Codon and amino-acid specificities of a transfer RNA are both converted by a single post-transcriptional modification.

An Escherichia coli isoleucine transfer RNA specific for the codon AUA (tRNA(2Ile) or tRNA(minorIle] has a novel modified nucleoside, lysidine in the first position of the anticodon (position 34), which is essential for the specific recognition of the codon AUA. We isolated the gene for tRNA(2Ile) (ileX) and found that the anticodon is CAT, which is characteristic of the methionine tRNA gene. Replacement of L(34) of tRNA(2Ile) molecule enzymatically with unmodified C(34) resulted in a marked reduction of the isoleucine-accepting activity and, surprisingly, in the appearance of methionine-accepting activity. Thus, both the codon and amino-acid specificity of this tRNA are converted by a single post-transcriptional modification of the first position of the anticodon during tRNA maturation.