The sldes shares about the negative and positive regulation of lac operon under different concentration of glucose/lactose.

1. Prepared by: Deepali RahiPrepared by: Deepali Rahi
2015-11-1022015-11-102
CPBMB, CoHCPBMB, CoH
Course Title:Course Title:
Fundamentals of Molecular BiologyFundamentals of Molecular Biology
MBB 502MBB 502
(3+0)(3+0)
lac Operon

2. Gene reGulation
 Constitutive gene expression and Regulated
gene expression
Gene expression can be controlled in many
stages
Bacteria- the basic concept for the way of
transcription controlled – operon
Prokaryotes - polycistronic

3.  Nobel Prize → Jacob and Monod (1965)
 Lactose metabolism – E. coli - Lactose Operon
 “Operon”→ Proceedings of the French Academy of
Sciences in 1960
 Group of genes that are transcribed at the same time
Introduction

4.  Regulatory GeneRegulatory Gene
 Promoter GenePromoter Gene
 Operator GeneOperator Gene
 Structural GeneStructural Gene
ComponentsComponents
JacobJacob MonodMonod

5. LACTOSE METABOLISMLACTOSE METABOLISM

6.  Inducible operon (switch off in normal condition)
 Regulated by presence of substrate/inducer i.e. lactose (but
it was later discovered allolactose is the true inducer)
 Presence of inducer/effector makes it switch on. This is
called induction/ derepression
 This is negative regulation of lac operon
Mechanism ofMechanism of LacLac OperonOperon

8. •
• Two types of binding sites
• Dual function
• Allosteric control
repressorrepressor

9. DNA-REPRESSOR BINDING

10. Structure of a monomer of lac repressor obtained from PDBStructure of a monomer of lac repressor obtained from PDB

11.  Known as catabolite repression/glucose effect
 Growing E.coli in a medium with lactose+glucose, the
organism utilizes glucose first and after that it goes for
lactose
 When glucose is available in the environment, synthesis of
β-galactosidase is repressed
lac Operon under positive regulation

12. Control ofControl of laclac by CRP-cAMPby CRP-cAMP
COMPLEXCOMPLEX
• positive regulator protein- catabolic repressor
protein (CRP) or catabolic activator protein (CAP)
– activator– activator
• second messenger- cyclic AMP (cAMP)––
coactivatorcoactivator
• CRP is active when bound to cAMP and form a
complexcomplex-small inducer and act as a positive
control
• cAMP binding alter – structure of CRP

13. CRP homodimer
•RED- bound
molecule of cAMP
•YELLOW – RNA
polymerase
interation
•Bending of DNA
around protein
•Binding is helix-
turn-helix motif
within DNA

14.  Binding of complex- enhance RNA transcription 50 folds
 The presence of the higher level of glucose the synthesis of
cAMP is inhibited
 CRP binding to the DNA declines. Thus decreasing the lac
operon activity
 Strong induction of lac operon- lactose (to inactivate
repressor)+ low level of glucose (increase CRP-cAMP
complex)
Control ofControl of laclac by CRP-Camp COMPLEXby CRP-Camp COMPLEX

16.  Glucose must be absent and lactose must be presentGlucose must be absent and lactose must be present
simultaneously for full induction of lac operon systemsimultaneously for full induction of lac operon system
 Positive and negative regulation must occur simultaneouslyPositive and negative regulation must occur simultaneously
SummarySummary
Glucose Lactose cAMP Lac operon
Present Absent low No mRNA
Absent Absent high No mRNA
Present
(high level)
Present low Basal level of
mRNA synthesis
Present
(low level)
Present High Rapid mRNA
synthesis

17. Thank YouThank You