title: 70-S ribosomes of Escherichia coli have an additional site for deacylated tRNA binding.
abstract: Escherichia coli 70-S ribosomes contain a third site for tRNA binding, additional to the A and P sites. This conclusion is based on several findings. Direct measurements showed that in the presence of poly(U), when both A and P sites are occupied by Ac[14C]Phe-tRNAPhe, ribosomes are capable of binding additionally deacylated non-cognate [3H]tRNA. If ribosomes in the preparation are active enough, the total binding of labeled ligands amounted to 2.5 mol/mol ribosomes. In the absence of poly(U), when the A site can not bind, the P site and the 'additional' site can be filled simultaneously with Ac[14C]Phe-tRNAPhe and deacylated [3H]tRNA, or with [3H]tRNA alone; the total binding exceeds in this case 1.5 mol/mol ribosomes. The binding at the 'additional' site is not sensitive to the template. [3H]tRNA bound there is able to exchange rapidly for unlabeled tRNA in solution. Deacylated tRNA is preferred to the aminoacylated one. The binding of AcPhe-tRNAPhe was not observed there at all. The 3'-end adenosine is essential for the affinity. The function of the 'additional' site is not known, but its existence has to be considered when tRNA . ribosome complexes are studied.
mesh_terms: D000215:Acylation; D001665:Binding Sites; D004926:Escherichia coli / Q000378:metabolism*; D007700:Kinetics; D011072:Poly U / 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; C006383:tRNA, N-acetylphenylalanine-; D011072:Poly U; D012343:RNA, Transfer