Visualization of elongation factor Tu on the Escherichia coli ribosome

The delivery of a specific amino acid to the translating ribosome is fundamental to protein synthesis. The binding of aminoacyl-transfer RNA to the ribosome is catalysed by the elongation factor Tu (EF-Tu). The elongation factor the aminoacyl-tRNA and GTP form a stable 'ternary' complex that binds to the ribosome. We have used electron cryomicroscopy and angular reconstitution(1) to visualize directly the kirromycin-stalled ternary complex in the A site of the 70S ribosome of Escherichia coli. Electron cryomicroscopy had previously given detailed ribosomal structures at 25 and 23 Angstrom (refs 2, 3) resolution, and was used to determine the position of tRNAs on the ribosome(4,5). In particular, the structures' of pre-translocational (tRNAs in A and P sites) and post-translocational ribosomes (P and E sites occupied) were both visualized at a resolution of similar to 20 Angstrom. Our three-dimensional reconstruction at 18 Angstrom resolution shows the ternary complex spanning the inter-subunit space with the acceptor domain of the tRNA reaching into the decoding centre. Domain I (the G domain) of the EF-Tu is bound both to the L7/L12 stalk and to the 50S body underneath the stalk, whereas domain 2 is oriented towards the S12 region on the 30S subunit.