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Lac operon
The document discusses gene regulation and the lactose operon. It defines positive and negative control of gene expression, where activator and repressor proteins respectively allow or hinder transcription. The lactose operon was the first discovered operon in E. coli and contains three genes for lactose metabolism. It is regulated by the lactose repressor protein which provides negative control and the catabolite activator protein which enables positive control and transcription in the presence of lactose.
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Lac operon
- 4. Positive Control • Regulationof 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
- 5. Types of Genes (basedon 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.
- 9. 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
- 11. Lactose operon • Firstoperon 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”
- 13. Components of theLac 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
- 14. (Continuation) – LacP • Bindingsite of RNA Polymerase – LacO • Binding site of Lac repressor – LacZ • Encodes β-galactosidase – LacY • Encodes lactose permease – LacA • Encodes galactoside transacetylase
- 15. Lactose metabolism inEscherichia 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
- 17. Negative Control • Allostericregulation • 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
- 18. Positive Control • Cataboliterepression • 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
- 20. 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.