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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. Absence of lactose. z. y. a. i. p. o. Active. No lac mRNA.

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Genetic Regulatory Mechanisms


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inducible genes operon model
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

z

y

a

i

p

o

Active

No lac mRNA

Presence of lactose

z

y

a

i

p

o

Inactive

-Galactosidase

Permease

Transacetylase

Lactose Operon
  • Inducer -- lactose
    • Absence
      • Active repressor
      • No expression
  • Presence
    • Inactivation of repressor
    • Expression
  • Negative control
positive gene regulation cap

Operator

RNA

polymerase

can bindand transcribe

Promoter

DNA

lacl

lacZ

CAP-binding site

ActiveCAP

cAMP

Inactive lac

repressor

InactiveCAP

(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.

Figure 18.23a

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)
slide5

Promoter

Operator

DNA

lacl

lacZ

CAP-binding site

RNA

polymerase

can’t bind

InactiveCAP

Inactive lac

repressor

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.

(b)

Figure 18.23b

  • When glucose is abundant,
    • CAP detaches from the lac operon, which prevents RNA polymerase from binding to the promoter
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.

Induction of Lac Operon

Transacetylase

Lac repressor

Lac repressor

Lactose

b-galactosidase

lac operon

lac operon

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 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