Abstract
Decoding of the 61 sense codons of the genetic code requires a variable number of tRNAs that establish codon-anticodon interactions. Thanks to the wobble base pairing at the third codon position, less than 61 different tRNA isoacceptors are needed to decode the whole set of codons. On the tRNA, a subtle distribution of nucleoside modifications shapes the anticodon loop structure and participates to accurate decoding and reading frame maintenance. Interestingly, although the 61 anticodons should exist in tRNAs, a strict absence of some tRNAs decoders is found in several codon families. For instance, in Eukaryotes, G34-containing tRNAs translating 3-, 4- and 6-codon boxes are absent. This includes tRNA specific for Ala, Arg, Ile, Leu, Pro, Ser, Thr, and Val. tRNAGly is the only exception for which in the three kingdoms, a G34-containing tRNA exists to decode C3 and U3-ending codons. To understand why G34-tRNAGly exists, we analysed at the genome wide level the codon distribution in codon +1 relative to the four GGN Gly codons. When considering codon GGU, a bias was found towards an unusual high usage of codons starting with a G whatever the amino acid at +1 codon. It is expected that GGU codons are decoded by G34-containing tRNAGly, decoding also GGC codons. Translation studies revealed that the presence of a G at the first position of the downstream codon reduces the +1 frameshift by stabilizing the G34•U3 wobble interaction. This result partially explains why G34-containing tRNAGly exists in Eukaryotes whereas all the other G34-containing tRNAs for multiple codon boxes are absent.
Highlights
The process of translation is the last stage in the genetic information transfer and it depends upon the correct matching between mRNA codons and corresponding tRNA anticodons within the ribosomal complex
TRNAGly, which belongs to the 4-codon box group, clearly escapes this global trend and 6636 sequences harbouring an anticodon G34CC are found in the database
When a G residue was in +1 of an intergenic region (IGR) construct reprogrammed with a GGU codon, the translational activity of the reporter renilla luciferase (R-Luc) was improved in the 0 frame and frameshifting activity in the +1 frame dropped to a minimum
Summary
The process of translation is the last stage in the genetic information transfer and it depends upon the correct matching between mRNA codons and corresponding tRNA anticodons within the ribosomal complex. During the initial selection step, the ternary complex of aa-tRNA with EF-Tu–GTP (in procaryotes) binds to the ribosome and forms a readily reversible complex that dissociates rapidly when there is no codon/anticodon match. Discrimination of correct and incorrect substrates is achieved in two consecutive steps, initial selection and proofreading which are separated by GTP hydrolysis All these events between the initial binding and accommodation of the cognate aa-tRNA have been dissected into several distinguishable steps (reviewed in [2,3])
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