THE CRYSTAL-STRUCTURE OF ELONGATION-FACTOR-G COMPLEXED WITH GDP, AT 2.7-ANGSTROM RESOLUTION

Elongation factor G (EF-G) catalyzes the translocation step of protein synthesis in bacteria, and like the other bacterial elongation factor , EF-Tu-whose structure is already known-it is a member of the GTPase superfamily. We have determined the crystal structure of EF-G-GDP from Thermus thermophilus. It is an elongated molecule whose large, N-term inal domain resembles the G domain of EF-Tu, except for a 90 residue i nsert, which covers a surface that is involved in nucleotide exchange in EF-Tu and other G proteins. The tertiary structures of the second d omains of EF-G and EF-Tu are nearly identical, but the relative placem ent of the first two domains in EF-G-GDP resembles that seen in EF-Tu- GTP, not EF-Tu-GDP. The remaining three domains of EF-G look like RNA binding domains, and have no counterparts in EF-Tu.