ABSTRACT Modifications in the tRNA anticodon, adjacent to the three-nucleotide anticodon, influence translation fidelity by stabilizing the tRNA to allow for accurate reading of the mRNA genetic code. One example is the N1-methylguaonosine modification at guanine nucleotide 37 (m 1 G37) located in the anticodon loop, immediately adjacent to the anticodon nucleotides 34-36. The absence of m 1 G37 in tRNA Pro causes +1 frameshifting on polynucleotide, slippery codons. Here, we report structures of the bacterial ribosome containing tRNA Pro bound to either cognate or slippery codons to determine how the m 1 G37 modification prevents mRNA frameshifting. The structures reveal that certain codon-anticodon contexts and m 1 G37 destabilize interactions of tRNA Pro with the peptidyl site, causing large conformational changes typically only seen during EF-G mediated translocation of the mRNA-tRNA pairs. These studies provide molecular insights into how m 1 G37 stabilizes the interactions of tRNA Pro with the ribosome and the influence of slippery codons on the mRNA reading frame. IMPACT STATEMENT Chemical modifications near the tRNA anticodon and specific mRNA-tRNA pairs combine to control the ribosomal three-nucleotide mRNA reading frame, essential for the sequential addition of amino acids into polypeptide chains. Data deposition Crystallography, atomic coordinates, and structure factors have been deposited in the Protein Data Bank, www.pdb.org (PDB codes 6NTA, 6NSH, 6NUO, 6NWY, 6O3M, 6OSI)

Structural insights into mRNA reading frame regulation by tRNA modification and slippery codon-anticodon pairing