Helix-Turn-Helix DNA binding domain of the MerD transcription regulator
Helix-turn-helix (HTH) transcription regulator MerD. The putative secondary regulator of mercury resistance (mer) operons, MerD, has been shown to down-regulate the expression of this operon in gram-negative bacteria. It binds to the same operator DNA as MerR that activates transcription of the operon in the presence of mercury ions. The MerD protein shares the N-terminal DNA binding domain with other transcription regulators of the MerR superfamily, which promote transcription by reconfiguring the spacer between the -35 and -10 promoter elements. A typical MerR regulator is comprised of two distinct domains that harbor the regulatory (effector-binding) site and the active (DNA-binding) site. Their N-terminal domains are conserved and contain predicted winged HTH motifs that mediate DNA binding, while the dissimilar C-terminal domains bind specific coactivator molecules such as metal ions, drugs, and organic substrates.