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Based on the 1 H-15 N NMR spectroscopy data, the three-dimensional structure and internal dynamic properties of ribosomal protein L7 from Escherichia coli were derived. The structure of L7 dimer in solution can be described as a set of three distinct domains, tumbling rather independently and linked via flexible hinge regions. The dimeric N-terminal domain (residues 1-32) consists of two antiparallel ␣-␣-hairpins forming a symmetrical four-helical bundle, whereas the two identical C-terminal domains (residues 52-120) adopt a compact ␣/␤-fold. There is an indirect evidence of the existence of transitory helical structures at least in the first part (residues 33-43) of the hinge region. Combining structural data for the ribosomal protein L7/L12 from NMR spectroscopy and x-ray crystallography, it was suggested that its hinge region acts as a molecular switch, initiating "ratchet-like" motions of the L7/L12 stalk with respect to the ribosomal surface in response to elongation factor binding and GTP hydrolysis. This hypothesis allows an explanation of events observed during the translation cycle and provides useful insights into the role of protein L7/L12 in the functioning of the ribosome.

Elongation factor G (EF-G) is a G protein factor that catalyzes the translocation step in protein synthesis on the ribosome. Its GTP conformation in the absence of the ribosome is currently unknown. We present the structure of a mutant EF-G (T84A) in complex with the non-hydrolysable GTP analogue GDPNP. The crystal structure provides a first insight into conformational changes induced in EF-G by GTP. Comparison of this structure with that of EF-G in complex with GDP suggests that the GTP and GDP conformations in solution are very similar and that the major contribution to the active GTPase conformation, which is quite different, therefore comes from its interaction with the ribosome.

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Ultracentrifuge studies of intact protein L7/L12, of its fragments 27-120, 1-74 and 74-120 and of protein L7/L12 with oxidized methionine residues, indicate that the N-terminal sequence of the protein L7/L12 is responsible for its dimerization. The symmetry model of the dimer is discussed.Protein L7/L12 from Escherichia coli ribosomes is known to have a dimeric form in solution [l]. The peculiarities of the protein L7/L12 primary structure [2] permit the isolation of different derivatives convenient for studying interactions between the subunits. Three methionine residues (14, 17 and 26) can be oxidized by hydrogen peroxide into their sulfoxides [3]. Fragment 27-120 can be obtained by treatment with cyanogen bromide. The presence of the only arginine residue in position 73 allows the isolation of the fragments 1-73 and 74-120.Ultracentrifugation studies of the protein L7/L12 and its fragments led to the conclusion that the region responsible for its dimerization is located in its Nterminal sequence. MATERIALS AND METHODS The buffer solutions: buffer Isolation and Purification of Protein L7/L12The fraction of acidic ribosomal proteins was isolated by column chromatography with carboxymethylcellulose [4]. It was desalted on a Sephadex G-25 column in buffer D, lyophilized and further separation was carried out in conditions as described [l]. To remove aggregates, the proteins L7/L12 were subjected to column chromatography with Sephadex G-100 in buffer D. Oxidation of Protein L7/L12In order to oxidize methionine residues the protein was dissolved in a small amount of buffer D ; 50 % acetic acid containing hydrogen peroxide was added and the mixture was kept overnight at 4 "C. The final concentration of protein in solution was 2-3 mg/ml and that of hydrogen peroxide 0.7 -1 .O mg/ml.