Prokaryotic elongation factors
In prokaryotes, three elongation factors are required: EF-Tu, EF-Ts, and EF-G.
The prokaryotic factor EF-Tu mediates the entry of the aminoacyl tRNA into a free site of the ribosome. EF-Tu functions by binding an aminoacylated, or charged, tRNA molecule in the cytoplasm. This complex transiently enters the ribosome, with the tRNA anticodon domain associating with the mRNA codon in the ribosomal A site. If the codon-anticodon pairing is correct, EF-Tu hydrolyzes GTP into GDP and inorganic phosphate, and changes in conformation to dissociate from the tRNA molecule. The aminoacyl tRNA then fully enters the A site, where its amino acid is brought near the P-site polypeptide and the ribosome catalyzes the covalent transfer of the polypeptide onto the amino acid.
EF-Tu contributes to translational accuracy in three ways. It delays GTP hydrolysis if the tRNA in the ribosome’s A site does not match the mRNA codon, thus preferentially increasing the likelihood for the incorrect tRNA to leave the ribosome. It also adds a second delay (regardless of tRNA matching) after freeing itself from tRNA, before the aminoacyl tRNA fully enters the A site. This delay period is a second opportunity for incorrectly-paired tRNA (and their bound amino acids) to move out of the A site before the incorrect amino acid is irreversibly added to the polypeptide chain. A third mechanism is the less well understood function of EF-Tu to crudely check amino acid-tRNA associations, and reject complexes where the amino acid is not bound to the correct tRNA coding for it.
EF-Ts serves as the guanine nucleotide exchange factor for EF-Tu, catalyzing the release of GDP from EF-Tu. This enables EF-Tu to bind to a new GTP molecule, release EF-Ts, and go on to catalyze another aminoacyl tRNA addition.
The factor EF-G catalyzes the translocation of the tRNA and mRNA down the ribosome at the end of each round of polypeptide elongation. Homologous to EF-Tu + tRNA, EF-G also binds to the ribosome in its GTP-bound state. When it associates with the A site, EF-G causes the tRNA previously occupying that site to occupy an intermediate A/P position (bound to the A site of the small ribosomal subunit and to the P site of the large subunit), and the tRNA in the P site is shifted to a P/E hybrid state. EF-G hydrolysis of GTP causes a conformation change that forces the A/P tRNA to fully occupy the P site, the P/E tRNA to fully occupy the E site (and exit the ribosome complex), and the mRNA to shift three nucleotides down relative to the ribosome due to its association with these tRNA molecules. The GDP-bound EF-G molecule then dissociates from the complex, leaving another free A-site where the elongation cycle can start again.