The frequency of errors upon decoding of messenger RNA by the bacterial ribosome is low, with one misreading event per 1 × 10 4 codons. In the universal genetic code, the AUN codon box specifies two amino acids, isoleucine and methionine. In bacteria and archaea, decoding specificity of the AUA and AUG codons relies on the wobble avoidance strategy that requires modification of C34 in the anticodon loop of isoleucine transfer RNA Ile CAU (tRNA Ile CAU ). Bacterial tRNA Ile CAU with 2-lysylcytidine (lysidine) at the wobble position deciphers AUA while avoiding AUG. Here we report cryo-electron microscopy structures of the Escherichia coli 70S ribosome complexed with elongation factor thermo unstable (EF-Tu) and isoleucine-tRNA Ile LAU in the process of decoding AUA and AUG. Lysidine in tRNA Ile LAU excludes AUG by promoting the formation of an unusual Hoogsteen purine-pyrimidine nucleobase geometry at the third position of the codon, weakening the interactions with the mRNA and destabilizing the EF-Tu ternary complex. Our findings elucidate the molecular mechanism by which tRNA Ile LAU specifically decodes AUA over AUG.
Organizational Affiliation:
Department of Microbiology and Immunology, University of Texas Medical Branch, Galveston, TX, USA.
Department of Microbiology and Immunology, University of Texas Medical Branch, Galveston, TX, USA. magagnon@utmb.edu.
Department of Biochemistry and Molecular Biology, University of Texas Medical Branch, Galveston, TX, USA. magagnon@utmb.edu.
Sealy Center for Structural Biology and Molecular Biophysics, University of Texas Medical Branch, Galveston, TX, USA. magagnon@utmb.edu.
Institute for Human Infections and Immunity, University of Texas Medical Branch, Galveston, TX, USA. magagnon@utmb.edu.
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