Binding of spermidine to transfer ribonucleic acid

Abstract

The binding of spermidine to yeast tRNAPhe and Escherichia coli tRNAGlu at low and high ionic strength was studied by equilibrium dialysis. Once corrected for the expected Donnan effect, the binding at low ionic strength obeys the simple relationship of equivalent binding sites, and cooperative binding of spermidine to tRNA could not be detected. At low ionic strength (0.013 M Na+ ion), tRNAPhe (yeast) had 13.9 .+-. 2.3 strong spermidine binding sites per molecule with Kd = 1.39 .times. 10-6 M and a few weak spermidine binding sites which were inaccessible to experimentation; tRNA2Glu (E. coli) has 14.8 .+-. 1.6 strong spermidine binding sites and 4.0 .+-. 0.1 weak spermidine binding sites with Kd = 1.4 .times. 10-6 M and Kd = 1.23 .times. 10-4 M, respectively. At high ionic strength (0.12 M monovalent cation) and 0.01 M Mg2+, tRNAPhe (yeast) has .apprx. 13 strong spermidine binding sites with an apparent Kd = 3.4 .times. 10-3 M while the dimeric complex tRNAPhe.cntdot.tRNA2Glu has 10.4 .+-. 1.2 strong spermidine binding sites per monomer with an apparent Kd = 2.0 .times. 10-3 M. In the presence of increasing Na+ ion or K+ ion concentration, spermidine binding data do not fit a model for competitive binding to tRNA by monovalent cations. Rather, analysis of binding data by the Debye-Huckel approximation results in a good fit of experimental data, indicating that monovalent cations form a counterion atmosphere about tRNA that decreases electrostatic interactions. On the basis of equilibrium binding analyses, it is proposed that the binding of spermidine to tRNA occurs predominantly by electrostatic forces.