Mechanism of protein synthesis. The cycle begins with peptidyl-tRNA in the P site. (1) An aminoacyl-tRNA binds in the A site. (2) With both sites occupied, a new peptide bond is formed. (3) The tRNAs and the mRNA are translocated through the action of elongation factor G, which moves the deacylated tRNA to the E site. (4) Once there, the tRNA is free to dissociate to complete the cycle.