In the lac operon, the operator region is located between the promoter and the structural genes. The operator region contains the binding site for the lac repressor protein, which can inhibit the expression of the structural genes by preventing RNA polymerase from binding to the promoter.
If the operator region was placed before the promoter of the structural genes (i.e., if the operator was positioned upstream of the promoter), it would likely result in the constitutive expression of the structural genes. Here's a possible scenario:
1. In the absence of lactose: Normally, in the absence of lactose, the lac repressor protein binds to the operator, physically blocking the binding of RNA polymerase to the promoter. This prevents transcription of the structural genes (lacZ, lacY, and lacA), and the lac operon remains in a repressed state.
2. Placing operator before the promoter: If the operator region is positioned upstream of the promoter, the repressor protein could continuously bind to the operator, effectively blocking RNA polymerase from binding to the promoter. This would result in a constant inhibition of the operon, irrespective of the presence or absence of lactose.
3. Constitutive expression: Without the repressor protein being able to dissociate from the operator and allow RNA polymerase to bind, the lac operon would be constitutively expressed. The structural genes (lacZ, lacY, and lacA) would be continuously transcribed, leading to the synthesis of their respective enzymes, such as β-galactosidase and lactose permease, even in the absence of lactose.
