Dependence of tRNA structure in solution upon ionic condition of the solvent. Fluorescence studies of monovalent cation binding to tRNAPhe from barley embryos.

Dependence of barley phenylalanine tRNA (tRNAPhe) fluorescence intensity at 430 nm upon LiCl, NaCl, KCl, CsCl or NH4Cl concentration was measured in 0.01 M Tris-HCl, pH 7.5, 0.001 M Na2EDTA solutions. Increase of monovalent cation concentration in the solvent from 0 to 2 M induced about 3-fold fluorescence intensity enhancement. The fractional fluorescence change was used as a measure of bound ligand concentration. Fluorescence Scatchard plots revealed three classes of monovalent cation binding sites on the tRNA molecule: interacting (strong) and independent (weak and very weak) sites. Calculated from Scatchard plots binding constants (K), for strong and weak binding of monovalent cations (in the case of Na+ binding: Ks = 26 M-1 and Kw = 4.3 M-1 respectively) exhibit linear dependence upon ionic radius (r). Two limiting values obtained from the plot of K versus r: K(max) at r = 0 r(max) at K = 0, characterize additionally strong and weak monovalent cation binding sites (Ks(max) = 42 M-1 Kw(max) = 8.5 M-1, rs(max) = 0.23 nm and rw(max) = 0.22 nm). A model of the relationship between weak Mg2+ binding sites and monovalent cation binding sites as well as of monovalent cations binding to tRNA is proposed.