Fluorimetric Study of Yeast tRNA Phe CCF in the Complex with Phenylalanyl‐tRNA Synthetase

The fluorescence properties of yeast tRNA Phe CCF (tRNA Phe in which the 3′‐terminal adenosine has been replaced by formycin) and tRNA Phe CCFoxi‐red after periodate oxidation followed by borohydride reduction) were studied in the complex with the cognate ammoacyl‐tRNA synthetase. In both cases a conformational change affecting the 3′ end was observed in a magnesium concentration range close to 1 mM. The modification of formycin fluorescence could be ascribed simultaneously to the existence of a tautomeric equilibrium of the fluorescent probe and to a pH effect arising from a prototropic effect at the active site of phenylalanyl‐tRNA synthetase, and to a partial destacking of the 3′‐formycin from the adjacent C residue. The observed transconformation, which can be related to the structure modification of the anticodon loop previously reported [Ehrlich, Lefevre, and Remy (1980) Eur. J. Biochem. 103 , 145–153], takes place in the magnesium concentration range allowing the transfer of the activated amino acid from the adenylate to the tRNA. The interconnection between the anticodon loop and the accepting end was further supported by the observation that wybutine excision hinders the specific structure modification of 3′‐formycin upon binding to the synthetase. The tRNA Phe transconformations occurring in the complex with the cognate synthetase probably reflect a reciprocal adaptation of both macromolecules which might lead to the optimal aminoacylation velocity and thus contribute to the specificity of aminoacylation. since it was previously established that this specificity relies more strongly on the kinetics of the reaction than on a discrimination of tRNAs according to different affinities [Ebel et al. (1973) Biochimie Paris 55 , 547–557].