the operon concept - google sitessites.google.com/site/brmohanty/theoperonconcept.pdf · the lac...

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The Operon Concept

B. R. Mohanty

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Why - regulate gene expression?

To adjust to sudden changesTo conserve energyTo save resources

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Where – regulate gene expression?

Block the transcription of mRNA for that proteinHydrolyze the mRNA after it is made and prior to translationPrevent translation of the mRNA at the ribosomeHydrolyze the protein after it is madeInhibit the function of the protein

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How – regulate a metabolic pathway?

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An operon

Promoter (DNA)– where RNA polymerase binds

Operator (DNA)– lies between promoter and structural genes

Repressor (protein)– binds to operator to block transcription

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The lac operon of E. coli – an inducible system

The operon containing the genes for the three lactose metabolizing proteins of E. coli is called the lacoperonβ-galactosidase (lacZ) - catalyzes the hydrolysis of lactose to glucose and galactose.β-galactoside permease (lacY) is a carrier protein in the bacterial plasma membrane that moves the sugar into the cells.β-galactoside transacetylase (lacA), that transfers acetyl group from acetyl-CoA to β-galactosides.

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Like all other genes, the i (inducibility) gene itself has a promoter, pi. There is no operator between pi and the i gene. Therefore, the repressor of the lac operon is a constitutive protein. Pi does not bind to RNA polymerase effectively.

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1,6-allolactose

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The lac operon – summary

In the absence of inducer, the operon is turned off.Control is exerted by a regulatory protein—the repressor—that turns the operon off.Regulatory genes produce proteins whose sole function is to regulate the expression of other genes.Certain other DNA sequences (operators and promoters) do not code for proteins, but are binding sites for regulatory or other proteins.Adding inducer turns the operon on.

Operator-repressor control that induces transcription

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The trp operon of E. coli – a repressible system

In repressible systems, the repressor protein cannot shut off its operon unless it first binds to a corepressor, which may be either the metabolic end product itself (tryptophan in this case) or an analog of it.

Operator-repressor control that represses transcription

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Inducible Vs Repressible systems

In inducible systems, the substrate of a metabolic pathway (the inducer) interacts with a regulatory protein (the repressor) to render it incapable of binding to the operator, thus allowing transcription.In repressible systems, the product of a metabolic pathway (the corepressor) interacts with a regulatory protein to make it capable of binding to the operator, thus blocking transcription.In both kinds of systems, the regulatory molecule functions by binding to the operator.

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Promoter mediated control

Promoter-CRP-cAMP complex – cataboliterepression

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The inducible lac and repressible trp systems—the two operator–repressor systems—are examples of negative control of transcription because the regulatory molecule (the repressor) in each case prevents transcription. The promoter– catabolite repression system is an example of positive control of transcription because the regulatory molecule (the CRP–cAMP complex) enhances transcription.

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The ara operon

Arabinose isomerase (araA) - converts arabinose to ribuloseRibulokinase (araB)- phosphorylates ribulose

Ribulose-5-phosphate epimerase (araD) -converts ribulose-5-phosphate to xylulose-5-phosphate which can then be metabolized via the pentose phosphate pathwayaraC gene product regulates these 3 structural genes

Ellis Englesberg worked out ara operon in E. coli

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Arabinose absent

No transcription

AraC

ara operon in E. coli

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Arabinose ( ) present

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ara operon exhibits both positive & negative controlAraC is a autoregulatory proteinCRP doesn’t assist RNA polymerase here but assists in rearrangement of AraC

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Viral gene regulation

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Y~diëX

Thanks to all the authors for their original works

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