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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

Glucose

Lactose

CAP Binds?


Repressor Binds?

Transcription Level

+

-

No

Yes

No transcription

+

+

No

No

Low transcription

-

-

Yes

Yes

No transcription

-

+

Yes

No

Strong transcription

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.


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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

Glucose

Lactose

CAP Binds?

Repressor Binds?

Transcription Level

+

-

No

Yes

No transcription

+

+

No

No

Low transcription

-

-

Yes

Yes

No transcription

-

+

Yes

No

Strong transcription

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.