Three-dimensional reconstruction of the Escherichia coli 30 S ribosomal subunit in ice.

Three-dimensional (3D) reconstructions of both the heat-activated and non-activated 30 S subunit of the Escherichia coli 70 S ribosome were obtained from a frozen hydrated specimen preparation at 1/37 A-1 resolution. Well-characterized features that can be identified in both reconstructions are the head, the base, the platform and the cleft formed between the head and the platform. Comparisons between the 3D maps of 30 S subunits at 0 degree C, heat-activated at 37 degrees C, and the 30 S subunit portion identified in the cryo-3D map of 70 S ribosome reveal conformational changes the subunit probably undergoes during inactive-active transition and upon association with the 50 S subunit. These comparisons also allow us to localize the sites of association of 30 S and 50 S subunits.