Elongation cycle of protein synthesis. The ribosome is shown in top view, with the small subunit (transparent yellow) below the large subunit (transparent blue). (i) The ribosome in the pre-translocational state with tRNAs in the A (magenta) and P (green) sites. After spontaneous peptidyl transfer, the nascent peptide is covalently attached to the A-site tRNA. (ii) The elongation factor EF-G in complex with GTP (blue) has bound to the ribosome to facilitate the translocation of tRNAs to the P (green) and E (yellow) sites. The translocation is induced by GTP hydrolysis accompanied by large transient conformational changes in the EF-G and the ribosome. (iii) The release of the EF-G after GTP hydrolysis leaves the ribosome in the post-translocational state. It is now ready to accept a new aminoacyl-tRNA (white) presented to the ribosome by the ternary complex, comprising, in addition to the new aminoacyl-tRNA, the the elongation factor EF-Tu and a GTP (red). (iv) The ribosome with a bound ternary complex (magenta and red) in place. It has been suggested that when the ternary complex binds to the ribosome, the E-site tRNA moves further away from the P site to the so-called E2 site (orange). The snapshot shown here is part of the decoding step, where the aminoacyl-tRNA whose anticodon matches the next codon in the mRNA is selected to enter the A site, accompanied by GTP hydrolysis and conformational changes. In case of a match, EF-Tu with the hydrolyzed GTP and the E-site tRNA leave the ribosome, leaving it in the pre-translocational state (i). This figure is adapted from Frank, J. in Conformational Proteomics of Macromolecular Architechtures. World Scientific Publishing Comp. Singapore, 2004.