RESUMEN
Estimates for the phases of the X-ray diffraction data from the 50S ribosomal particle of Thermus thermophilus has been made to an effective resolution around 80 A using the few-atoms-modes ab initio technique [Lunin, Lunina, Petrova, Vernoslova, Urzhumtsev & Podjarny (1995). Acta Cryst. D51, 896-903]. This technique models the density with a small number of Gaussian spheres to generate a large number of possible phase sets and then uses clustering algorithms to identify the best ones. Independently, an envelope obtained from electron-micrograph image reconstruction [Yonath, Leonard & Wittmann (1987). Science, 236, 813-816] was oriented and positioned using the molecular-replacement technique, specially adapted to the very low resolution case [Urzhumtsev & Podjarny (1995). Acta Cryst. D51, 888-895]. The two methods show similar packing arrangements. The electron density calculated by the few-atoms-models technique without any assumption on the number of molecules in asymmetric unit or on their shape shows recognizable features of the particle.
RESUMEN
A method is proposed for the solution of the phase problem at very low resolution for macromolecules. It generates randomly a very large number of models, each consisting of a few (two to ten) pseudo-atoms. The corresponding amplitudes are used for selecting a subset of 'best' models by choosing those with the highest correlation with experimental values. The phases calculated from these 'best' models are analysed by a clusterization procedure leading to a few possible solutions, from which the correct one can be recognized by simple additional criteria. This method has been successfully applied to the neutron diffraction data of the AspRS-tRNA(Asp) complex at 50 A resolution and to data calculated from a model ribosome crystal at 60 A resolution.
RESUMEN
The crystal structure of gamma-crystallin IIIb (gamma C) from calf eye lens has been refined at 2.5 A resolution. The molecule of about 21 kDa consists of two similar domains. Each domain is composed of two motifs with the 'Greek key' topology which form a pair of four-stranded beta-sheets with an antiparallel packing. The molecule has three hydrophobic cores: one within each domain and one between them. Six of the eight functionally important cysteines are located within the N-domain, and only two in the C-domain. Several large clusters of charged residues are at the surface of the molecule. Surface residues Val 101, Met 103 and Leu 155 are important for packing of molecules in crystal medium and possibly in the lens. Features of the gamma-crystallin IIIb molecule which may be related to its function in the vertebrate eye lens are briefly discussed. An attempt has been made to correlate molecular characteristics with some general properties of the eye lens such as high density and refractive index gradients and strong stability of the lens during an organism's lifetime.