<p>The MarR-type HTH domain is a DNA-binding, winged helix-turn-helix (wHTH) domain of about 135 amino acids present in transcription regulators of the MarR/SlyA family, involved in the development of antibiotic resistance. This family of transcription regulators is named after <taxon tax_id="562">Escherichia coli</taxon> MarR, a repressor of genes which activate the multiple antibiotic resistance and oxidative stress regulons, and after slyA from <taxon tax_id="602">Salmonella typhimurium</taxon> and E. coli, a transcription regulator that is required for virulence and survival inthe macrophage environment. Regulators with the MarR-type HTH domain arepresent in bacteria and archaea and control a variety of biological functions,including resistance to multiple antibiotics, household disinfectants, organicsolvents, oxidative stress agents and regulation of the virulence factorsynthesis in pathogens of humans and plants. Many of the MarR-like regulatorsrespond to aromatic compounds [<cite idref="PUB00015407"/>, <cite idref="PUB00015408"/>, <cite idref="PUB00015410"/>].</p><p>The crystal structures of MarR, MexR and SlyA have been determined and show a winged HTH DNA-binding core flanked by helices involved in dimerisation. The DNA-binding domains are ascribed to the superfamily of wingedhelix proteins, containing a three (four)-helix (H) bundle and a three-stranded antiparallel beta-sheet (B) in the topology: H1-(H1')-H2-B1-H3-H4-B2-B3-H5-H6. Helices 3 and 4 comprise the helix-turn-helix motif and the beta-sheet is called the wing. Helix 4 is termed the recognition helix, like in other HTHs where it binds the DNA major groove. The helices 1, 5 and 6 are involved in dimerisation, as most MarR-like transcription regulators form dimers [<cite idref="PUB00015410"/>, <cite idref="PUB00015409"/>].</p>