Operon
Cluster of genes that are transcribed into one long mRNA allowing the genes of a single pathway to be controlled with a single on/off switch
Operator
Segment of DNA that acts as a switch to control access of RNA polymerase to the gene
Activator
Protein that binds to DNA and stimulates gene transcription
Negative regulation
Bound repressor protein blocks transcription
Positive regulation
Bound activator protein promotes transcription
LAC OPERON
- Set of genes that code for proteins necessary for the bacterium to use the sugar lactose as an energy source
Structure
- 3 genes: lacZ (beta-galactosidase), lacY (lactose permease) and lacA (galactoside acetyltransferase)
- Promoter region:
- CAP-binding site
- Operator
- lacI gene – prior to CAP-binding site; codes for repressor protein; under control of a different promoter
High glucose, no lactose
- CAP-binding site empty (inactive catabolite activator protein due to low cAMP levels)
- Repressor is bound to operator (no allolactose present to inactivate repressor)
- No transcription
No glucose, no lactose
- CAP is bound to CAP-binding site (low glucose means high levels of cAMP)
- Repressor is bound to operator
- No transcription
High glucose, lactose available
- Cap-binding site empty
- Operator is empty (allolactose present inactivates repressor protein)
- Low-level transcription
No glucose, lactose available
- CAP is bound to CAP-binding site
- Operator is empty
- High levels of transcription
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