Initiation factor IF-3: a specific inhibitor of ribosomal subunit association

At Mg²⁺ concentrations optimal for protein synthesis on natural messenger UNA, initiation factor IF-3^† effectively inhibits the association of 30 s and 50 s ribosomal subunits but does not cause the dissociation of single ribosomes. The rate of exchange between single ribosomes and ribosomal subunits is very slow at these concentrations of Mg²⁺, but increases markedly at lesser concentrations of Mg²⁺. The net dissociation of single ribosomes observed at low concentrations of Mg²⁺ in the presence of IF-3 is not stimulated by this factor, but proceeds only to the extent of spontaneous dissociation of single ribosomes, with IF-3 subsequently intercepting the ribosomal subunits. The inhibition of ribosomal subunit association is displayed by highly purified IF-3, but not by IF-1 or IF-2. It can be overcome in the presence of 0.01 mspermidine, but not by putrescine. Ribosomal subunit association also is inhibited by tetracycline; this inhibition likewise is overcome by spermidine.