Regulation of Gene Expression

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Protein

DNA

DNA

mRNA 5

Protein

Active

repressor

RNA

polymerase

Regulatory

gene

Promoter

Operator

mRNA

5

3

Inactive

repressor

Allolactose

(inducer)

5

3

No

RNA

made

RNA

polymerase

Permease

Transacetylase

lac operon

-Galactosidase

lacY

lacZ

lacA

lacI

lacI

lacZ

Slide 16

Fig. 18-4a

(a) Lactose absent, repressor active, operon off

DNA

Protein

Active

repressor

RNA

polymerase

Regulatory

gene

Promoter

Operator

mRNA

5

3

No

RNA

made

lacI

lacZ

Slide 17

Fig. 18-4b

(b) Lactose present, repressor inactive, operon on

mRNA

Protein

DNA

mRNA 5

Inactive

repressor

Allolactose

(inducer)

5

3

RNA

polymerase

Permease

Transacetylase

lac operon

-Galactosidase

lacY

lacZ

lacA

lacI

Slide 18

Inducible enzymes usually function in catabolic pathways; their synthesis is induced by a chemical signal

Repressible enzymes usually function in anabolic pathways; their synthesis is repressed by high levels of the end product

Regulation of the trp and lac operons involves negative control of genes because operons are switched off by the active form of the repressor

Slide 19

Positive Gene Regulation

Some operons are also subject to positive control through a stimulatory protein, such as catabolite activator protein (CAP), an activator of transcription

When glucose (a preferred food source of E. coli) is scarce, CAP is activated by binding with cyclic AMP

Activated CAP attaches to the promoter of the lac operon and increases the affinity of RNA polymerase, thus accelerating transcription

Slide 20

When glucose levels increase, CAP detaches from the lac operon, and transcription returns to a normal rate

CAP helps regulate other operons that encode enzymes used in catabolic pathways

Slide 21

Fig. 18-5

(b) Lactose present, glucose present (cAMP level

low): little lac mRNA synthesized

cAMP

DNA

Inactive lac

repressor

Allolactose

Inactive

CAP

lacI

CAP-binding site

Promoter

Active

CAP

Operator

lacZ

RNA

polymerase

binds and