Controls the permeability of the cell envelope to hydrophobic compounds such as antibiotics and detergents (PubMed:8196548). Represses transcription of the mtrCDE-encoded efflux pump by binding within the mtrCDE promoter (PubMed:9209024, PubMed:23221802). Also negatively regulates the expression of farR, by binding to its promoter region, leading indirectly to the positive regulation of expression of the farAB-encoded efflux pump (PubMed:14645274). DNA binding is affected significantly by increasing the NaCl concentration. Homodimer. Binds to DNA as a pair of dimers. Deletion of the gene results in increased resistance to antibiotics and detergents. The MtrA and MtrR-binding sites are sterically close and addition of an effector increases the affinity of MtrA for the mtrCDE promoter such that MtrR binding is negatively impacted.