Molecular Morphology of Ribosomes

The effect of chain initiation factor 3 (IF-3) and ribosomal protein S1 on ribosome conformation was measured by alterations in the reactivity of individual ribosomal proteins to lactoperoxidasecatalyzed iodination. Escherichia coli ribosomes, under ionic conditions conducive to the binding of IF-3 and S1, were enzymatically iodinated in the presence and absence of IF-3 and/or S1. Our results demonstrate that the binding of ribosomal protein S1 to isolated 30-S subunits induces a conformational change in the small subunit as reflected by the altered reactivity of proteins S3, S7, S12, S19 and S21 to enzymatic iodination. We have also previously shown [FEBS Lett. 71, 347–350 (1976)] that the binding of IF-3 to isolated 30-S subunits alters the reactivity of ribosomal proteins S2, S5, S8, S11 and S17. These data suggest that the factor does not influence 30-S subunit conformation in the same manner as ribosomal protein S1. The binding of IF-3 or S1 to 70-S ribosomes results in the same conformational change to 30-S subunits as that obtained with isolated 30-S subunits. This effect is additive when both proteins are bound to the 70-S particle. Ribosomal protein S1 does not appear to influence 50-S conformation. However, the binding of IF-3 to 50-S and 70-S particles results in the altered reactivity of several 50-S ribosomal proteins, the most prominent of which is L10. Our results suggest that IF-3 interacts with 30-S, 50-S and 70-S particles, and that part of the 70-S binding site of the factor is in the region of L7/L12 on the 50-S subunit.