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Accuracy of codon reading by transfer RNAs

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Translation of the genetic code on ribosomes requires recognition of 61 sense codons by amino acid carrying transfer RNAs. mRNA translation has evolved to high speed and high accuracy. The accuracy of sense codon reading relies on Watson-Crick base pairing between the three codon bases of mRNA and the three anticodon bases of tRNAs cognate to the codons, but also on ribosome aided stereo-selection of the codon-anticodon helix. The accuracy of tRNA recognition is greatly enhanced by a two-step mechanism, consisting of initial selection and proofreading selection, where the two steps are separated by GTP -hydrolysis on elongation factor Tu in ternary complex with tRNA and GTP . This separation of steps by a large chemical potential decrease allows for repeated selection of the very same standard free energy difference between cognate and non-cognate codon-anticodon interactions at transition states for GTP -hydrolysis (initial selection) and aa-tRNA accommodation and peptidyl transfer (proofreading selection). Structural elements of codon-anticodon interactions of importance for the accuracy of genetic code translation define the intrinsic single step selectivity of codon reading (d-value), the actual (current) selectivity of single step reading (I-value for initial and F-value for proofreading). We will discuss the speed-accuracy trade-off of single step codon reading: when the I- or F-value approaches the d-value the rate of codon translation necessarily goes to zero. The working principle of proofreading and its manifestation in codon translation will be discussed along with a novel type of biochemical experiments to estimate I- and F-values as well as intrinsic d-values: the rate-accuracy trade off leads to linear plots, from which the d-values of discrimination against near-cognate codons by Lys-tRNALys in ternary complex with EF-Tu and GTP . It seems that this method can be generalized to all aminoacyl-tRNAs and all ribosomal contexts, as determined by mutations, modification deficiencies, drugs etc. Calibration of biochemical systems to the accuracy of codon reading in the living cell will be discussed along with criteria for optimal translation accuracy for maximal fitness of growing cells.

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