Importance of tRNA anticodon loop modification and a conserved, noncanonical anticodon stem pairing in tRNA for decoding.
Publication Type:Journal Article
Source:J Biol Chem (2019)
<p>Modification of anticodon nucleotides allows tRNAs to decode multiple codons, expanding the genetic code. Additionally, modifications located in the anticodon loop but outside the anticodon itself, stabilize tRNA-codon interactions, increasing decoding fidelity. Anticodon loop nucleotide 37 is 3' to the anticodon and in tRNA , is methylated at the N1 position in its nucleobase (mG37). The mG37 modification in tRNA stabilizes its interaction with the codon and maintains the mRNA frame. However, it is unclear how mG37 affects binding at the decoding center to both cognate and +1 slippery codons. Here, we show that the tRNA mG37 modification is important for the association step during binding to a cognate CCG codon. In contrast, mG37 prevented association to a slippery CCC-U or +1 codon. Similar analyses of frameshift suppressor tRNASufA6, a tRNA derivative containing an extra nucleotide in its anticodon loop that undergoes +1 frameshifting, revealed that mG37 destabilizes interactions with both the cognate CCG and slippery codons. One reason for this destabilization was the disruption of a conserved U32·A38 nucleotide pairing in the anticodon stem through insertion of G37.5. Restoring the tRNA U32·A37.5 pairing resulted in a high-affinity association on the slippery CCC-U codon. Further, an X-ray crystal structure of the 70S ribosome bound to tRNA U32·A37.5 at 3.6-Å resolution showed a reordering of the anticodon loop consistent with the findings from the high-affinity measurements. Our results reveal how the tRNA modification at nucleotide 37 stabilizes interactions with the mRNA codon to preserve the mRNA frame.</p>