A 9-ANGSTROM RESOLUTION X-RAY CRYSTALLOGRAPHIC MAP OF THE LARGE RIBOSOMAL-SUBUNIT

The 50S subunit of the ribosome catalyzes the peptidyl-transferase rea ction of protein synthesis. We have generated X-ray crystallographic e lectron density maps of the large ribosomal subunit from Haloarcula ma rismortui at various resolutions up to 9 Angstrom using data from crys tals that diffract to 3 Angstrom. Positioning a 20 Angstrom resolution EM image of these particles in the crystal lattice produced phases ac curate enough to locate the bound heavy atoms in three derivatives usi ng difference Fourier maps, thus demonstrating the correctness of the EM model and its placement in the unit cell. At 20 Angstrom resolution , the X-ray map is similar to the EM map; however, at 9 Angstrom it re veals long, continuous, but branched features whose shape, diameter, a nd right-handed twist are consistent with segments of double-helical R NA that crisscross the subunit.