Lac repressor
![Annotated crystal structure of dimeric LacI. Two monomers (of four total) co-operate to bind each DNA operator sequence. Monomers (red and blue) contain DNA binding and core domains (labeled) which are connected by a linker (labeled). The C-terminal tetramerization helix is not shown. The repressor is shown in complex with operator DNA (gold) and ONPF (green), an anti-inducer ligand (i.e. a stabilizer of DNA binding)](/Images/godic/202501/23/LacI_Dimer_Structure_Annotated3735.png")
![Tetrameric LacI binds two operator sequences and induces DNA looping. Two dimeric LacI functional subunits (red+blue and green+orange) each bind a DNA operator sequence (labeled). These two functional subunits are coupled at the tetramerization region (labeled); thus, tetrameric LacI binds two operator sequences. This allows tetrameric LacI to induce DNA looping.](/Images/godic/202501/23/Annotated_Theoretical_Model_of_Bound_Tetrameric_Lac_Repressor3735.png")
The lac repressor is a DNA-binding protein which inhibits the expression of genes coding for proteins involved in the metabolism of lactose in bacteria. These genes are repressed when lactose is not available to the cell, ensuring that the bacterium only invests energy in the production of machinery necessary for uptake and utilization of lactose when lactose is present. When lactose becomes available, it is converted into allolactose, which inhibits the lac repressor's DNA binding ability. Loss of DNA binding by the lac repressor is required for transcriptional activation of the operon.