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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.

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SUMMARYThe 32000 mol. wt. (32K) non-structural protein of alfalfa mosaic virus, P3, has previously been detected in a crude membrane fraction of infected tobacco leaves, where it accumulated transiently at the beginning of the infection period. We show here, by immunoblotting with an antiserum to a synthetic peptide corresponding to the C terminus of the protein, that the majority of P3 is in the cell wall fraction where it remains throughout infection, both at 25 °C and at 10 °C. The cell wall-associated, P3-related material is heterogeneous and contains polypeptide species of slightly lower electrophoretic mobility than the major in vitro translation product of RNA 3, which suggests that P3 may be post-translationally modified.