Solution structure of the ribosome-binding domain of E. coli translation initiation factor IF3. Homology with the U1A protein of the eukaryotic spliceosome.

Initiation of translation in prokaryotes requires the formation of a complex between the messenger RNA, the 30 S ribosomal subunit and the initiator tRNA(fMet). Initiation factor IF3 binds to the 30 S ribosomal subunit and proof-reads the initiation complex, thereby ensuring the accuracy of this step. IF3 also plays a pleiotropic role in the regulation of translation, as a result of differential influences exerted on the levels of the initiation of translation of genes or groups of genes. IF3 is composed of two independent domain or roughly identical sizes. We have expressed and purified the C-terminal domain of E. coli IF3 and shown that it retains both the 30 S particle binding and 70 S ribosome dissociating activities of the native protein. We have obtained 1H and 15N NMR resonance assignments and its 3D solution structure was calculated using 551 restraints. It is composed of a mixed beta-sheet backed by two alpha-helices. It shows a striking resemblance to the U1A small nuclear ribonucleoprotein structure, which binds to the U1 snRNA in the eukaryotic spliceosome. This suggests a convergent evolution process for these two proteins that are associated with ribonucleoproteic complexes.