regulation of lac operon positive nd negative

Keshav Narayan PaiMSc II

DOS in BotanyManasagangotri

REGULATION OF

Content• Introduction• Negative Regulation• Positive Regulation• Difference between Positive and negative type.

Introduction

• OPERON MODEL - Francois Jacob and Jaques Monod (1961).

• Regulated genes can be switched on/off depending on cell's metabolic needs.

Francois Jacob

Jaques Monod

• An Operon is a group of structural genes function as a single unit.

• It contains Structural genes i.e lac Z, lac Y and lac A.

LacZ gene- codes beta-galactosidase.LacY gene- codes galactose permease.LacA gene- codes thio galactoside

transacetylase.• Operator• Promoter• Regulator gene

Designation of gene

Codes for enzyme Function of the enzyme

lac Z β-galactosidase Enzymatically cleaves lactose into glucose & galactose.

lac Y galactoside permease

a membrane protein required for the active transport of lactose into the cytoplasm of the bacterium

lac A Thio-galactoside transacetylase

help to get rid of the toxic thiogalactoside

The lac promotor have 2 regulatory sites, known as operator and CAP site.

lac I gene (Regulator gene ) encodes the lac repressor, a regulatory protein that binds to the DNA and prevents transcription.

Operator (lac O) is a binding site for repressor.

• CAP( Catabolite Activator Protein ) acts as an activator.

• CAP binds DNA and activate the lac genes in the absence of glucose.

• The lac repressor binds DNA and repress transcription only in the absence of lactose.

Types•There are 2 types of regulation of the lac

operon.

• Negative Regulation- When the expression of genetic information is quantitatively decreased by the presence of specific regulatory element, it is called as negative regulation

• Positive Regulation- When the expression of genetic information is quantitatively increased by the presence of specific regulatory element, it is called as positive regulation

Negative Regulation•Negative form of regulation involves

the lac repressor protein which binds to the sequence of nucleotides found within the lac operator site.

•Once the lac repressor binds, it prevents RNA polymerase to transcribe the lacZ, lacY and lacA genes.

Lac repressor binds as a tetramer.So each Operator is contacted by the repressor dimer.

•The ability of the lac repressor to bind to the operator site depends on whether or not allolactose is bound to it.

• Allolactose acts as an inducer and binds to the repressor.

• When allolactose binds to the repressor, a change occurs that prevents the lac repressor from binding to the operator site.

• RNA polymerase is now free to transcribe the operon.

• In presence of lactose – repressor protein binds to allactose and forms Repressor-Lactose complex. RNA polymerase binds to P site and genes transcribed.

• In absence of lactose – repressor protein binds to O site of lac operon , block the RNA polymerase from reaching P site. No transcription .

Positive Mechanism• This type of regulation is influenced by the

presence of sugar glucose.• The presence of glucose ultimately leads to

the repression of the lac operon.

• Catabolite repression Is a type of +ve control in the lac operon.

• cAMP binds to an activator protein known as catabolite activator protein(CAP) binds to a site near the promoter & stimulates the binding of RNA polymerase.

ATP synthesizes cAMP in the presence of the enzyme Adenyl cyclase.

Binding of the CAP-CAMP to the CAP site causes the DNA to bend by more than 90 degrees.

Binding enhances the transcription by which more DNA will be available for the RNA polymerase.

CRP

• CRP is a cAMP receptor Protein.• It is necessary for the high level transcription.• It alone cannot bind the DNA nor regulate

transcription.• It can form CRP-cAMP complex to bind to the

DNA.

Conclusion

ReferenceLodish.H,Berk.A,Kaiser.C.A,Krieger.M,Scott.

M.P,Bretscher.A, Ploegh.H and Matsudaira.P. 2008. Molecular Cell Biology, 6th Edition.W.H.Freeman and Company.Pp:271-273.

Watson.D.J, Baker.T.A, Bell.S.P, Gann.A, Levine.M, Losick.R.2004.Molecular Biology of the Gene,5th Edition.CSHL Press.Pp:489-495.

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