Properties of the elongation factors from Escherichia coli. Exchange of elongation factor G during elongation of polypeptide chain.

Abstract

The interaction of elongation factor G (EF‐G) with ribosomes has been studied during or in the absence of elongation of the polypeptide chain in a system from Escherichia coli in vitro.Mutual saturation curves of ribosomes and EF‐G suggest a rapid exchange of the factor from ribosome to ribosome. In the GTPase reaction, V/2 was reached at [ribosomes]/[EF‐G] ∼ 14 and at [EF‐G]/[ribosomes] ∼ 33. In the poly(U)‐directed poly(phenylalanine) synthesis, V/2 was obtained at [ribosomes]/[EF‐G] ∼ 12 and at [EF‐G]/[ribosomes] ∼ 6.Kinetics of the exchange of EF‐G between two pure acetyl‐aminoacyl‐tRNA ·mRNA · ribosome complexes containing synthetic homo‐ or heteropolymeric mRNAs were investigated by following their respective EF‐G‐dependent incorporation of differently labelled amino acids into the corresponding polypeptides in experimental conditions in which EF‐G was either added free or already prebound to one of the complexes prior to addition of the second one.In all experiments the rates of incorporation showed that the exchange of EF‐G between the two complexes was immediate and complete. These observations indicate that EF‐G behaves as a virtually free component which interacts catalytically with ribosomes during each round of elongation of the polypeptide chain.