Reversible dissociation of wheat germ ribosomal subunits: cation-dependent equilibria and thermodynamic parameters

The influence of magnesium ion concentration upon the equilibrium between the wheat germ ribosome and its subunits has been studied by light scattering. The curves obtained for ribosome dissociation and subunit reassociation were identical and were independent of the origin and direction of the magnesium ion titration, suggesting that the wheat germ ribosomes are behaving as a homogeneous population equivalent to Escherichia coli type A ribosomes. Increasing K+ concentrations in the presence of Mg2+ favored ribosome dissociation. Polyamines favored subunit reassociation, with 0.1 mM spermine acting as effectively as 1 mM spermidine. Thermodynamic parameters have been determined from the temperature-dependent equilibria and have been compared to those of E. coli type A ribosomes. The association of the 40S and 60S subunits is exothermic. At 1.5 mM Mg2+ and 100 mM K+, the entropy term is negative, favoring ribosome dissociation, and contributes less to the free energy than the enthalpy term. The determination of these thermodynamic parameters was based on molecular weights of 1.2 x 10(6) and 2.3 x 10(6) for the 40S and 60S subunits, respectively, calculated from laser light scattering data.