title: Allosteric interactions between the ribosomal transfer RNA-binding sites A and E.
abstract: We have previously proposed a three-site model for the elongation cycle. The model is characterized by the presence of two tRNAs on the ribosome before and after translocation. We have already shown a first consequence of the model, namely that the translocation reaction is not coupled with a release of deacylated tRNA. Here we demonstrate the following conclusions. Occupation of the A site triggers the tRNA release from the E site, i.e. the A site occupation induces a drastic decrease in the affinity of the E site for deacylated tRNA. In the concentration range of deacylated tRNA in which a ribosome binds a second tRNA in addition to that one already present at the P site the deacylated tRNA does not compete for one and the same binding site with an A site ligand (AcPhe-tRNA) at 37 degrees C. It follows that the second deacylated tRNA binds to a site, the E site, which is physically distinct from the A site. When the ribosome binds a deacylated tRNA at the E site (in addition to a tRNA at the P site), the A site cannot be occupied by AcPhe-tRNA at 0 degree C and only poorly by the ternary complex elongation factor Tu . Phe-tRNA . guanyl-5'-yl imidodiphosphate. At 37 degrees C a significant A site binding is observed, with a corresponding tRNA release from the E site. In contrast, if the E site is free and only the P site occupied, the A site can bind significant amounts of charged tRNA already at 0 degree C. It follows that an occupied E site induces a low-affinity state of the A site. Thus, the ribosome always contains two high-affinity binding sites, which are A and P sites before and P and E sites after translocation. A and E sites are allosterically linked in a bidirectional manner.
mesh_terms: D000215:Acylation; D000495:Allosteric Site; D004926:Escherichia coli / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000378:metabolism; D006165:Guanylyl Imidodiphosphate / Q000378:metabolism; D010441:Peptide Chain Elongation, Translational; D010444:Peptide Elongation Factor Tu / Q000378:metabolism / Q000494:pharmacology; D012329:RNA, Bacterial / Q000378:metabolism*; D012343:RNA, Transfer / Q000378:metabolism*; D012346:RNA, Transfer, Amino Acyl / Q000378:metabolism; D012270:Ribosomes / Q000378:metabolism*
chemical_list: D012329:RNA, Bacterial; D012346:RNA, Transfer, Amino Acyl; D006165:Guanylyl Imidodiphosphate; D006160:Guanosine Triphosphate; D012343:RNA, Transfer; D010444:Peptide Elongation Factor Tu