Lac Operon
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
Transcriptional Regulation in Prokaryotes
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
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.
Inducible genes
Repressible genes
Operon
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
Lac Operon
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”
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
(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
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
Regulation of Lac Operon
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
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
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.
Thank You!!!
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