We have determined the equilibrium constants for the binding of AEDANS-labelled SI to Sl-depleted 30S and 70S ribosomes. For "tight" ribosomes,2+the association of SI increases with the sixth power of Mg concentration, but for 30S subunits and "loose" ribosomes, there is virtually no dependence of the association on Mg2+ over the same concentration range, 2-10 mM in Mg2+. The binding of SI to 70S ribosomes at 10 mM Mg2+ 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 intact ribosomes, perhaps including rRNA, and the binding of SI.
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