Step by step Lac operon

  1. Initiation of Transcription:

    • The lac operon is normally repressed in the absence of lactose. The Lac repressor protein, encoded by the lacI gene, binds to the operator region (lacO) adjacent to the promoter, preventing RNA polymerase from initiating transcription.

  2. Presence of Lactose:

    • When lactose is present in the environment, it can enter the bacterial cell.

  3. Lactose Metabolism:

    • Within the cell, lactose is converted into allolactose by the enzyme β-galactosidase, encoded by the lacZ gene.

  4. Allolactose and Repressor Inactivation:

    • Allolactose acts as an inducer by binding to the Lac repressor, causing a conformational change that inactivates the repressor.

  5. Repressor-DNA Dissociation:

    • The inactivated Lac repressor can no longer bind to the operator region. This dissociation allows RNA polymerase to access the promoter and initiate transcription of the lac operon.

  6. Transcription of lac Operon Genes:

    • RNA polymerase binds to the promoter region and transcribes the structural genes lacZ, lacY, and lacA.

  7. lacZ: β-Galactosidase Production:

    • The lacZ gene encodes β-galactosidase, which hydrolyzes lactose into glucose and galactose. This allows the cell to utilize lactose as a carbon source.

  8. lacY: Permease Production:

    • The lacY gene encodes permease, a protein that facilitates the entry of lactose into the bacterial cell.

  9. lacA: Transacetylase Production:

    • The lacA gene encodes transacetylase, which is involved in the removal of toxic by-products from lactose metabolism.

  10. Role of cAMP-CAP Complex:

    • Simultaneously, the presence of lactose induces the production of cyclic AMP (cAMP) within the cell.

  11. cAMP-CAP Binding to Promoter:

    • cAMP binds to CAP (catabolite activator protein). The cAMP-CAP complex binds to a specific site near the lac promoter, enhancing the affinity of RNA polymerase for the promoter.

  12. Enhanced Transcription:

    • The binding of the cAMP-CAP complex to the promoter enhances the transcription of the lac operon genes, further increasing the efficiency of lactose metabolism.