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The complex formation between elongation factor Tu (EF-Tu) . GTP and valyl-tRNA:"' has been investigated using the small-angle X-ray scattering titration technique. The main species observed is a 1 : 1 complex with a stability constant log K 2 6. The corresponding interaction between EF-Tu . GTP and non-aminoacylated tRNA appears to be much weaker with an estimated log K z 4.The radius of gyration determined for the EF-Tu . GTP -valyl-tRNA:"' complex is larger ( R = 3.6 nm) than that of EF-Tu . GTP ( R = 2.5 nm). Likewise, the maximum distance within this complex is larger (D,,, = 12.5 nm) than the one within EF-Tu I GTP (D,,, = 8.5 nm). These data as well as the p ( r ) curve are consistent with a multiellipsoid model for the complex. From this model it is indicated that the acceptor stem of tRNA is attached to EF-Tu and that the anticodon stem and loop protrude into the solution.During the chain-elongation cycle ofprotein biosynthesis in prokaryotes, a ternary complex involving the elongation factor Tu (EF-Tu), GTP and aminoacyl-tRNA is an obligatory intermediate in the binding of aminoacyl-tRNA to the ribosome. On the ribosome, G T P is cleaved to G D P and, as a result, the EF-Tu . G D P complex and Pi are released. Further interaction with the elongation factor, EF-Ts, and GTP are necessary in order to regenerate the ternary complex [I, 21. Although these elongation events are functionally understood, there is only limited information regarding the structures of the elongation factors and how they correlate with their functions.The recent analyses by X-ray crystallography of the trypsinmodified form of EF-TU . G T P represent encouraging progress in this aspect [3 -51. The results indicate that EF-Tu consists of two domains. The larger domain exhibits several features of secondary structure. The structure of the smaller domain, on the other hand, appears to be loose and poorly organized and the polypeptide chain can only be partially resolved at the present stage of resolution [3 -51. High-resolution structures of yeast tRNAPh" and Escherichia coli tRNAyet have been obtained by X-ray crystallography [6-81. However, so far the series of complexes involved in the elongation cycle and other events of the translation process have not been crystallized in a form amenable to X-ray crystallographic analysis. For structural information of these fairly complicated systems attention has therefore been focused on X-ray scattering and neutron scattering methods. These methods have the great advantage that the functionally active complexes can be studied in solution under conditions similar to those existing in the cell. We have previously studied the formation of a complex between aminoacyl-tRNA ligase and its cognate tRNA using X-ray scattering techniques [9]. In this paper we present the results of an X-ray scattering titration study of the complex formation between EF-Tu . GTP and valyl-tRNA\j"' from E. coli.Ahhreviurion. EF-Tu, elongation factor Tu; Hepes, 4-(2-hydroxyethy1)-1 -piperazineethanesulphonic acid.

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