA) which is closely linked to strA (LD1). We further demonstrate that the weak suppression of a lacZ_UGA mutation in a suppressor-free strain, which probably takes place by normal tRNA^Trp, can be detected by the use of the chromagenic substance x-gal (5-Bromo-4-chloro-3-indolyl-β-d-Galactopyranoside).