Negative Regulation of Lactose Operon
A repressor protein encoded by the i gene prevents the structural genes from being expressed. Close to the promoter is another cis-acting site called operator. The repressor binds to the operator and prevents RNA polymerase from initiating transcription (Figure 7.3). The gene expression is, therefore, turned off. The expression of lac i gene is constitutive, i.e., it is always expressed. The lac repressor protein is an allosteric protein. Negative regulation takes place in the absence of lactose and the repressor protein is strongly bound to the operator.
In the above figure, L denotes lactose/allolactose.
Figure 7.3 Negative regulation of Lac operon
REGULATION OF LACTOSE OPERON