Editing Lac repressor (section)

{{Short description|DNA-binding protein}}
{{redirect-distinguish|lacI|lac I|Laci (disambiguation){{!}}Laci}}
[[Image:LacI Dimer Structure Annotated.png|thumb|right|300px|'''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)]]

The '''''lac'' repressor''' (LacI) is a DNA-binding protein that inhibits the [[Gene expression|expression]] of [[gene]]s 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 firstly converted into [[allolactose]] by [[Beta-galactosidase|β-Galactosidase]] ([[Lac operon|lacZ]]) in bacteria. The DNA binding ability of ''lac'' repressor bound with allolactose is inhibited due to [[allosteric regulation]], thereby genes coding for proteins involved in lactose uptake and utilization can be expressed.