Mechanism of codon recognition by transfer RNA and codon-induced tRNA association

The steps of UUC recognition by tRNAPhe were analysed by temperature-jump measurements. At ion concentrations close to physiological conditions we found three relaxation processes, which we assigned to (1) formation of codon-anticodon complexes, (2) a conformational change of the anticodon loop coupled with Mg2+ binding, and (3) codon-induced association of tRNA. The relaxation data were evaluated both by the usual procedure (fitting the exponentials evaluated from the individual experiments of a set to a reaction model) and by "global fitting", i.e. fitting a set of relaxation curves obtained at various concentrations directly to a reaction model, thus leaving out the intermediate exponential fitting step. The data can be represented quantitatively by a three-step model: the codon binds to the anticodon at a rate of 4 X 10(6) to 6 X 10(6) M-1S-1 as is usual for the formation of oligomer helices; the conformation change of the anticodon loop is associated with inner sphere complexation of Mg2+ at a rate of 10(3) S-1; the codon-tRNA complexes form dimers at a rate of 5 X 10(6) to 15 X 10(6) M-1S-1. A similar mechanism is found for the binding of the wobble codon UUU to tRNAPhe at increased concentrations of Mg2+. Measurements at different Mg2+ concentrations demonstrate the distinct role of this ion in the codon recognition and the codon-induced tRNA dimerization. We propose a simple mechanism, based upon the special properties of magnesium ions, for long-distance transfer of reaction signals along nucleic acid chains.