Editing Lac repressor (section)

== Function ==
The ''lac'' repressor (LacI) operates by a [[helix-turn-helix]] [[Structural motif|motif]] in its [[DNA-binding domain]], binding base-specifically to the [[major groove]] of the [[Operon|operator region]] of the [[lac operon|''lac'' operon]], with base contacts also made by residues of symmetry-related alpha helices, the "hinge" helices, which bind deeply in the minor groove.<ref>{{cite journal | vauthors = Schumacher MA, Choi KY, Zalkin H, Brennan RG | title = Crystal structure of LacI member, PurR, bound to DNA: minor groove binding by alpha helices | journal = Science | volume = 266 | issue = 5186 | pages = 763–70 | date = November 1994 | pmid = 7973627 | doi = 10.1126/science.7973627 | bibcode = 1994Sci...266..763S }}</ref> This bound repressor can reduce [[transcription (genetics)|transcription]] of the Lac proteins by occluding the [[RNA polymerase]] binding site or by prompting DNA looping.<ref>{{cite journal | vauthors = Razo-Mejia M, Boedicker J, Jones D, DeLuna A, Kinney J, Phillips R | title = Comparison of the theoretical and real-world evolutionary potential of a genetic circuit | journal = Physical Biology | volume = 1 | issue = 2 | pages = 026005 | date = 2014 | pmid = 24685590 | pmc = 4051709 | doi = 10.1088/1478-3975/11/2/026005| bibcode = 2014PhBio..11b6005R }}</ref> When lactose is present, allolactose binds to the ''lac'' repressor, causing an [[allosteric]] change in its shape. In its changed state, the ''lac'' repressor is unable to bind tightly to its cognate operator. Thus, the gene is mostly off in the absence of inducer and mostly on in the presence of inducer, although the degree of gene expression depends on the number of repressors in the cell and on the repressor's DNA-binding affinity.<ref>{{cite journal | vauthors = Razo-Mejia M, Barnes S, Belliveau N, Chure G, Einav T, Lewis M, Phillips R | title = Tuning Transcriptional Regulation through Signaling: A Predictive Theory of Allosteric Induction | journal = Cell Systems | volume = 6 | issue = 4 | pages = 456–469 | date = 2018 | pmid = 29574055 | pmc = 5991102 | doi = 10.1016/j.cels.2018.02.004 }}</ref> [[Isopropyl β-D-1-thiogalactopyranoside]] (IPTG) is a commonly used allolactose mimic which can be used to induce transcription of genes being regulated by ''lac'' repressor.