BIOB11H3 Lecture Notes - Lecture 9: Lac Repressor, Lac Operon, Camp Receptor Protein
by Elton Yeung
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BIOB11-L9: Gene Regulation (Promoters and Control Circuits)
Collection of components that’s responsible to organize response to a particular stimulus
Components: Promotor, Operator, Structural genes, Regulatory gene
Regulatory gene encode for Regulatory protein which interact operator sequence
Control the expression of Structural genes (Code for enzymes of the same metabolic pathway)
- Repressor protein is active when its translated, will be inactivated when bound to Lac
- When Lactose is present, it binds to repressor and inactivate it
induced Operon, transcription of mRNA
- Translation of mRNA yields 3 enzymes that convert Lactose to Glucose & Galactose
What happens when Lactose supply is reduced?
Binding of lactose to Lac repressor is transient,
When lactose conc. falls, repressor becomes active,
Repressor now able to bind to operator
Repression occurs (Transcription blocked)
Lactose Metabolism in E. Coli
Lactose = Disaccharide of Galactose + Glucose covalently linked together
Lactose (act as an inducer) β-galactosidase expressed Lactose metabolized into Glucose + Galactose
Rapid expression of β-galactosidase mRNA after inducer is added
mRNA level crashes when inducer is eliminated Level of β-galactosidase will fall after time
(not shown in graph)
Cis vs Trans: “Cis-acting promotor sequence to which a trans-acting transcription factor binds”
Trans: Trans-acting / Transcription factor etc. Soluble and Able to diffuse around
Cis: On the same strand, e.g. DNA sequence that serves as a binding site for a TF, TATA box… (usually nearby the promotor region)
Controlling when and where the coding region should be expressed
Positive vs Negative control
Depends on the active form of the trans-acting factor (e.g. repressor), and its effect upon binding to
its target cis-acting sequence.
(For almost every gene, theres both positive and negative influences on activation and expression)
Bacterial Biochemical Logic for LAC=GLU + GAL
1. If glucose is available, why expend energy to make enzymes to catabolize lactose?
2. If lactose is absent, why expend energy to make enzymes to catabolize it?
Both positive & negative control involved
Positive control: If glucose level is low, cAMP level is high; cAMP binds to CRP (cAMP Receptor Protein), and complex activates the lac operon
Positive control mediated by cAMP/CRP // Negative control mediated by lac repressor
*CRP is also called CAP (Catabolite Activator Protein)
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