Helix-turn-helix (HTH) DNA binding domain of the LacI family of transcriptional regulatorsHTH-DNA binding domain of the LacI (lactose operon repressor) family of bacterial transcriptional regulators and their putative homologs found in plants. The LacI family has more than 500 members distributed among almost all bacterial species. The monomeric proteins of the LacI family contain common structural features that include a small DNA-binding domain with a helix-turn-helix motif in the N-terminus, a regulatory ligand-binding domain which exhibits the type I periplasmic binding protein fold in the C-terminus for oligomerization and for effector binding, and an approximately 18-amino acid linker connecting these two functional domains. In LacI-like transcriptional regulators, the ligands are monosaccharides including lactose, ribose, fructose, xylose, arabinose, galactose/glucose, and other sugars, with a few exceptions. When the C-terminal domain of the LacI family repressor binds its ligand, it undergoes a conformational change which affects the DNA-binding affinity of the repressor. In Escherichia coli, LacI represses transcription by binding with high affinity to the lac operon at a specific operator DNA sequence until it interacts with the physiological inducer allolactose or a non-degradable analog IPTG (isopropyl-beta-D-thiogalactopyranoside). Induction of the repressor lowers its affinity for the operator sequence, thereby allowing transcription of the lac operon structural genes (lacZ, lacY, and LacA). The lac repressor occurs as a tetramer made up of two functional dimers. Thus, two DNA binding domains of a dimer are required to bind the inverted repeat sequences of the operator DNA binding sites.