Discovery and characterization of tRNAIle lysidine synthetase (TilS)

In the bacterial decoding system, the AUA codon is deciphered as isoleucine by tRNAIle bearing lysidine (L, 2-lysyl-cytidine) at the wobble position. Lysidine is an essential modification that determines both the codon and amino acid specificities of tRNAIle. We identified an enzyme named tRNAIle lysidine synthetase (TilS) that catalyzes lysidine formation by using lysine and ATP as substrates. Biochemical studies revealed a molecular mechanism of lysidine formation that consists of two consecutive reactions involving the adenylated tRNA intermediate. In addition, we deciphered how Escherichia coli TilS specifically discriminates between tRNAIle and the structurally similar tRNAMet, which bears the same anticodon loop. Recent structural studies unveiled tRNA recognition by TilS, and a molecular basis of lysidine formation at atomic resolution.