Download - Lac operon
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Lac Operon
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Table of Contents• Transcriptional Regulation in prokaryotes– Types of Regulation– Types of genes: Based on the small effector molecule
• Operon– Definition and Discovery
• Lac operon– Components of the Lac Region– Lac metabolism in Escherichia coli
• Regulation of Lac Operon– Negative Control– Positive Control
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Transcriptional Regulation in Prokaryotes
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Types of regulationPositive Control
• Regulation of gene expression by a regulatory protein called activator protein.
• Activator protein – allows transcription
Negative Control• Regulation of gene
expression by a regulatory protein called repressor protein.
• Repressor protein – hinders transcription
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Types of Genes(based on the small effector molecules)
Inducible Genes• Genes that are transcriptionally
regulated by an inducer.• Inducer – a small effector
molecule that binds to:– an activator protein which
allows binding of the protein to the DNA.
– A repressor protein which hinders binding of the protein to the DNA.
Repressible Genes• Genes that are transcriptionally
regulated either by a corepressor or an inhibitor.
• Corepressor – a small effector molecule which binds to a repressor protein that allows binding of the protein to the DNA.
• Inhibitor – a small effector molecule which binds to an activator protein that restricts binding of the protein to the DNA.
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Inducible genes
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Repressible genes
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Operon
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Definition and Discovery
Definition• Refers to two or more
contigious genes and the genetic elements that regulate their transcription in a coordinate fashion
• Consists of promoter, operator, and structural genes
• Encodes proteins (enzymes) that have common function
Discovery• François Jacob and Jacques
Monod (1940)• Via combination of
biochemical and genetic analyses
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Lac Operon
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Lactose operon• First operon to be discovered• François Jacob and Jacques Monod (1940)
• Contains three genes that codes for proteins involved in lactose metabolism in Escherichia coli that plays a role in bacterial growth
• It is said to be inducible– Due to its inducer “Lactose”
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Components of the Lac Region
• Regulatory gene (LacI)– Expresses the Lac repressor• A regulatory protein of Lac Operon• Responsible for the Negative Control of Lac Operon
• Lac Operon– CAP site
• The binding site of CAP–Catabolite Activator Protein–Another regulatory protein of Lac Operon–Responsible for the Positive Control of lac Operon
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(Continuation)– LacP
• Binding site of RNA Polymerase
– LacO• Binding site of Lac repressor
– LacZ• Encodes β-galactosidase
– LacY• Encodes lactose permease
– LacA• Encodes galactoside transacetylase
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Lactose metabolism in Escherichia coli
• Involves the three enzymes expressed by the lac operon– β-galactosidase
• Catalyzes Lactose → Glucose + Galactose• Catalyzes Lactose → Allolactose (side reaction)
– Lactose permease• Serves as the channel protein of lactose to the cell
– Galactoside transacetylase• Acetylation of lactose and lac analogs
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Regulation of Lac Operon
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Negative Control• Allosteric regulation• Lac Repressor– Responsible for the negative control of lac operon– Presence of lactose and lac analogs
• Binding to the allosteric site• Conformational change in the active site• Hindrance of lac repressor binding• Transcription occurs
– Absence of lactose and lac analogs• Binding of lac repressor = No Transcription
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Positive Control• Catabolite repression• CAP– Catabolite Activator Protein• Responsible for the positive control of lac operon• Activated by cAMP (Cyclic Adenosine
Monophosphate)• Can be deactivated by glucose
– Inhibits adenylyl cyclase via glucose transport and a signalling pathway
– Adenylyl cyclase catalyzes the conversion of ATP to cAMP
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References
• Weaver RF. 2012. Molecular Biology 5th Ed. New York: McGraw-Hill. 914 P.
• Karp G. 2010. Cell and Molecular Biology: Concepts and Experiments 6th ed. New Jersey: John Wiley & Sons, Inc. 837 P.
• Brooker RJ. 2012. Genetics: Analysis & Principles 4th Ed. New York: McGraw-Hill. 868 P.
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Thank You!!!