Cryo-EM reveals multiple mechanisms of ribosome inhibition by doxycycline.

Antimicrobial resistance is driving the search for new antibiotics and a greater understanding of their mechanism of action. Doxycycline is amongst the most-prescribed antimicrobials. It demonstrates a particularly low minimum inhibitory concentration against the zoonotic pathogen Coxiella burnetii. Doxycycline canonically targets the bacterial ribosome by blocking tRNA binding at the decoding centre (A site) of the small subunit. Using cryo-electron microscopy, we analysed doxycycline binding to C. burnetii and Escherichia coli ribosomes. Both structures reveal doxycycline binding at the exit tunnel in the large subunit. In C. burnetii three doxycycline molecules stack to block the tunnel. In E. coli one doxycycline molecule triggers a major change in the conformation of the ribosome. This rearrangement of the peptidyl transferase centre blocks tRNA binding and nascent chain accommodation, abolishing interactions that are fundamental to ribosome function. We identify a distinct ribosomal protein in the C. burnetii large subunit and characterise an additional member of the prokaryotic ribosome hibernation-promoting factor family. These insights into ribosome function and antibiotic action may aid the development of new ribosome inhibitor antibiotics.