The lac repressor is a protein that represses (inhibits) transcription of the lac operon. When lactose is not available, the lac repressor binds tightly to the operator, preventing transcription by RNA polymerase. However, when lactose is present, the lac repressor loses its ability to bind DNA.Click to see full answer. Similarly, you may ask, where does the lac repressor bind to DNA?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.Likewise, what is the function of lac repressor system in E coli? Repressor. The lac repressor of E. coli is a well-studied example of a repressor whose effector is an inducer. The lac repressor controls the expression of the lactose operon, which is responsible for the metabolism of lactose. Also, when the lac repressor protein binds to lactose why does it fall off its binding site at the operator? Concept 6: The Effect of Lactose on the lac Operon Small amounts of allolactose are formed when lactose enters E. coli. Allolactose binds to an allosteric site on the repressor protein causing a conformational change. As a result of this change, the repressor can no longer bind to the operator region and falls off.How is lac operon regulated? Regulation of the lac Operon The activity of the promoter that controls the expression of the lac operon is regulated by two different proteins. One of the proteins prevents the RNA polymerase from transcribing (negative control), the other enhances the binding of RNA polymerase to the promoter (positive control).