LmrR Is a Transcriptional Repressor of Expression of the Multidrug ABC Transporter LmrCD in Lactococcus lactis▿

LmrCD is an ABC-type multidrug transporter in Lactococcus lactis. LmrR encodes a putative transcriptional regulator. In a ΔlmrR strain, lmrCD is up-regulated. LmrR binds the promoter region of lmrCD and interacts with drugs that cause lmrCD up-regulation. This suggests that LmrR is a drug-dependent transcriptional regulator of lmrCD expression

Structure of the transcriptional regulator LmrR and its mechanism of multidrug recognition

LmrR is a PadR-related transcriptional repressor that regulates the production of LmrCD, a major multidrug ABC transporter in Lactococcus lactis. Transcriptional regulation is presumed to follow a drug-sensitive induction mechanism involving the direct binding of transporter ligands to LmrR. Here, we present crystal structures of LmrR in an apo state and in two drug-bound states complexed with Hoechst 33342 and daunomycin. LmrR shows a common topology containing a typical β-winged helix-turn-helix domain with an additional C-terminal helix involved in dimerization. Its dimeric organization is highly unusual with a flat-shaped hydrophobic pore at the dimer centre serving as a multidrug-binding site. The drugs bind in a similar manner with their aromatic rings sandwiched in between the indole groups of two dimer-related tryptophan residues. Multidrug recognition is facilitated by conformational plasticity and the absence of drug-specific hydrogen bonds. Combined analyses using site-directed mutagenesis, fluorescence-based drug binding and protein–DNA gel shift assays reveal an allosteric coupling between the multidrug- and DNA-binding sites of LmrR that most likely has a function in the induction mechanism