 Once the peptidyl transferase reaction has occurred, the tRNA in the P site is deacetylated that is it is no longer attached to an amino acid and the growing polypeptide chain is linked to tRNA in the A site. For a new round of peptide chain elongation to occur, the P site tRNA must move to the E site and the A site tRNA must move to the P site. At the same time the mRNA must move by three nucleotides to expose the new codon. These movements are coordinated within the ribosome and are collectively referred to as translocation. The initial steps of translocation are coupled to the peptidyl transferase reaction. Once the growing peptide chain has been transferred to the A site tRNA, the A site and P site tRNAs have a preference to occupy new positions in the large subunit. The three prime end of the A site tRNA is bound to the growing polypeptide chain and prefers to bind in the P site of the large subunit. The now deacetylated P site tRNA is no longer attached to the growing polypeptide chain and prefers to bind in the E site of the large subunit. In contrast, at this time the anticodons of these tRNAs remain in their initial location in the small subunit bound to the mRNA. Thus translocation is initiated in the large subunit before the small subunit and the tRNA are set to be in hybrid state. Their three prime ends have shifted into a new location but their anticodon ends are still in their pre-peptidyl transferase position. Importantly this change is associated with a counterclockwise rotation of the small subunit relative to the large subunit facilitating interaction of tRNAs with distinct tRNA binding sites in the different subunits. The completion of translocation requires the action of a second elongation factor which is called EFG. Initial binding of EFG to the ribosome occurs when associated with GTP. After the peptidyl transferase reaction, EFG GTP binds to and stabilizes the ribosome in the rotated hybrid state. When EFG GTP binds it contacts the factor binding center of the large subunit which stimulates GTP hydrolysis. GTP hydrolysis changes the conformation of EFG with two consequences. First interaction between EFG GTP and the ribosome are thought to unlock the ribosome. Structural studies reveal that there are gates that separate the A, P and E sites and EFG GTP is said to unlock the ribosome by opening these gates. Second, the changed EFG GTP conformation binds to the A site of the decoding center. This interaction competes with the TRNA for binding to the A site of the decoding center. Because the ribosome is unlocked, the formerly A site TRNA can move into the P site allowing EFG GTP to bind the A site. Since the ribosome is unlocked, the gates are open between A and P site, P and E site. This means that the A site can easily move to the P site and EFG GTP will be attached to the A site. Completion of translocation is accompanied by a clockwise rotation of the small subunit back to its starting position. When the translocation is complete, the small subunit will be reversed. This means that the translocation was counterclockwise rotated and the translocation is completed. The resulting ribosome structure has dramatically reduced the affinity for EFG GTP. Now, when the ribosome will arrive in the real state or starting position, the affinity for EFG GTP will be greatly reduced. This means that the EFG GTP will be released. The release of EFG results in the return of the ribosome to a locked state in which the TRNAs and MRNA are once again tightly associated with the small subunit decoding center and the gates between A, P and E sites are closed. The gate which had been opened and the ribosome was unlocked, now due to the return of the small subunit and due to the release of the EFG, the gates are closed and the ribosome is locked again. Together, these events result in the translocation of the A site TRNA into the P site, the P site TRNA into the E site and the movement of MRNA by exactly 3 BA spares. This is the end of the translocation. The ribosome is now ready for a new cycle of amino acid addition to begin.