Two T4-coded nonsense suppressors, psu+a and psu+b, have beenisolated and characterized. Both were isolated as pseudo-wild typerevertants 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 growthon both B and K strains of E. coli bacteria. psu+b may be either an amberor on an ochre suppressor which occurs at a frequency of 10-7 - 10-10 andmakes small plaques on B strains but grows very poorly or not at allon K strains. Phage with the characteristics of psu+a occure in populationsof psu+b phage at a frequency of 10-4. Both suppressors insertserine 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. On the basis of their initial frequencies of appearance and the frequency of psu+b mutation topsu+a, we speculate that psu+a is derived from the wild type ser-tRNA by two base changes in the anticodon and that psu+b is a one-base-changeintermediate.
151 independent suppressor-negative derivatives of psu+b phage havebeen isolated and characterized. They fall into two complementation groups. One, designated mb (modifier of psu+b phenotype), is unlinked topsu+b and has been located about 10 map units from rII. The secondgroup, designated psu+b, is made up of deletions and single base changeswhich affect sites within 0.2 map units of the original mutation. Thosepsu-b mutants which still contain the original mutation, psub, have beenmapped relative to psub and each other by a series of two-factor andintragenic three-factor crosses. 32P-labeles tRNA from mb, psu-b andwild type infected cells have been compared by polyacrylamide gel electrophoresis. In mb-infected cells several of the tRNA species are missing, while in psu-b-infected cells only the ser-tRNA is clearly absent. These studies suggest there are only two phage genes which areessential for the production of functional ser-tRNA. One is the structural gene for the ser-tRNAand the second plays an undefined role which affects several tRNAs.
E. coli cells infected with phage strains carrying a large deletionof gene e or gene psu+b, are missing most if not all of the phage tRNAsnormally present in wild type infected cells. By DNA-RNA hybridizationwe have demonstrated that the DNA corresponding to the missing tRNAs isabsent. Thus the genes for these tRNAs must be clustered in the sameregion of the genome as the ser-tRNA gene. We have been able to locateand to define a maximum size for the cluster by physically mapping thedeletions of genes e and psu+b by examination of heteroduplex DNA in theelectron microscope. That such deletions can be isolated indicates that thephage-specific tRNAs from this cluster are dispensable.
