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Lac repressor
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.
==Function==
The ''lac'' repressor (LacI) operates by a helix-turn-helix motif in its DNA binding domain binding base-specifically to the major groove of the operator region of the ''lac'' operon, with base contacts also made by residues of symmetry-related alpha helices, the "hinge" helices, which bind deeply in the minor groove. This DNA binding causes the specific affinity of RNA polymerase for the promoter sequence to increase sufficiently that it cannot escape the promoter region and enter elongation, and so prevents transcription of the mRNA coding for the Lac proteins.〔http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2715899/〕 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. This effect is referred to as induction, because it ''induces'', rather than represses, expression of the metabolic genes. ''In vitro,'' 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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