includes both the mechanism of positive and negative regulation

1. Keshav Narayan Pai
MSc II
DOS in Botany
Manasagangotri
REGULATION
OF

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

3. 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

4. • AnOperon is a groupofstructuralgenes functionasasingleunit.
• ItcontainsStructuralgenesi.elacZ,lacYandlacA.
LacZgene-codesbeta-galactosidase.
LacYgene-codes galactosepermease.
LacAgene-codes thiogalactosidetransacetylase.
• Operator
• Promoter
• Regulatorgene

6. 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

9. 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.

10. • 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.

11. 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

12. 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.

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

16. • 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.

17. • 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.

19. • 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 .

21. 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.

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

24. 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.

26. 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.

31. Reference
Lodish.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.