where does the lac repressor bind to dna
Lac repressor 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.[1] 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.[2] 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.
RNA polymerase RNA polymerase (ribonucleic acid polymerase), both abbreviated RNAP or RNApol, official name DNA-directed RNA polymerase, is a member of a family of enzymes that are essential to life: they are found in all organisms (-species) and many viruses. RNAP locally opens the double-stranded DNA (usually about four turns of the double helix) so that one strand of the exposed nucleotides can be used as a template for the synthesis of RNA, a process called transcription. A transcription factor and its associated transcription mediator complex must be attached to a DNA binding site called a promoter region before RNAP can initiate the DNA unwinding at that position. RNAP has intrinsic helicase activity, therefore no separate enzyme is needed to unwind the DNA (in contrast to DNA polymerase). RNAP not only initiates RNA transcription, it also guides the nucleotides into position, facilitates attachment and elongation, has intrinsic proofreading and replacement capabilities, and termination recognition capability. In eukaryotes, RNAP can build chains as long as 2.4 million nucleotides.