The relative contributions of the three domains of elongation factor Tu (EF-Tu) to the factor's function and thermal stability were established by dissecting the domains apart with recombination techniques. Domain 1 (EF-Tu'), domains 1/11 (EF-Tu'"') and domain 111 (EF-Tu"') of the EF-Tu from Thermus thermophilus HB8 comprising the amino acids 1-211, 1-312 and 317-405, respectively, were overproduced in Escherichia coli and purified. A polypeptide consisting of domain I1 and I11 (EF-Tu""") was prepared by limited proteolysis of native EF-Tu with V8 protease from Staphylococcus aureus [Peter, M. E., Reiser, C. 0. A,, Schirmer, N. K., Kiefhaber, T., Ott, G., Grillenbeck, N. W. & Sprinzl, M. (1990) Nucleic Acids Rex 18, 6889-68931. As determined by circular dichroism spectrometry, the isolated domains have the secondary structure elements found in the native EF-Tu. GTP and GDP binding as well as GTPase activity are maintained by the EF-Tul and EF-Tu""; however, the rate of GDP dissociation from EF-Tu' . GDP and EF-Tu"" . GDP complex is increased as compared to native EF-Tu . GDP, reflecting a constraint imposed by domain 111 on the ability to release the nucleotide from its binding pocket located in domain I. A weak interaction of Tyr-tRNATy' with the EF-Tu' . GTP suggests that domain I provides a part of the structure interacting with aminoacyl-tRNA. The domain 111 is capable of regulating the rate of GTPase in EF-Tu, since the polypeptide consisting only of domains 1/11 has a 39-fold higher intrinsic GTPase compared to the native EF-Tu. No in vitro poly(U)-dependent poly(Phe) synthesis was detectable with a mixture of equimolar amounts of domains I/II and domain 111, demonstrating the necessity of covalent linkage between the domains of EF-Tu for polypeptide synthesis. In contrast to native EF-Tu and EFTu""", EF-Tu' and, to a lesser extent the polypeptide consisting of domains ILI, are unstable at elevated temperatures. This indicates that domains II/III strongly contribute to the thermal stability of this 7: thermophilus EF-Tu. Deletion of amino acid residues 181 -190 from domain I of 7: thermophilus EF-Tu decreases the thermostability to that of EF-Tu from E. coli, which does not have these residues. lnterdomain interactions must be important for the stabilisation of the structure of domain I, since isolated 7: thermophilus EF-Tu' is thermolabile despite the presence of the 181 -190 loop. Nucleotide exchange factor Ts (EF-Ts), which promotes the dissociation of the nucleotide from native EF-Tu, does not accelerate the nucleotide exchange from EF-Tu' . GDP and EF-Tu"" . GDP polypeptides.