Broad ligand specificity of the transcriptional regulator of the Bacillus subtilis multidrug transporter Bmr.

The expression of the Bacillus subtilus multidrug-efflux transporter Bmr can be induced by two of its structurally dissimilar substrates, rhodamine 6G and tetraphenylphosphonium, through their direct interaction with the transcriptional regulator BmrR (Ahmed et al., J. Biol. Chem. 269, 28506). Here, by screening a chemical library, we identified four additional ligands of BmrR inducing Bmr expression at micromolar concentrations. BmrR ligands, although sharing a positive charge and moderate hydrophobicity, are structurally very diverse. At the same time, not all hydrophobic positively charged compounds, including many structural analogs of the inducers, induce Bmr expression, thus suggesting that local chemical interactions and not merely physical properties of the ligands are important for their recognition by BmrR. These results confirm that this soluble protein, like the membrane transporter it regulates, has a uniquely broad substrate specificity.