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Genetic Regulatory Mechanisms
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
Lactose Operon
Absence of lactose
• Inducer -- lactose
– Absence
i
p
y
a
No lac mRNA
– Presence
• Negative control
z
Active
• Active repressor
• No expression
• Inactivation of
repressor
• Expression
o
Presence of lactose
i
p
o
z
y
a
Inactive
-GalactosidasePermease Transacetylase
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
DNA
lacl
lacZ
CAP-binding site
cAMP
Inactive
CAP
Figure 18.23a
RNA
Operator
polymerase
can bind
Active
and transcribe
CAP
Inactive lac
repressor
(a) 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.
• When glucose is abundant,
– CAP detaches from the lac operon, which
prevents RNA polymerase from binding to the
promoter
Promoter
DNA
lacl
lacZ
CAP-binding site
Operator
RNA
polymerase
can’t bind
Inactive
CAP
Inactive lac
repressor
(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.
Figure 18.23b
Induction of Lac Operon
Transacetylase
Lac repressor
b-galactosidase
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.
Lactose
Lac repressor
Lactose permease
2. lac operon proteins
are synthesized. This
promotes the efficient
metabolism of lactose.
4. Most proteins involved
with lactose utilization
are degraded.
Lac repressor
3. The lactose is depleted.
Allolactose levels decrease.
Allolactose is released from
the repressor, allowing it to
bind to the operator site.