RIBOSOME SUBSTRUCTURE INVESTIGATED BY SCANNING FORCE MICROSCOPY AND IMAGE-PROCESSING

Scanning force microscopy was used to study the ultrastructure of euka ryotic ribosomes from Chironomus pallidivitatus in the polysomal compl ex. Positively stained polysomes were imaged, and the resulting three- dimensional ribosomal structures were further processed by statistical analyses of virtual cross-sections parallel to the substrate plane. S tructural investigations were based on parameters which are minimally influenced by the tip geometry, like section plane centre or axis rati o. In the lower part of the structure a shift of the section centres w as observed, suggesting an attached structure. The geometry of the sec tions revealed an elliptical shape in the upper part (axis ratio 1.52 +/- 0.22), with a less elongated shape in the lower region (axis ratio 1.41 +/- 0.18). A model for the surface topography of a positively st ained ribosome exhibiting a small subunit attached along the long side of an elliptical large structure is proposed.