Fluorimetric Study of the Complex between Yeast Phenylalanyl‐tRNA Synthetase and tRNA^Phe

Abstract

The interaction between yeast tRNA Phe and phenylalanyl‐tRNA synthetase was studied by monitoring different emission properties of the wybutine residue. The following results were established: (a) the isolated tRNA Phe exists in solution under at least two forms in a magnesium‐dependent equilibrium; (b) in the complex with the cognate synthetase, the tRNA still has two different conformations, slightly different from the above conformers. Mg 2+ ions appear to play a crucial role in the adaptation of both macromolecules one to another: for concentrations of the order of 1 mM, magnesium ions trigger a conformational change of the anticodon loop of tRNA, resulting in the expulsion of the wybutine from a stacked region. Furthermore, experimental evidence suggests that the anticodon region lies in a groove of the protein. This local conformation change, occurring in the anticodon loop, can be correlated to structural modifications of the enzyme as shown by the modification of circular dichroism and tryptophan fluorescence. During these different conformation changes, an energy transfer from tryptophan residues to wybutine is triggered in the critical magnesium concentration range.