Steps of mRNA translocation in protein biosynthesis.

The translocation of the messenger RNA relative to the ribosome during peptide synthesis represents an example of a mechano-chemical reaction in which the chemical bond energy of GTP is transformed into coordinated motion. Such transformations also occur during the beating of cilia and flagellae, the contraction of muscle and the migration of chromosomes in cell division. In protein synthesis the functional geometric and energetic conditions for this transformation are well defined. For each peptide bond formed, the ribosome moves one codon along the mRNA (towards the 3' end) and one molecule of GTP is hydrolysed. Although the basic requirements of this reaction have been elucidated, the mechanism is still unresolved. We demonstrate here that translocation can be analysed as a series of binding equilibria shifted by one irreversible, GTP-consuming step. The shift in the binding equilibrium is induced by the transfer of the peptidyl moiety to the (A) site-bound aminoacyl (AA)-tRNA. This results in the A site-bound tRNA having an increased affinity for the high-affinity (P) site, and a strengthened association with the mRNA. Elongation factor (EF) G . GPT catalyses removal of the deacylated tRNA, empties the P site and at the same time loosens ribosome-mRNA association. The result of these changes is that peptidyl(PP)-tRNA . mRNA is shifted from the A site to the P site, binding of AA-tRNA . EF-Tu . GPT to the vacant A site ensuring that the process is irreversible.