A STEM approach to the study of the proteinaceous architecture of a ribosome.

We used a high-resolution STEM to attempt to recognize the spatial localization of the macromolecular components of ribosomes isolated from Tetrahymena pyriformis. Images obtained either by the annular dark field detector or by the ratio-contrast mode (Z-contrast) show a ribosome as constituted of rounded dark areas, the number and size of which are compatible with those of the protein molecules recognized by biochemistry. Images of different ribosomes, when similar orientations are observed, exhibit a roughly constant pattern of the dark areas. A set of images of a tilted ribosome allows to recognize that some masses are in fact two more superimposed components. We propose that the dark areas represent essentially the proteinaceous architecture of the ribosome.