Fig. 4: Structure of the ribosome-associated TAC toxin with its native cspA substrate.

a Electron density map of the complex (transparent gray), showing HigB^TAC [K95A] (red), its target cspA mRNA (purple), fMet-tRNA^fMet (orange), and the atomic models for the 50S (blue) and 30S (yellow) ribosomal subunits. b Crystal structure of HigB^TAC [K95A]. The N-terminus, C-terminus, and the secondary structure elements are indicated. The sequence of the toxin is shown underneath with the secondary structure element highlighted (α-helix in pink, β-strand in yellow). The first six residues (light gray) are not visible in the crystal structure. c Close-up of the interactions between HigB^TAC [K95A], the cspA mRNA, and the translating ribosome, rotated by 90° with respect to the structure in a. The HigB^TAC [K95A] is red; the P-site fMet-tRNA^fMet is orange; the cspA mRNA is purple; the uS13 ribosomal protein is green; the 16S rRNA is light yellow; the 23S rRNA is light blue; and the cryo-electron density map is a gray mesh. For clarity, 16S and 23S α-helices are labeled.