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Further studies with labelled polypeptide-chain-initiation factor 3 (IF-3).

The preparation of 35S-labelled initiation factor 3 (IF-3) from Escherichia coli is described. As reported earlier, labelled IF-3 binds to 30-S, but not to 50-S ribosomal subunits or 70-S ribosomes, and is released upon formation of a 70-S couple. In continuation of this work we find that the 30-S binding of IF-3 is rather sensitive to aurin-tricarboxylic acid, a powerful inhibitor of polypeptide chain initiation, but relatively insensitive to streptomycin. IF-3 binds weakly to both 23-S and 16-S ribosomal RNA and to 23-S but not 43-S ribosomal cores, Free IF-3 readily exchanges with the 30-S-bound factor even at low temperature. There is also an exchange of 50-S or 30-S ribosomal subunits labelled with [3H]uracil into 70-S ribosomes in the absence of the reactions of protein synthesis. This exchange is markedly temperature-dependent and strongly inhibited at high (20 mM) Mg2+ concentrations.