Crystal structure of an initiation factor bound to the 30S ribosomal subunit

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