A Gripping Tale of Ribosomal Frameshifting: Extragenic Suppressors of Frameshift Mutations Spotlight P-Site Realignment

Abstract

SUMMARYMutants of translation components which compensate for both −1 and +1 frameshift mutations showed the first evidence for framing malleability. Those compensatory mutants isolated in bacteria and yeast with altered tRNA or protein factors are reviewed here and are considered to primarily cause altered P-site realignment and not altered translocation. Though the first sequenced tRNA mutant which suppressed a +1 frameshift mutation had an extra base in its anticodon loop and led to a textbook “yardstick” model in which the number of anticodon bases determines codon size, this model has long been discounted, although not by all. Accordingly, the reviewed data suggest that reading frame maintenance and translocation are two distinct features of the ribosome. None of the −1 tRNA suppressors have anticodon loops with fewer than the standard seven nucleotides. Many of the tRNA mutants potentially affect tRNA bending and/or stability and can be used for functional assays, and one has the conserved C74 of the 3′ CCA substituted. The effect of tRNA modification deficiencies on framing has been particularly informative. The properties of some mutants suggest the use of alternative tRNA anticodon loop stack conformations by individual tRNAs in one translation cycle. The mutant proteins range from defective release factors with delayed decoding of A-site stop codons facilitating P-site frameshifting to altered EF-Tu/EF1α to mutant ribosomal large- and small-subunit proteins L9 and S9. Their study is revealing how mRNA slippage is restrained except where it is programmed to occur and be utilized.

Structural insights into +1 frameshifting promoted by expanded or modification-deficient anticodon stem loops

“…Although it is unlikely that a single mechanism exists to explain all occurrences of mRNA frameshifting (11,57), this study represents a first step toward understanding the molecular details required for +1 frameshifting. We envisage that most +1 frameshift suppressor tRNAs adopt noncanonical conformations in the 5′ region of the ASL that mediate a rearrangement in the P site.…”

“…Based on decades of genetic, biochemical, and structural studies, two likely possibilities exist to explain the molecular basis of +1 frameshifting (11). The first possibility is an alteration in translocation of the tRNA-mRNA pair on the 30S from the A to P site by translation factor EF-G (Fig.…”

“…Over the next several decades, genetic characterization of these suppressors played a large role in the fundamental understanding of tRNA structure, decoding, and other general features of ribosome function (reviewed in ref. 11). …”

Global shape mimicry of tRNA within a viral internal ribosome entry site mediates translational reading frame selection

“…These results also imply that interactions of SLIII with the ribosome are not required for, or that the loss of these interactions underlie, +1 frame translation. The P site tRNA interacts with several ribosomal proteins and both small and large rRNAs that may contribute to reading frame maintenance (42). One proposed model for reading frame maintenance is the "ribosomal grip"-the interaction of the ribosome with the peptidyl-tRNA-which prevents slippage of the reading frame during translation (43).…”

Celebrating wobble decoding: Half a century and still much is new

“…The quadruplet translocation theory is used to deduce the pairing of these four cytidines with four bases of the codon in the A site. The tRNA anticodon sequence has been suggested to act as a molecular ruler which determines the codon size during translation, within certain limits, 69 thereby restoring some ribosomes to the wild type frame. Thus, the nature of the N 32 ○N 38 base interaction affects the binding of the anticodon to the codon suggesting that the intraloop hydrogen bonding alters the conformation and dynamics of the anticodon stem and loop domain within the ribosomal complex.…”