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

Abstract

An Escherichia coli isoleucine transfer RNA specific for the codon AUA (tRNA₂^Ile or tRNA_minor^Ile (ref. 1) has a novel modified nucleo-side, lysidine (L; ref. 2) (Fig. la) in the first position of the anticodon (position 34), which is essential for the specific recognition of the codon AUA (ref. 1). We isolated the gene for tRNA₂^Ile (ileX) and found that the anticodon is CAT, which is characteristic of the methionine tRNA gene. Replacement of L(34) of tRNA₂^Ile 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.