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Abstract

The low resolution structure of a trypsin-modified form of elongation factor EF-Tu from Escherichia coli has been determined by X-ray crystallographic methods. The crystals belong to space group P212121 with two molecules in the asymmetric unit. The phase determination was based on three isomorphous heavy-atom derivatives. The quality of the resulting electron density map at 6 Å was sufficient to identify the molecules. The two molecules in the asymmetric unit are related by a non-crystallographic 2-fold rotation. A molecular model was derived by averaging the electron density of the two molecules at equivalent points. Its overall dimensions are 75 Å × 50 Å × 35 Å. The molecule consists of a compact globular head of dimensions 45 Å × 40 Å × 40 Å and a curled tail of diameter 25 Å and length 55 Å. There is a second connection between head and tail, probably an α-helix, such that the molecule forms a ring. The large groove in the centre could accommodate a RNA double helix. The head has a high α-helical content whereas the tail seems to be helix-free. A molecular weight of 43,000 was derived from the electron density map indicating that no major part of the molecule is missing. Possible interactions between EF-Tu and transfer RNA are discussed.