Regulation of Gene Expression

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Slide 9

Fig. 18-3

Polypeptide subunits that make up

enzymes for tryptophan synthesis

(b) Tryptophan present, repressor active, operon off

Tryptophan

(corepressor)

(a) Tryptophan absent, repressor inactive, operon on

No RNA made

Active

repressor

mRNA

Protein

DNA

DNA

mRNA 5

Protein

Inactive

repressor

RNA

polymerase

Regulatory

gene

Promoter

Promoter

trp operon

Genes of operon

Operator

Stop codon

Start codon

mRNA

trpA

5

3

trpR

trpE

trpD

trpC

trpB

A

B

C

D

E

Slide 10

Fig. 18-3a

Polypeptide subunits that make up

enzymes for tryptophan synthesis

(a) Tryptophan absent, repressor inactive, operon on

DNA

mRNA 5

Protein

Inactive

repressor

RNA

polymerase

Regulatory

gene

Promoter

Promoter

trp operon

Genes of operon

Operator

Stop codon

Start codon

mRNA

trpA

5

3

trpR

trpE

trpD

trpC

trpB

A

B

C

D

E

Slide 11

Fig. 18-3b-1

(b) Tryptophan present, repressor active, operon off

Tryptophan

(corepressor)

No RNA made

Active

repressor

mRNA

Protein

DNA

Slide 12

Fig. 18-3b-2

(b) Tryptophan present, repressor active, operon off

Tryptophan

(corepressor)

No RNA made

Active

repressor

mRNA

Protein

DNA

Slide 13

Repressible and Inducible Operons: Two Types of Negative Gene Regulation

A repressible operon is one that is usually on; binding of a repressor to the operator shuts off transcription

The trp operon is a repressible operon

An inducible operon is one that is usually off; a molecule called an inducer inactivates the repressor and turns on transcription

Slide 14

The lac operon is an inducible operon and contains genes that code for enzymes used in the hydrolysis and metabolism of lactose

By itself, the lac repressor is active and switches the lac operon off

A molecule called an inducer inactivates the repressor to turn the lac operon on

Slide 15

Fig. 18-4

(b) Lactose present, repressor inactive, operon on

(a) Lactose absent, repressor active, operon off

mRNA