Crystal Structure of an Initiation Factor Bound to the 30 S Ribosomal Subunit

Initiation of translation at the correct position on messenger RNA is essential for accurate protein synthesis. In prokaryotes, this process requires three initiation factors: IF1, IF2, and IF3. Here we report the crystal structure of a complex of IF1 and the 30 S ribosomal subunit. Binding of IF1 occludes the ribosomal A site and flips out the functionally important bases A1492 and A1493 from helix 44 of 16 S RNA, burying them in pockets in IF1. The binding of IF1 causes long-range changes in the conformation of H44 and leads to movement of the domains of 30 S with respect to each other. The structure explains how localized changes at the ribosomal A site lead to global alterations in the conformation of the 30 S subunit.