Ribosomal subunit interaction as studied by light scattering. Evidence of different classes of ribosome preparations.

The Mg2+-dependent equilibrium of ribosomal subunits was studied by light scattering in the absence of any other factor of protein synthesis. The difference of intensity in the light scattered by dissociated 30S-50S couples and by 70S particles was used as a method to measure the percentage of association as a function of Mg2+ under conditions of nonperturbation of the equilibrium. We found that pressure-resistant (type A) and non-pressure-resistant (type B) ribosomes may be characterized by the different behavior of their association equilibrium curves. The thermodynamic parameters of this equilibrium and their comparison for the two classes of ribosomes were calculated from experiments at different temperatures.