Leftward ribosome frameshifting at a hungry codon.

J Mol Biol

Department of Genetics, University of Washington, Seattle 98195.

Published: January 1992

Previous experiments have shown that limitation for certain aminoacyl-tRNA species results in phenotypic suppression of a subset of frameshift mutant alleles, including members in both the (+) and (-) incorrect reading frames. Here, we demonstrate that such phenotypic suppression can occur through a ribosome reading frame shift at a hungry AAG codon calling for lysyl-tRNA in short supply. Direct amino acid sequence analysis of the product and DNA sequence manipulation of the gene demonstrate that the ribosome frameshift occurs through a movement of one base to the left, so as to decode the triplet overlapping the hungry codon from the left or 5' side, followed by continued normal translation in the new, shifted reading frame.

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http://dx.doi.org/10.1016/0022-2836(92)90713-tDOI Listing

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In each round of translation elongation, the ribosome translocates along the mRNA by precisely one codon. Translocation is promoted by elongation factor G (EF-G) in bacteria (eEF2 in eukaryotes) and entails a number of precisely-timed large-scale structural rearrangements. As a rule, the movements of the ribosome, tRNAs, mRNA and EF-G are orchestrated to maintain the exact codon-wise step size.

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Programmed ribosomal frameshifting (PRF) is a key mechanism that viruses use to generate essential proteins for replication, and as a means of regulating gene expression. PRF generally involves recoding signals or frameshift stimulators to elevate the occurrence of frameshifting at shift-prone 'slippery' sequences. Given its essential role in viral replication, targeting PRF was envisioned as an attractive tool to block viral infection.

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The process of translation is characterized by irregularities in the local decoding rates of specific mRNA codons. This includes the occurrences of long pauses that can take place when ribosomes decode certain peptide sequences, encounter strong RNA secondary structures, or decode "hungry" codons. Examples are known where such pausing or stalling is used for regulating protein synthesis.

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