Reactivity of ribosomal sulfhydryl groups in 30S ribosomal subunits of Escherichia coli and 30S-IF-3 complexes.

The reaction of 30S subunits with the SH group reagent N-ethylmaleimide (NEM) causes the loss of approximately 60% of their synethetic activity, but has little or no effect on the ribosomal binding of initiation factor IF-3. The ribosomal binding of this factor, on the other hand, was found to significantly influence the rate and the extent to which several 30S ribosomal proteins react with radioactively labeled NEM when the reaction kinetics of individual ribosomal proteins toward NEM were compared in 30S and 30S-IF-3 complexes. Of the nine 30S ribosomal proteins which react with NEM, proteins S1, S11, S12, and S18 were found to have lower reactivities, while proteins S4 and S21 displayed higher reactivity in the presence of IF-3. The reactivity of proteins S8, S13, and S17, on the other hand, appeared to be influenced little or not at all by the presence of the factor. These results are interpreted as supporting evidence for the premise that the binding of IF-3 results in a conformational change of the 30S subunit.