Stoichiometry of GTP hydrolysis in a poly(U)-dependent cell-free translation system. Determination of GTP/peptide bond ratios during codon-specific elongation and misreading

The Reverse Side of a Coin: "Factor-Free" Ribosomal Protein Synthesis In Vitro is a Consequence of the In Vivo Proofreading Mechanism

This paper elucidates a close connection between two well-known facts that until now have seemed independent: (i) the quality control ("proofreading") of the emerging amino acid sequence, occurring during the normal, elongation-factor-dependent ribosomal biosynthesis, which is performed by removing those Aa-tRNAs (aminoacyl tRNAs) whose anticodons are not complementary to the exhibited mRNA codons, and (ii) the in vitro discovered existence of the factor-free ribosomal synthesis of polypeptides. It is shown that a biological role of proofreading is played by a process that is exactly opposite to the step of factor-free binding of Aa-tRNA to the ribosome-exposed mRNA: a factor-free removal of that Aa-tRNA whose anticodon is not complementary to the ribosome-exhibited mRNA codon.

Stoichiometry of elongation factor G function in translation

A steady-state translation system has been used in vitro to measure the stoichiometry with which elongation factor G-GTP complexes are dissipated during polypeptide elongation. It has been possible to separate this dissipation from that associated with elongation factor Tu function. Our measurements for the wild-type as well as for two mutant variants indicate that there is one elongation factor G-GTP complex dissipated per peptide bond in the steady-state.

The excess GTP hydrolyzed during mistranslation is expended at the stage of EF-Tu-promoted binding of non-cognate aminoacyl-tRNA

The system of translation of Sepharose-bound poly(U) in which all ribosomes are active in peptide elongation was used to determine the stoichiometry of GTP hydrolysis at the stage of EF-Tu-promoted aminoacyl-tRNA binding. The ratio of GTP hydrolyzed at this stage per peptide bond was assayed during codon-specific elongation (polyphenylalanine synthesis) and misreading (polyleucine synthesis). It was demonstrated directly that the excess GTP hydrolyzed during misreading [(1984) FEBS Letters 178, 283-287] is expended at the stage of Ef-Tu-promoted binding of non-cognate aminoacyl-tRNA.

Effect of streptomycin on the stoichiometry of GTP hydrolysis in a poly(U)-dependent cell-free translation system

The technique of Sepharose-bound template translation has been used to estimate the stoichiometry of GTP hydrolysis during peptide elongation in the presence of streptomycin. The presence of streptomycin has been shown to have no great effect on the elongation rate and the stoichiometry of GTP hydrolysis during codon-specific peptide elongation in the poly(U)-directed translation system: the molar ratio of hydrolysed GTP to incorporated phenylalanine was about 2. At the same time streptomycin exerted a significant effect during misreading when a ribosome-bound peptide in the poly(U)-programmed system was elongated by leucine or isoleucine residues: the miselongation was stimulated and hence the ratio of hydrolysed GTP per peptide bond was strongly reduced, as compared with the excessive GTP hydrolysis which is characteristic of the misreading system in the absence of streptomycin [(1984) FEBS Lett. 178, 283-287]. The conclusion has been made that streptomycin blocks the stage of correction ('proof-reading') following GTP hydrolysis during EF-Tu-dependent aminoacyl-tRNA binding.