A structure-based strategy to identify new molecular scaffolds targeting the bacterial ribosomal A-site.

The need for novel antibiotics is widely recognized. A well validated target of antibiotics is the bacterial ribosome. Recent X-ray structures of the ribosome bound to antibiotics have shed new light on the binding sites of these antibiotics, providing fresh impetus for structure-based strategies aiming at identifying new ribosomal ligands. In that respect, the ribosomal decoding region of the aminoacyl-tRNA acceptor site (A-site) is of particular interest because oligonucleotide model systems of this site are available for crystallography, NMR and compound binding assays. This work presents how these different resources can be combined in a hierarchical screening strategy which has led to the identification of new A-site ligands. The approach exploits an X-ray structure of the A-site against which large and diverse libraries of compounds were computationally docked. The complementarity of the compounds to the A-site was assessed using a scoring function specifically calibrated for RNA targets. Starting from approximately 1 million compounds, the computational selection of candidate ligands allowed us to focus the experimental work on 129 compounds, 34 of which showed affinity for the A-site in a FRET-based binding assay. NMR experiments confirmed binding to the A-site for some compounds. For the most potent compound in the FRET assay, a tentative binding mode is suggested, which is compatible with the NMR data and the limited SAR in this series. Overall, the results validate the screening strategy.