1. Genetic Regulatory Mechanisms

2. Inducible Genes - Operon Model
Definition: Genes whose expression is turned on by the presence of some substance
Lactose induces expression of the lac genes
Catabolic pathways

3. Lactose Operon Inducer -- lactose Negative control Absence Presence pz y a
No lac mRNA
Absence of lactose
Active
Inducer -- lactose
Absence
Active repressor
No expression
Presence
Inactivation of repressor
Expression i p o z y a
-Galactosidase
Permease
Transacetylase
Presence of lactose
Inactive
Negative control

4. Positive Gene Regulation- CAP
In E. coli, when glucose is always the preferred food source
When glucose is scarce, the lac operon is activated by the binding of the catabolite activator protein (CAP)
Promoter
Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized. If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces large amounts of mRNA for the lactose pathway.
(a)
CAP-binding site
Operator
RNA
polymerase
can bind and transcribe
Inactive CAP
Active CAP
cAMP
DNA
Inactive lac
repressor
lacl
lacZ
Figure 18.23a

5. When glucose is abundant,
CAP detaches from the lac operon, which prevents RNA polymerase from binding to the promoter
Figure 18.23b
(b)
Lactose present, glucose present (cAMP level low): little lac mRNA synthesized. When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription.
Inactive lac
repressor
Inactive CAP
DNA
RNA
polymerase
can’t bind
Operator
lacl
lacZ
CAP-binding site
Promoter

6. Lac - operon
High level of glucose, not present lactose, negative regulation on and there is not a positive regulation
High level glucose and lactose, negative regulation and positive regulation are off, lac operon is not active
In cells is a low level of glucos and high level of lactose, there is not negative regulation and there is positive regulation, all enzymes are in very high amount
Low level of glucose and lactose – not active

7. Induction of Lac Operon
1. When lactose becomes
available, a small amount of it
is taken up and converted to
allolactose by β-galactosidase.
The allolactose binds to the
repressor, causing it to fall off
the operator site.
Transacetylase
Lac repressor
b-galactosidase
Lactose
Lac repressor
lac operon
lac operon
Lactose permease
4. Most proteins involved
with lactose utilization
are degraded.
2. lac operon proteins
are synthesized. This
promotes the efficient
metabolism of lactose.
lac operon
lac operon
3. The lactose is depleted.
Allolactose levels decrease.
Allolactose is released from
the repressor, allowing it to
bind to the operator site.
Lac repressor