Characterization and preliminary attempts for derivatization of crystals of large ribosomal subunits from Haloarcula marismortui diffracting to 3 Å resolution

An improved form of crystals of large (50 S) ribosomal subunits from Haloarcula marismortui, formally named Halobacterium marismortui, diffracting to 3 Å resolution, has been obtained by the addition of 1 m m-Cd 2+ to the crystallization medium, which contained more than 1·9 m of other salts. The improved crystals, grown from functionally active particles to an average size of 0·3 mm × 0·3 mm × 0·08 mm, are isomorphous with the previously reported ones, which diffracted to 4·5 Å. They are of space group C222 1, cell dimensions a = 210 A ̊ , b = 300 A ̊ , c = 581 A ̊ , and contain one particle in the asymmetric unit. Their superior internal order is reflected not only in their high resolution, but also in their reasonable mosaicity (less than 0·3 °). In contrast to the previously grown crystals, the new ones are of adequate mechanical strength and survive well the shock-cooling treatment. Due to their weak diffracting power, all crystallographic studies have been performed with synchrotron radiation. At cryotemperature, these crystals showed no measurable decay for a few days of irradiation and a complete diffraction data set could be collected from a single crystal. Efforts for initial phasing by specific and quantitative derivatization with superdense heavy-atom clusters are in progress.