W0001

Ab-initio Phasing at 30 Å of the t50S Ribosomal Particle with the Few Atoms Model Method. A.D. Podjarny, N. Lunina, A.G. Urzhumtsev, E.A. Vernoslova, T. Petrova and V. Lunin, UPR de Biologie Structurale, IGBMC, B.P. 163, 67404 Illkirch Cedex, C.U de Strasbourg, France and IMPB, Pushchino, Moscow Region, Russia

The Few-Atoms Model ab initio technique (Lunin et al., Acta Cryst. D51, 896-903, 1995 has been applied to the X-ray diffraction data from the 50S ribosomal particle of Thermus thermophilus (Von Bohlen et al, J. Mol. Biol., 222, 11, 1991) in collaboration with A. Yonath and coworkers. The first application used 60 Å resolution data, and generated randomly one million models consisting of 5 pseudo atoms each and selected the 560 which fitted best the observed amplitudes. The selected models were grouped with a clusterisation procedure, which reduced the possible solutions to 10. The most adequate solution was chosen by imposing the additional constraint that there should be no strong densities on symmetry axes, leading to an 80 Å image. To refine this result at higher resolution, further work was done using data to 20 Å resolution. This new application imposed density constraints using the envelope from the first one, stronger amplitude correlations for the highest resolution data and maximum likelihood techniques. This improvements of the FAM method led to a map with an effective resolution about 30Å, shown in the figures below.

Single Particle Packing