30S crystal form likely prevents toxin-engaged mRNA in the A site. (A) In the context of the 30S crystal form, the 30S spur from a crystallographic symmetry mate packs in the P site and interacts with the 3′-end of the 16S rRNA, mimicking a P-site tRNA–mRNA pair. This arrangement only allows mRNA to be programmed starting at the A site in the 30S crystal form. (B) HigB binding to the ribosomal A site facilitates the mRNA being pulled into its active site in the context of a 70S-HigB ΔH92 complex in a precleavage state (PDB code 4YPB). The orientation that the mRNA adopts in the absence of toxin or tRNA is shown for comparison as light purple (PDB code 4V6G).
