The effect of aminoacylation on the structure of yeast phenylalanine tRNA was evaluated by laser light scattering. In these experiments, the translational diffusion coefficient (D20,w) of phenylalanyl-tRNA was monitored continuously during spontaneous deacylation in a variety of solution conditions. The results reveal that significant changes can occur in the hydrodynamic volume and electric charge as a consequence of aminoacylation but that the effects are magnesium dependent. At neutral pH, 20 degrees C, and 0.1 M salt, the D20,w value increased by 18% when deacylation occurred in 2--10 mM Mg2+ concentrations while no change in diffusivity was observed for tRNA deacylating in 0.5--1.0 mM Mg2+. The Mg2+ concentration dependence of the D20,w changes behaves in highly cooperative manner. The electric charges of aminoacyl-tRNA and nonacylated tRNA in 1 and 10 mM Mg2+ were estimated from the diffusive virial coefficients. In the higher Mg2+ conditions, aminoacyl-tRNA has a charge of 15 +/- 2e- while that of the nonacylated form is 10 +/- 2e-; both acylated and nonacylated tRNA have a charge of 11 +/- 4e- in 1 mM Mg2+. Taken together, the results indicate that aminoacylation permits the binding of additional Mg2+, resulting, in turn, in the formation of a more extended conformer of lower diffusivity and greater negative charge. The results also provide a possible explanation for several contradictory results in the literature.