MerR

MerR is a family of metal-responsive transcriptional regulators found in bacteria. The prototype is MerR, which regulates the mercury resistance operon by sensing Hg2+ and activating detoxification genes such as merA, which encodes mercuric reductase, and merT and merP, which encode transport proteins.

MerR family proteins function as homodimers that bind specifically to operator sequences overlapping the promoter region of target operons. In the absence of inducer, MerR-like regulators often repress transcription by distorting the promoter and preventing efficient RNA polymerase binding. Upon binding a metal ion or related inducer, the regulator undergoes a conformational change that remodels the DNA to optimally align the -35 and -10 elements, enabling transcription from otherwise weak promoters.

Structurally, MerR proteins have an N-terminal DNA-binding helix-turn-helix domain, a central dimerization/coil region, and a C-terminal metal-sensing effector domain that coordinates metal ions via conserved ligands such as cysteine or histidine residues. The metal-binding site governs DNA distortion and transcriptional output.

Beyond mercury resistance, the MerR family includes regulators that respond to other metals and metalloids, such as Zn2+ and Cu+/Ag+ sensors. Some representatives regulate lead resistance or other metal homeostasis genes, illustrating the family’s diversity.

MerR regulons contribute to bacterial detoxification of toxic metals and to metal homeostasis, and are commonly carried on chromosomes or plasmids near metal resistance operons.

MerR-based biosensors exploit the metal-responsive DNA-distortion mechanism to detect mercury contamination with high sensitivity and tunability.