Effects of spermine and magnesium ions on the aminoacylation of yeast tRNA Tyr

Abstract

Stimulatory effects of Mg 2+ and spermine on the kinetics of the aminoaculation of tRNA Tyr were examined using purified yeast tRNA Tyr and tyrosyl-tRNA synthetase. The apparent K m for tRNA Tyr was the lowest at Mg 2+ concentrations between 2 and 5 mM and was not influenced by spermine. In the absence of spermine, the apparent V max was the highest at Mg 2+ concentrations of 5 mM or higher, whereas the presence of spermine strongly stimulated the reaction at lower Mg 2+ concentrations. Spermine alone could not substitute for Mg 2+, nor was it able, at any Mg 2+ concentration, to increase the reaction rate above the level reached at high concentrations of Mg 2+ alone. Calculations of the concentration of Mg 3·tRNA Tyr complex as a function of initial Mg 2+ concentration, using the binding constants derived from physical measurements, allow the conclusion that spermine exerts its stimulatory activity by creating strong binding sites for Mg 2+; this would enable the tRNA to assume the conformation required for optimal aminoacylation. The conformational requirement for the first tRNA: synthetase encounter is obviously less stringent, since the apparent K m for tRNA Tyr is not influenced by spermine.