# Doublet decoding of tRNASer3 demonstrates plasticity of ribosomal decoding center

## TL;DR

Scientists used cryo-EM to show how a tRNA can read a two-base codon instead of a three-base one, causing frameshifts in bacteria.

## Contribution

The study reveals how a ribosomal monitoring base enables non-cognate tRNA to decode a two-base codon, allowing frameshifting.

## Key findings

- A1493 in 16S rRNA forms a Hoogsteen base pair with tRNA's U36, mimicking a codon base.
- This interaction positions the first two mRNA codon bases for pairing with tRNA's C35 and G34.
- Doublet decoding is enabled by ribosomal structural flexibility.

## Abstract

Frameshifts can be caused by specific combinations of tRNA and mRNA. The wildtype AGC-decoding E. coli tRNASer3GCU has been shown to induce −1 ribosomal frameshifting on GCA alanine codons, and proposed to read a two-base codon instead of a canonical triplet. However, it has remained unclear whether this type of non-cognate decoding can be accommodated by the ribosome. Here, we perform single-particle cryo-EM reconstructions on E. coli 70S ribosomes with the frameshift-inducing tRNASer3 bound to the non-cognate GCA codon or the cognate AGC codon in the ribosomal A site. The structures demonstrate that doublet decoding is made possible when A1493, the conserved monitoring base in 16S rRNA, mimics a first codon base, forming a Hoogsteen base pair with U36 from the anticodon and stacking with the mRNA. This interaction pushes the first two bases of the A-site codon in position for base pairing with C35 and G34 of the anticodon.

The ribosomal decoding center monitors accurate translation of 3-base mRNA codons. Here, the authors use cryo-EM to show how one of the monitoring bases of the ribosome enables a frameshift-inducing tRNA to instead read a 2-base codon.