Selection of the initiator tRNA by Escherichia coli initiation factors.

We have developed a new technique, called 'toeprinting,' which has allowed a study of the tRNA-binding properties of Escherichia coli translation initiation complexes. In response to natural mRNAs, the initiator tRNA and a variety of elongator tRNAs bind to the same tRNA-binding site on the 30S ribosomal subunit as long as a cognate codon is present near the Shine and Dalgarno sequence. The selection of the initiator tRNA in 30S initiation complexes is accomplished by initiation factors IF2 and IF3. 70S ribosomes accept both initiator tRNA and elongator tRNAs on natural mRNAs, much like 30S ribosomal subunits; IF3 and IF2 do not, however, select the initiator tRNA on 70S initiation complexes unless the initiation factor IF1 is present.