The ribosome is a massive RNA-protein complex that catalyzes template-directed protein synthesis in all cells. As the machine that converts genetic information into a biological phenotype, understanding the structural basis for ribosome function is of central importance to biology. We have grown square prisms of T. thermophilus 70S ribosomes ranging in size from 300-500 micrometers. On a rotating-anode source, these crystals diffract to approximately 25 Angstrom resolution. However, based on data collected at CHESS beamline F1, these same crystals diffract to ~8 Angstrom resolution. We used the beamtime at CHESS for two purposes. First, we wanted to determine the diffraction limit of our present 70S ribosome crystals. We found that, although our native crystals varied in quality, we were able to obtain data with mean I/sigma(I) > 2 at 8.2 Angstrom resolution. Second, we wanted to continue screening for potential heavy-atom derivatives to phase the structure factor amplitudes. By using molecular replacement phases, we had identified heavy-atom derivatives in a previous crystal form. We are therefore repeating the search in these new crystals. Several partial and complete data sets collected at CHESS are being used for this purpose. A molecular replacement solution has been determined and difference Fourier analyses are presently underway.
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