New aspects of the ribosomal elongation cycle.

The ribosomal elongation cycle represents a series of reactions during which the polypeptide is prolonged by one amino acid and after which the prolonged polypeptidyl residue is again ready to accept the next aminoacyl residue. It is generally believed that the ribosome carries two tRNA binding sites, the A site for aminoacyl-tRNA and the P site for peptidyl-tRNA, leading to the classical two-site model of the ribosome as a description for the elongation cycle. However, evidence is accumulating which is in conflict with the classical two-site model. These conflicts are resolved in a new three-site model which is discussed in detail in this paper.