Bacteriophage T4 transfer RNA. I. Isolation and characterization of two-phage-coded nonsense suppressors

Two phage-coded nonsense suppressors, psuf_a⁺ and psu_b⁺, have been isolated and characterized. Both were isolated as pseudo-wild type revertants of phage strains which carry multiple amber mutations. psu_a⁺ is an amber suppressor which occurs at a frequency of 10⁻¹¹ to 10⁻¹² and is indistinguishable from wild type phage in its growth on both B and K strains of Escherichia coli bacteria. psu_b⁺ may be either an amber or an ochre suppressor, which occurs at a frequency of 10⁻⁷ to 10⁻¹⁰ and makes small plaques on B strains, but grows very poorly or not at all on K strains. Phage with the characteristics of psu_a⁺ occur in populations of psu_b⁺ phage at a frequency of 10⁻⁴. Both suppressors insert serine in response to the amber codon at an efficiency of about 45%.