EF-G (elongation factor G, historically known as translocase) is a prokaryotic elongation factor involved in protein translation. As a GTPase, EF-G catalyzes the movement (translocation) of transfer RNA (tRNA) and messenger RNA (mRNA) through the ribosome. Structure Encoded by the fusA gene on the str operon, EF-G is made up of 704 amino acids that form 5 domains, labeled Domain I through Domain V. Domain I may be referred to as the G-domain or as Domain I(G), since it binds to and hydrolyzes guanosine triphosphate (GTP). Domain I also helps EF-G bind to the ribosome, and contains the N-terminal of the polypeptide chain. Domain IV is important for translocation, as it undergoes a significant conformational change and enters the A site on the 30S ribosomal subunit, pushing the mRNA and tRNA molecules from the A site to the P site. The five domains may be also separated into two super-domains. Super-domain I consists of Domains I and II, and super-domain II consists of Domains III - IV. Throughout translocation, super-domain I will remain relatively unchanged, as it is responsible for binding tightly to the ribosome. However, super-domain II will undergo a large rotational motion from the pre-translocational (PRE) state to the post-translocational (POST) state. Super-domain I is similar to the corresponding sections of EF-Tu. Super-domain II in the POST state mimics the tRNA molecule of the EF-Tu • GTP • aa-tRNA ternary complex. EF-G on the ribosome Binding to L7/L12 L7/L12 is only a multicopy protein on the large ribosomal subunit of the bacterial ribosome that binds to certain GTPases, like Initiation Factor 2, Elongation factor-Tu, Release Factor 3, and EF-G. Specifically, the C-terminal of L7/L12 will bind to EF-G and is necessary for GTP hydrolysis. Interaction with the GTPase Associated Center The GTPase Associated Center (GAC) is a region on the large ribosomal subunit that consists of two smaller regions of 23S ribosomal RNA called the L11 stalk and the sarcin-ricin loop (SRL). As a highly conserved rRNA loop in evolution, the SRL is critical in helping GTPases bind to the ribosome, but is not essential for GTP hydrolysis. There is some evidence to support that a phosphate oxygen in the A2662 residue of the SRL may help hydrolyze GTP. Function in protein elongation EF-G catalyzes the translocation of the tRNA and mRNA down the ribosome at the end of each round of polypeptide elongation. In this process, the