Targeting exposed RNA regions in crystals of the small ribosomal subunits at medium resolution

Within the framework of ribosomal crystallography, the small subunits are b eing analyzed, using crystals diffracting to 3 Angstrom resolution. The med ium resolution electron density map of this subunit, obtained by multiple i somorphous replacement, show recognizable morphologies, strikingly similar to the functional active conformer of the small ribosomal subunit. It conta ins elongated dense features, traceable as RNA chains as well as globular r egions into which the structures determined for isolated ribosomal proteins , or other known structural motifs were fitted. To facilitate unbiased map interpretation, metal clusters are being covalently attached either to the surface of the subunits or to DNA oligomers complementary to exposed riboso mal RNA. Two surface cysteines and the 3' end of the 16S ribosomal RNA have been localized. Targeting several additional RNA regions shed light on the ir relative exposure and confirmed previous studies concerning their functi onal relevance.