The small molecule regacin binds a conserved pocket in RegA to inhibit the expression of bacterial virulence factors.

Widespread antibiotic resistance is an urgent threat to public health. Small molecules that specifically target virulence factors or their expression are promising alternative therapeutics to antibiotics. The small molecule regacin was identified as a potent inhibitor of RegA, a transcription factor required for the expression of numerous virulence genes of Citrobacter rodentium. This demonstrates that targeting virulence regulators is a viable strategy for developing clinically useful therapeutics. RegA is a member of the AraC/XylS superfamily of transcription factors and with homologs in numerous pathogens that contribute to human morbidity and mortality worldwide. The development of antivirulence therapeutics that target these regulators could be accelerated by understanding the structural basis of inhibition. To that end we present the first high resolution structural analysis of RegA with and without bound regacin. RegA is structurally homologous to ToxT of Vibrio cholerae and Rns of enterotoxigenic Escherichia coli. Regacin was bound within a conserved binding pocket within the RegA amino-terminal domain. Structure guided mutagenesis of key pocket residues confirmed their role in regacin mediated inhibition. These findings define the regacin binding site, the structural basis for RegA inhibition, and provide a framework for designing inhibitors that target a vast family of virulence regulators essential for the pathogenesis of numerous bacterial pathogens.