Specificity and properties of the destabilization, induced by initiation factor IF-3, of ternary complexes of the 30-S ribosomal subunit, aminoacyl-tRNA and polynucleotides.

Initiation factor IF-3 causes the destabilization of preformed ternary complexes of 30-S ribosomal subunit, codons and aminoacyl-tRNAs or peptidyl-tRNA. This destabilization is dilution-dependent and affects all ternary complexes with the exception of those containing the initiator fMet-tRNA, which remain more resistant to IF-3-induced destabilization under the various conditions studied. Several possible reasons for this specificity have been examined. It was found that the basis for the specificity is not: (a) an intrinsic greater stability of the ternary complexes containing fMet-tRNA, (b) the amoung of aminoacyl-tRNA bound to the ribosome, (c) the conditions under which the ternary complex is made or (d) the formylation of the amino group. On the other hand, the nature of the polynucleotide in response to which the ternary complex is formed was found to influence the amount of aminoacyl-tRan bound to the ribosome, and to some extent the amount of aminoacyl-tRNA which can be relased. The ternary complex containing the mischarged initiator tRNA fVal-tRNAfMet displays greater resistance to the IF-3-induced destabilization than the complex containing fVal-tRNAVal. These results indicate that the specificity of the IF-3 activity is due to the special structural feature of the initiator tRNA molecule and to some extent to the nature of the polynucleotide. The IF-3-induced destabilization of ternary complexes was found to be little affected by variations in reaction conditons, so that this IF-3 activity can be used to measure the stoichiometric binding of IF-3 to the ribosome over a broad range of pH and K+ and Mg2+ concentrations. Several antibiotics have been tested for their capacity to interfere with this reaction; only high concentrations of tetracycline blocked this IF-3 activity.