Low resolution structure of partially trypsin-degraded polypeptide elongation factor, EF-TU, from Escherichia coli

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

An electron-density map of tobacco mosaic virus at 10 Å resolution

Thirty-five years ago Bernal began investigating diffraction from oriented gels of tobacco mosaic virus (TMV) (Bernal and Fankuchen, 1941). Bernal and Fankuchen realized that the diffraction diagram showed TMV to be composed of an identical array of subunits. This realization was of considerable significance in the development of molecular biology; however, the exact nature of the ordering of the subunits in TMV eluded them. This was in fact elucidated by Watson (1954). Using the theory of diffraction from a helix developed by Cochran, Crick, and Vand (1952), he was able to show that TMV is a helical array of subunits, that the virus repeated exactly after three turns of the helix, and that the number of subunits in three turns was probably of the form 3n + 1. On the basis of Watson's work, Rosalind Franklin began an analysis of the fiber diffraction pattern from TMV on a quantitative basis