Lac operon in E. coli : The operon controlling lactose metabolism consists of -
(a) A regulatory or inhibitor
(i) gene: Codes for the repressor.
(b) 3 structural genes :
(i) z gene : Codes for $-galactosidase (hydrolyze lactose to galactose and glucose).
(ii) y gene : Codes for permease (increase permeability of the cell to lactose).
(iii) a gene : Codes f.or a transacetylase.
1. The genes present in the operon function together in the same or related metabolic pathway. There is an operator region for each operon.
2. If there is no lactose (inducer), Lac operon remains switched off. The regulator gene synthesizes rnF, NIA to produce the repressor protein; this protein binds to the operator genes and blocks RNA polymerase movement. So the structural genes are not expressed.
3. If lactose is provided in the growth medium, the lactose is transported into the E. coli cells by the action of permease. Lactose (inducer) binds with repressor protein.
4. So repressor protein cannot bind to operator gene. The operator gene becomes free and induces the RNA polymerase to bind with promoter gene. Then transcription starts. Regulation of lac operon by repressor is called negative regulation.
In the absence of inducers :
