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.
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.
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.
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.
Glucose | Lactose | CAP Binds? | Repressor Binds? | Transcription Level |
+ | - | No | Yes | No transcription |
+ | + | No | No | Low transcription |
- | - | Yes | Yes | No transcription |
- | + | Yes | No | Strong transcription |
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.
The lac operon | Khan Academy
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.
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.
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.
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.
Glucose | Lactose | CAP Binds? | Repressor Binds? | Transcription Level |
+ | - | No | Yes | No transcription |
+ | + | No | No | Low transcription |
- | - | Yes | Yes | No transcription |
- | + | Yes | No | Strong transcription |
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.