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

Abstract 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%. psu a + and psu b + map less than 0.3 map units apart and are located between genes e and 57 about 8 map units from gene e. That psu a + and psu b + insert the same amino acid and map so close together suggests that they result from different mutations within the same gene. On the basis of their initial frequencies of appearance and the frequency of psu b + mutation to psu a + , we speculate that psu a + is derived from wild type by two base changes and that psu b + is a one-base-change intermediate.