Translational termination without a stop codon

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Science  01 Dec 2016:
DOI: 10.1126/science.aai9127

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Ribosomes stall when they encounter the end of mRNA without an in-frame stop codon. In bacteria, these nonstop complexes can be rescued by alternative ribosome-rescue factor A (ArfA). Here, using electron cryomicroscopy, we have determined structures of ArfA bound to the ribosome with 3′-truncated mRNA to resolutions from 3.0 to 3.4 Å. ArfA binds within the ribosomal mRNA channel and substitutes for the absent stop codon in the A site by specifically recruiting release factor 2 (RF2), initially in a compact pre-accommodated state. A similar conformation of RF2 may occur on stop codons, suggesting a general mechanism for release-factor-mediated translational termination in which a conformational switch leads to peptide release only when the appropriate signal is present in the A site.

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