The topology of the 30 S ribosomal subunit and a proposal for the surface distribution of its RNA by dark field electron microscopy.

A three-dimensional model of the Escherichia coli 30 S ribosome has been derived from an extensive dark field electron microscopic investigation of unstained and unshadowed ribosomes. Subsequent studies of specimens lightly stained with uranyl acetate have added information on the location of regions that preferentially bind uranyl ions and may represent surface RNA. The model is an elongated cone-shaped structure 180-200 A long and is divided into identifiable sections: a "head" partly encircled by a collar-like structure, a middle section carrying a 20-A protuberance, and a "tail." A groove separates the middle section from the tail. Uranyl-binding regions can be seen on the protuberance and the collar's ridge, in two bands below the collar and parallel to it, and in a patch on the back of the head. The staining technique described may offer a method for mapping the external nucleic acid of nucleoprotein complexes.