The identification of selected components in electron density maps of prokaryotic ribosomes at 7 angstrom resolution

Crystals of small and large ribosomal subunits from thermophilic and haloph ilic bacteria, diffracting to 3 Angstrom, are being subjected to structural analysis with synchrotron radiation. The bright beam necessary for detecti ng and collecting the diffraction at the higher-resolution shell causes sig nificant decay even at 25 K. Nevertheless, data collected from native and h eavy-atom-derivatized crystals led to the construction of electron density maps of both ribosomal subunits, showing recognizable morphologies and inte rnal features similar to those observed by EM reconstructions of the corres ponding ribosomal particle. The main features of these maps include elongat ed dense regions traceable as well separated RNA duplexes or single strands . Also seen are globular patches of lower density, readily distinguishable from the above, in which folds observed by NMR or crystallography in isolat ed ribosomal proteins at atomic resolution were detected. The intercomponen ts contacts identified so far reveal diverse modes of recognition. Metal cl usters, attached at selected sites on the particles, are being exploited to facilitate unbiased map interpretation. In this way, two surface proteins were located and several surface RNA strands were targeted.