The Cryo-EM Structure of a Translation Initiation Complex from Escherichia coli

The 70S ribosome and its complement of factors required for initiation of translation in E. coli were purified separately and reassembled in vitro with GDPNP, producing a stable initiation complex (IC) stalled after 70S assembly. We have obtained a cryo-EM reconstruction of the IC showing IF2•GDPNP at the intersubunit cleft of the 70S ribosome. IF2•GDPNP contacts the 30S and 50S subunits as well as fMet-tRNAfMet. IF2 here adopts a conformation radically different from that seen in the recent crystal structure of IF2. The C-terminal domain of IF2 binds to the single-stranded portion of fMet-tRNAfMet, thereby forcing the tRNA into a novel orientation at the P site. The GTP binding domain of IF2 binds to the GTPaseassociated center of the 50S subunit in a manner similar to EF-G and EF-Tu. Additionally, we present evidence for the localization of IF1, IF3, one C-terminal domain of L7/L12, and the N-terminal domain of IF2 in the initiation complex.