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 beta-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.
