Location of translational initiation factor IF3 on the small ribosomal subunit

The location of translational initiation factor IF3 bound to the 30S subuni t of the Thermus thermophilus ribosome has been determined by cryoelectron microscopy, Both the 30S IF3 complex and control 30S subunit structures,wer e determined to 27-Angstrom resolution. The difference map calculated from the two reconstructions reveals three prominent lobes of positive density. The previously solved crystal structure of IF3 fits very well irate two of these lobes, whereas the third lobe probably arises from conformational cha nges induced in the 30S subunit as a result of IF3 binding. Our placement o f IF3 on the 30S subunit allows an understanding in structural terms of the biochemical functions of this initiation factor, namely its ability to dis sociate 70S ribosomes into 30S and 50S subunits and the preferential select ion of initiator tRNA by IF3 during initiation.