3-DIMENSIONAL RECONSTRUCTION OF MAMMALIAN 40 S RIBOSOMAL-SUBUNIT EMBEDDED IN ICE

A platform-like structure, which appears equivalent to the platform or lobe structure of the 30 S subunit of the eubacterial ribosome, is ob served in the reconstruction of the small 40 S ribosomal subunit from images of ice-embedded particles. This cup-shaped structure, 15.0 nm i n side length and 13.5 nm wide at its rim, extends obliquely upward on the back of the subunit. Other previously characterized features of t he 40 S subunit can readily be identified: the head with its prominent beak structure, the body with its two back lobes expressed as relativ ely small-scale features, and the two widely separated feet that compr ise the base of the subunit.