The lac operon | Khan Academy

The Lac Operon Overview

• Regulates lactose metabolism in E. coli.

• Expressed only when:

  • Lactose is present.

  • Glucose is absent.

• Key Regulatory Proteins:

  • Lac Repressor: Inhibits transcription unless lactose is present.

  • Catabolite Activator Protein (CAP): Enhances transcription when glucose is low.

Structure of the Lac Operon

• Contains three genes:

  • lacZ: Encodes β-galactosidase (breaks down lactose into glucose & galactose).

  • lacY: Encodes lactose permease (transports lactose into the cell).

  • lacA: Encodes transacetylase (unclear role in lactose metabolism).

• Regulatory Sequences:

  • Promoter: RNA polymerase binding site.

  • Operator: Binding site for the lac repressor.

  • CAP binding site: Binding site for CAP that enhances transcription.

Regulatory Proteins and Their Roles

Lac Repressor (LacI Gene Product)

• Function: Blocks transcription by binding to the operator.

• Regulation:

  • Without lactose: Repressor binds to the operator, blocking RNA polymerase.

  • With lactose: Converted to allolactose, which binds to the repressor, freeing RNA polymerase.

Catabolite Activator Protein (CAP) and cAMP

• Function: Helps RNA polymerase bind to the promoter, increasing transcription.

• Regulation:

  • High glucose: Low cAMP → CAP inactive → No enhancement of transcription.

  • Low glucose: High cAMP → CAP binds to DNA → High transcription.

Expression Conditions and Transcription Levels

Key Takeaways

• The lac operon is an inducible operon (normally off but activated when conditions allow).

• It ensures efficient energy use:

  • Prefers glucose over lactose.

  • Activates when lactose is available and glucose is absent.

• Allolactose acts as an inducer by inactivating the repressor.

• cAMP levels regulate CAP activity based on glucose availability.