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In most organisms, the AUA triplet codes for isoleucine (Ile), whereas in a few organelles it codes for methionine (Met). In bacteria, this A-ending triplet is decoded by an unusual tRNA harboring a Met anticodon CAU, where cytidine at the wobble position 34 (C34) is posttranscriptionally modified to a 2-lysyl cytidine (lysidine), abbreviated as (k²C). Now, the bacterial gene tilS, which encodes the enzyme catalyzing the lysylation of C34 in the precursor tRNA^Ile(CAU), thereby leading to the formation of tRNA^Ile(k²CAU), has been identified. The formation of lysidine by this essential enzyme allows recognition of tRNA^Ile(k²CAU) by Ile-tRNA synthetase and switches the base pairing of the tRNA from AUG (Met) to AUA (Ile). This base change is reminiscent of C-to-U type of RNA editing of some mitochondrial tRNAs.