Maturation of precursor transfer RNA (pre-tRNA) includes excision of the 5′ leader and 3′ trailer sequences, removal of introns and addition of the CCA terminus1,2,3. Nucleotide modifications are incorporated at different stages of tRNA processing, after the RNA molecule adopts the proper conformation. In bacteria, tRNAIle2 lysidine synthetase (TilS) modifies cytidine into lysidine (L; 2-lysyl-cytidine) at the first anticodon of tRNAIle2 (refs 4–9). This modification switches tRNAIle2 from a methionine-specific to an isoleucine-specific tRNA9. However, the aminoacylation of tRNAIle2 by methionyl-tRNA synthetase (MetRS), before the modification by TilS, might lead to the misincorporation of methionine in response to isoleucine codons. The mechanism used by bacteria to avoid this pitfall is unknown. Here we show that the TilS enzyme specifically recognizes and modifies tRNAIle2 in its precursor form, thereby avoiding translation errors. We identified the lysidine modification in pre-tRNAIle2 isolated from RNase-E-deficient Escherichia coli and did not detect mature tRNAIle2 lacking this modification. Our kinetic analyses revealed that TilS can modify both types of RNA molecule with comparable efficiencies. X-ray crystallography and mutational analyses revealed that TilS specifically recognizes the entire L-shape structure in pre-tRNAIle2 through extensive interactions coupled with sequential domain movements. Our results demonstrate how TilS prevents the recognition of tRNAIle2 by MetRS and achieves high specificity for its substrate. These two key points form the basis for maintaining the fidelity of isoleucine codon translation in bacteria. Our findings also provide a rationale for the necessity of incorporating specific modifications at the precursor level during tRNA biogenesis.
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We thank the beamline staff at BL41XU of SPring-8 (Harima, Japan) and NW12 of PF-AR (Tsukuba, Japan) for help during data collection. We thank H. Aiba for providing the temperature-sensitive E. coli strain of RNase E, A. Soma and Y. Sekine for discussions, and T. Suzuki and Y. Sakaguchi for support with mass spectrometry. This work was supported by a SORST program grant from Japan Science and Technology to O.N., by a grant for the National Project on Protein Structural and Functional Analyses from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) to O.N. and T.S., by grants from MEXT to R.I. and O.N., and by Mitsubishi Foundation and Kurata Memorial Hitachi Science and Technology Foundation grants to O.N. L.B. and K.N. were supported by a postdoctoral fellowship for foreign researchers and a young scientist fellowship, respectively, from the Japan Society for the Promotion of Science.
Author Contributions K.N. performed purification, crystallization and structure determination of the complex, and biochemical analyses. L.B. performed purification, crystallization and structure determination of the ASB domain, and biochemical analyses. S.K. purified the native tRNAs and performed the mass spectrometry analysis under the supervision of T.S. R.I. performed the molecular dynamics analysis and assisted with the structural determination. O.N. assisted with the structure determination. All authors discussed the results and commented on the manuscript. O.N. supervised all the work.
This file contains a Supplementary Discussion, Supplementary Methods, Supplementary References, Supplementary Tables 1- 4 and Supplementary Figures 1-12 with Legends.
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Biochemical and structural characterization of oxygen-sensitive 2-thiouridine synthesis catalyzed by an iron-sulfur protein TtuA
Proceedings of the National Academy of Sciences (2017)