Domain III of elongation factor G from Thermus thermophilus is essential for induction of GTP hydrolysis on the ribosome

Two elongation factors (EF) EF-Tu and EF-G participate in the elongation ph ase during protein biosynthesis on the ribosome, Their functional cycles de pend on GTP binding and its hydrolysis. The EF-Tu complexed with GTP and am inoacyl-tRNA delivers tRNA to the ribosome, whereas EF-G stimulates translo cation, a process in which tRNA and mRNA movements occur in the ribosome. I n the present paper we report that: (a) intrinsic GTPase activity of EF-G i s influenced by excision of its-domain III; (b) the EF-G lacking domain III has a 10(3)-fold decreased GTPase activity on the ribosome, whereas its af finity for GTP is slightly decreased; and (c) the truncated EF-G does not s timulate translocation despite the physical presence of domain TV, which is also very important for translocation, By contrast, the interactions of th e truncated factor with GDP and fusidic acid-dependent binding of EF-G GDP complex to the ribosome are nest influenced. These findings indicate an ess ential contribution of domain III to activation of GTP hydrolysis. These re sults also suggest conformational changes of the EF-G molecule in the cours e of its interaction with the ribosome that might be induced by GTP binding and hydrolysis.