Fluorimetric study of the complex between yeast phenylalanyl-tRNA synthetase and tRNA-Phe. 2. Evidence for an asymmetric behaviour of the enzyme.

The variations of several spectroscopic properties of yeast tRNA-Phe and phenylalanyl-tRNA synthetase upon complex formation, were used to study the stoichiometry of the complex in different experimental conditions. In all cases, for the tRNA-Phe-enzyme complex, in the absence of other ligands, the saturations of the different conformational changes monitored for both macromolecules, are achieved at a 2:1 tRNA/enzyme stoichiometry. Phenylalanine does not modify this saturation. In contrast, the presence of 1 mM ATP induces an asymmetric behaviour of the synthetase: two tRNAs are still bound per enzyme molecule but the conformational change of the latter is completed upon binding of a single tRNA molecule.