A new crystal form of large ribosomal subunits from Halobacterium marismortui

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

An improved form of crystals of large (50 S) ribosomal subunits from Haloarcula marismortui, formally named Halobacterium marismortui, diffracting to 3 A resolution, has been obtained by the addition of 1 mM-Cd2+ 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 x 0.3 mm x 0.08 mm, are isomorphous with the previously reported ones, which diffracted to 4.5 A. 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 degrees). 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 super-dense heavy-atom clusters are in progress.

Approaching the molecular structure of ribosomes

Fifteen forms of three-dimensional crystals and three forms of two-dimensional sheets from ribosomal particles have been grown. In all cases only biologically active particles could be crystallized, the crystalline material retaining its integrity and biological activity for months. Cryastallographic data have been collected from crystals of 50 S ribosomal subunits, using synchrotron radiation, at temperatures between 19 and -180 degree C. Although at around 0 degrees C in the synchrotron X-ray beam the crystals rapidly lose their high-resolution reflections, at cryo-temperatures hardly any radiation damage occurs over long periods, and a complete set of diffraction data to about 6 A resolution could be collected from a single crystal. Heavy-atom clusters were used for soaking as well as for specific binding to the surface of the ribosomal subunits prior to crystallization. The 50 S ribosomal subunits from a mutant of Bacillus stearothermophilus which lacks the ribosomal protein BL11 crystallize isomorphously with the native form. Models of the entire 70 S ribosome and of the 50 S subunit have been reconstructed from two-dimensional sheets at 47 and 30 A, respectively. These models demonstrate the overall shape of the particles, the contact areas between large and small subunits, the space where protein biosynthesis may take place and a tunnel through the 50 S subunit which could provide a path for the nascent polypeptide chain.

The growth of ordered two-dimensional sheets of ribosomal particles from salt-alcohol mixtures

A procedure for the in vitro growth of well-ordered two-dimensional sheets from ribosomal particles using salts and salt-alcohol mixtures has been developed. Employing this procedure, ordered two-dimensional sheets of the wild type as well as of mutated 50 S ribosomal subunits from Bacillus stearothermophilus can readily be obtained. These sheets, stained with uranyl acetate or gold-thioglucose, are suitable for three-dimensional image reconstruction. They consist of relatively small unit cells with dimensions of 160 +/- 15 and 365 +/- 20 A. Diffraction patterns of electron micrographs of these sheets contain features to 25 A resolution.

A tunnel in the large ribosomal subunit revealed by three-dimensional image reconstruction

A better understanding of the molecular mechanism of protein biosynthesis depends on the availability of a reliable model for the ribosome particle. The application of a diffraction technique, namely, three-dimensional image reconstruction from two-dimensional sheets of the large ribosomal subunits of Bacillus stearothermophilus at a resolution of 30 angstroms is described. The resulting three-dimensional model shows at least four projecting arms, arranged radially near the presumed interface with the 30S subunit. The projecting arms are positioned around a cleft, which turns into a tunnel with a length of 100 to 120 angstroms and a diameter of up to 25 angstroms. This tunnel spans the particle and may provide the path taken by the nascent polypeptide chain.