The binding of ribosomal protein Si to Si-depleted 30S and 70S ribosomes. A fluorescence anisotropy study of the effects of Mg\u3csup\u3e2+\u3c/sup\u3e
We have determined the equilibrium constants for the binding of AEDANS-labelled Si to Sl-depleted 30S and 70S ribosomes. For “tight” ribosomes, the association of S1 increases with the sixth power of Mg2+ concentration, but for 30S subunits and loose ribosomes, there is virtually no dependence of the association on Mg2+ over the same concentration range, 2-1O mM in Mg2+. The binding of S1 to 70S ribosomes at 10 mM Mg is stabilized by 2 kcal/mol compared to the binding to 30S subunits. When intact Si binds to tight ribosomes, the fluorescence anisotropy is that for virtually complete rotational immobilization. The anisotropies vary considerably with the preparation and treatment of both Si and ribosomes and these variations are detailed here. The results suggest the linkage of Mg2+-dependent conformational changes in the i
