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 <2% and <1% of the AMP and CMP-incorporating activities, respectively. About 10% of the tRNA from this cell contained incomplete termini, and this level of defective tRNA was not affected by a variety of growth conditions or by transduction of the 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.