4. Positive Control
• Regulation of 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
(based on 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
• First operon 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 the Lac 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
• Binding site of RNA Polymerase
– LacO
• Binding site of Lac repressor
– LacZ
• Encodes β-galactosidase
– LacY
• Encodes lactose permease
– LacA
• Encodes galactoside transacetylase

15. Lactose metabolism in Escherichia
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
• Allosteric regulation
• 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
• Catabolite repression
• 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.