Lac Operon & Trp Operon

Question 1

Does E.coli metabolize lactose all the time?

Answer 1

No

• Growth environment changes all the time
• Production of protein costs energy & raw materials

Question 2

How does E.coli metabolize lactose?

Answer 2

• Lac operon is not continuously expressed when the repressor binds to the Operator region

Question 3

How does E.coli metabolize lactose?

Enzyme functions and which gene produces them

Answer 3

Repressor protein: produced by Lac I gene
Function - binds to the operator, preventing gene expression of the lac operon in the absence of lactose

Beta-galactosidase: produce by Lac Z gene
Function - breaks down lactose into glucose and galactose

Lactose permease: produce by lac Y gene
Function - transports lactose into the cell

Galactoside transacetylase: produced by lac A gene
Function - uncertain

Question 4

When the lac operon is switched off

lactose is absent

Answer 4

• RNA polymerase cannot recognize and bind to the promoter, cannot transcribe the structural genes to form mRNA
• Repressor protein binds to the operator, blocking the promoter region
• No transcription occurs, no mRNA produced and no protein is expressed

Question 5

When the lac operon is switched off

lactose is present

Answer 5

• Allolactose binds to the repressor protein
• Shape of the repressor protein changes, not able to bind to the operator region
• RNA polymerase recognizes and binds to the promoter region, initiating transcription
• More lactose is taken up and broken down for ATP production

Question 6

Absence of lactose permease

Answer 6

• Uptake of lactose by the cell is blocked
  Repressor bind to the operator region
• No transcription, no mRNA produced, no protein expressed

Question 7

Absence of repressor protein

Answer 7

• Genes will be expressed continuously
• Lactose is taken up and broken down
• Beta-galactosidase is produced continuously

Question 8

Use of lactose analog

Answer 8

IPTG

• binds to the repressor
• function as an inducer of gene expression

X-Gal

• can be hydrolyzed by beta-galactosidase
• producing a blue color product

Question 9

What type of operon is the lac and trp operon?

Answer 9

Lac operon:

• usually “off”
• turned “on” by inducer
• it is inducible

Trp operon:

• usually “on”
• turned “off” by corepressor
• it is repressible

Question 10

When the trp operon is switched on (tryptophan is absent)

Answer 10

• Repressor is inactive, cannot bind to the operator
• Transcription occurs
• Proteins Trp A, Trp B, Trp C, Trp D, and Trp E are produced, they make tryptophan together

Question 11

When the trp operon is switched off (tryptophan is present)

Answer 11

• Tryptophan binds to the repressor, causing confirmational change
• Active repressor binds to the operator
• Prevent RNA polymerase from binding and transcribing the operon
• Tryptophan is not produced