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Electrostatic effects in divalent ion binding to tRNA

The binding of Mn2+ to yeast tRNAPhe at 25°C is measured by esr, and found to depend strongly on the concentration of monovalent cations, showing the importance of electrostatic effects. In low sodium (<15mM/l.), the affinity is high and the Scatchard plots are distinctly curved. In high sodium (>50mM/l.), the affinity and the curvature are reduced. In a limited range of sodium concentrations (15–30mM/l.), the folding of tRNA which is induced by the divalent ions results in cooperative binding, leading to upwards convexity of the Scatchard plot. An electrostatic model is developed, based on a single type of binding site which we take to be the phosphates, with a binding constant for Mn2+ in the range of that found for ApA, 10 l./M. We compute the change in the binding constant due to the electrostatic potential of the distant charges (other phosphates and counterions), using a single set of parameters for all sodium concentrations. The model predicts that the plots in low sodium are curved, and a good fit to the experimental results is obtained: it is therefore not legitimate or necessary to interpret these results in terms of two types of binding sites. In high salt, the model gives plots that are only slightly curved, corresponding to weaker electrostatic effects. This shows that a search for sites with a special binding mode should be done in high salt. The computed plots are in good agreement with the data, except for slight differences concerning the first bound ions, which give a possible indication in favor of special binding. Given the observation of one special site for Mg2+ at 4°C in high sodium [Stein, A. & Crothers, D. M. (1976) Biochemistry 15, 157–160] in E. coli tRNAfMet, we have measured the binding of Mn2+ at lower temperature. At 12°C, in both yeast tRNAPhe and E. coli tRNAfMet, the plots clearly indicate special binding. A site found in high sodium is on a very different footing from the four to six so‐called strong sites unduly derived from low‐salt binding plots.