Transfer RNA metabolism in Escherichia coli cells deficient in tRNA nucleotidyltransferase

We have investigated the role of tRNA nucleotidyltransferase in transfer RNA metabolism using mutants ( cca mutants) of Escherichia coli deficient in this enzyme. Extracts of the most defective strain contained cca mutation into a variety of genetic backgrounds. However, tRNA from stationary phase cells was somewhat more defective. Aminoacylation studies revealed that most tRNAs were unaffected by the presence of the cca mutation, but a few species were as much as 20 to 60% defective, and this was true for all the isoacceptors for a given amino acid. Incubation of the mutant cells with the antibiotics chloramphenicol and rifampicin, alone or in combination, indicated that most defective tRNA species arose from end-turnover of the terminal AMP residue. However, the possibility exists that tRNA Cys , the most defective tRNA species, may require tRNA nucleotidyltransferase for its biosynthesis. No evidence was obtained for a role of the enzyme in the biosynthesis of any other tRNA species. End-turnover of terminal residues was shown to be related to the state of charging of a tRNA molecule, but the physiological significance of this process remains to be determined. Possible explanations for the apparently limited role of tRNA nucleotidyltransferase in E. coli are discussed.