VERY LOW RESOLUTION PHASING ATTEMPTS OF THE RIBOSOMAL 50S PARTICLE FROM T. THERMOPHILUS BY THE FEW ATOMS MODEL METHOD. A.D. Podjarny, A.G. Urzhumtsev and E.A. Vernoslova, UPR de Biologie Structurale, IGBMC, B.P. 163, 67404 Illkirch Cedex, C.U de Strasbourg, France

A suggestion for the phases for the 80 Å resolution X-ray diffraction data from the 50S ribosomal particle of Thermus thermophilus (Volkmann et al., J. Mol. Biol., 216, 239, 1990) has been made using the Few Atoms Model ab initio technique (Lunin et al., Acta Cryst., D51, 896, 1995), in collaboration with A. Yonath and coworkers. This technique generated randomly one million models consisting of 5 pseudo atoms each and selected the 560 solutions which fitted best the observed amplitudes to 60 Å resolution. The selected models were grouped with a clusterisation procedure in a small number of possible solutions. The most adequate one was chosen by imposing the additional constraint that there should be no strong densities on symmetry axes. To refine this result, a second model generation was done imposing stronger amplitude constraints between 120 and 60 Å and density constraints based on the result of the first generation. The map resulting from the second model generation (phased to 80 Å) is shown in the figure below. The position and features of the observed envelope agree with those obtained with other ab-initio solution methods and with molecular replacement using models from electron microscopy reconstructions (Volkmann et al., CCP4 Newsletter, 31, 23, 1995).