The function of the translating ribosome: allosteric three-site model of elongation.
During the last decade, a new model for the ribosomal elongation cycle has emerged. It is based on the finding that eubacterial ribosomes possess 3 tRNA binding sites. More recently, this has been confirmed for archaebacterial and eukaryotic ribosomes as well, and thus appears to be a universal feature of the protein synthetic machinery. Ribosomes from organisms of all 3 kingdoms harbor, in addition to the classical P and A sites, an E site (E for exit), into which deacylated tRNA is displaced during translocation, and from which it is expelled by the binding of an aminoacyl-tRNA to the A site at the beginning of the subsequent elongation round. The main features of the allosteric 3-site model of ribosomal elongation are the following: first, the third tRNA binding site is located 'upstream' adjacent to the P site with respect to the messenger, ie on the 5'-side of the P site. Second, during translocation, deacylated tRNA does not leave the ribosome from the P site, but co-translocates from the P site to the E site--when peptidyl-tRNA translocates from the A site to the P site. Third, deacylated tRNA is tightly bound to the E site in the post-translocational state, where it undergoes codon--anticodon interaction. Fourth, the elongating ribosome oscillates between 2 main conformations: (i), the pre-translocational conformer, where aminoacyl-tRNA (or peptidyl-tRNA) and peptidyl-tRNA (or deacylated tRNA) are firmly bound to the A and P sites, respectively; and (ii), the post-translocational conformer, where peptidyl-tRNA and deacylated tRNA are firmly bound to the P and E sites, respectively. The transition between the 2 states is regulated in an allosteric manner via negative cooperatively. It is modulated in a symmetrical fashion by the 2 elongation factors Tu and G. An elongating ribosome always maintains 2 high-affinity tRNA binding sites with 2 adjacent codon--anticodon interactions. The allosteric transition from the post- to the pre-translocational state is involved in the accuracy of aminoacyl-tRNA selection, and the maintenance of 2 codon--anticodon interactions helps to keep the messenger in frame during translation.
Rheinberger
H J
HJ
Max-Planck-Institut für Molekulare Genetik, Abteilung Wittmann, Berlin-Dahlem, Germany.
D000495:Allosteric Site; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D001483:Base Sequence; D008954:Models, Biological; D008969:Molecular Sequence Data; D010441:Peptide Chain Elongation, Translational; D014176:Protein Biosynthesis; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D012343:RNA, Transfer / Q000378:metabolism; D012270:Ribosomes / Q000187:drug effects / Q000378:metabolism*
D000900:Anti-Bacterial Agents; D012333:RNA, Messenger; D012343:RNA, Transfer