Cysteine transfer RNA of Escherichia coli: Nucleotide sequence and unusual metabolic properties of the 3′ C-C-A terminus

Abstract

Transfer RNA nucleotidyltransferase is required for the normal growth of Escherichia coli and for biosynthesis of some bacteriophage tRNAs. In order to investigate the involvement of this enzyme in the biosynthesis of E. coli tRNA we have studied the synthesis of these molecules in wild-type ( cca + ) and tRNA nucleotidyltransferase-deficient ( cca ) strains. We found that a small subset of tRNA species lack a complete C-C-A terminus in the cca strain. Within this subset of tRNAs, tRNA Cys was the most severely affected; nucleotide sequence analysis showed that 79% of tRNA Cys molecules lacked one or more of the 3′-terminal C-C-A residues. However, in this same cca strain, the tRNA Cys synthesized in the presence of chloramphenicol, an antibiotic which inhibits turnover of the terminal AMP residue of tRNAs, possessed a complete C-C-A terminus. These results indicate that tRNA nucleotidyltransferase is not required for the biosynthesis of tRNA Cys . We infer from these findings that the 3′ C-C-A sequence of tRNA Cys arises during transcription, and that the high proportion of defective tRNA Cys molecules found in cca cells results from unusually high turnover at the 3′ terminus of this tRNA. This conclusion must be stated with some reservation, for it is based on studies using a mutant strain which contains 2% residual tRNA nucleotidyltransferase activity. Regardless of its mode of biosynthesis, however, tRNA Cys does possess an unusual metabolic feature in that it is subject to rapid end-turnover. We have not determined why tRNA Cys exhibits such unusual behavior.