Codon-induced tRNA association: quantitative analysis by sedimentation equilibrium

It is shown by measurements of the sedimentation equilibrium that binding of the codon UUC to tRNAPhe from yeast induces an association of tRNA molecules. Sedimentation measurements at different concentrations demonstrate that the tRNA-codon complexes form dimers. The sedimentation profiles are analyzed quantitatively in terms of a simple monomer-dimer model as well as a model which considers the sedimentation of four species (tRNA, tRNA.UUC, (tRNA.UUC)2, and UUC) separately. The information resulting from the conservation of mass relation is used directly in the determination of equilibrium constants via integration o the sedimentation profiles. Using this procedure, we determine the equilibrium constants for dimerization of the tRNAPhe.UUC complex, KD = 8.6 x 10(4) M-1, and for the binding of UUC to tRNAPhe, KL = 1800 M-1 (5 degrees C). The large free energy for dimerization of the tRNA-codon complex suggests that interactions between adjacent tRNAs are important for the ribosomal translation process.