Figure 2.

Conformational changes in the position of the head domain of the 30S subunit induced by spectinomycin. a, b) Difference Fourier (F_obs − F_obs) electron density map in the context of the 30S ribosomal subunit (panel a) and in the proximity of the spectinomycin binding site in h34 (panel b, stereoview) of ribosome II. Positive (blue) and negative (red) difference densities are contoured at 3 SD from the mean. 16S rRNA is in light blue, and small subunit proteins are in dark blue. Spectinomycin is colored gold. For panel a, the direction of the view is indicated by the ribosome icon. c) Superposition of spectinomycin-bound (Spc) and unliganded (II) (13) conformations of ribosome II in the 70S ribosome crystals. The rRNA is shown in red and gray, and proteins are purple and gray for spectinomycin-bound (Spc) and unliganded (II) (13) conformations of ribo-some II, respectively. d) Superposition of 30S subunits within the 70S ribosome. Differences in the position of phosphorous atoms in 16S rRNA (light colors) and Cα positions in proteins S7, S13, and S19 (darker colors) in the 30S subunit head domain are shown as vectors, as follows. Vectors from apo-ribosome II (13) to spectinomycin-bound ribosome II, red; vectors from spectinomycin-bound ribosome II to spectinomycin-bound ribosome I, blue. The 50S subunit from ribosome I (13) is shown in gray (23S rRNA), purple (5S rRNA), and magenta (large subunit proteins). The direction of the conformational change induced by spectinomycin is indicated by the red arrow. The path tRNAs travel during translocation from the A site to the E site is indicated by the black arrow. The direction of the view is indicated by the ribosome icon.