Crystallographic studies on the ribosome, a large macromolecular assembly exhibiting severe nonisomorphism, extreme beam sensitivity and no internal symmetry

Crystals, diffracting best to around 3 Angstrom, have been grown from intac t large and small ribosomal subunits. The bright synchrotron radiation nece ssary for the collection of the higher-resolution X-ray diffraction data in troduces significant decay even at cryo temperatures. Nevertheless, owing t o the reasonable isomorphism of the recently improved crystals of the small ribosomal subunits, reliable phases have been extracted at medium resoluti on (5-6 Angstrom) and an interpretable five-derivative MIR map has been con structed. For the crystals of the large subunits, however, the situation is more complicated because at higher resolution (2.7-7 Angstrom) they suffer from substantial radiation sensitivity, a low level of isomorphism, instab ility of the longest unit-cell axis and nonisotropic mosaicity. The 8 Angst rom MIR map, constructed to gain insight into this unusual system, may prov ide feasible reasoning for the odd combination of the properties of these c rystals as well as hints for future improvement. Parallel efforts, in which electron-microscopy-reconstructed images are being exploited for molecular -replacement studies, are also discussed.