The Lac Operon

Question 1

What is the Lac Operon?

Answer 1

Contains 3 genes
- lacZ encoding beta-galactosidase
- lacY encoding beta-galactoside permease
- lacA encoding thiogalactoside transacetylase

The lacI gene is upstream + encodes the repressor

Question 2

What does beta-galactosidase do?

Answer 2

Catalyses reaction that cleaves the disaccharide (lactose) into beta glucose + galactose which can be converted to glucose - both used for energy

Question 3

What does beta-galactosidase permease do?

Answer 3

Allow uptake of lactose into E.coli

Question 4

Purpose of the lac operon

Answer 4

• Used to utilise lactose as a carbon/energy source when lactose is present + there is no glucose (preferred)
• Operon is switched on + off in response to changes in lactose + glucose conc.
  -Transcribed when glu. is low + lac. is high

Question 5

In absence of lactose..

Answer 5

• lacI is constitutively expressed
• Lac repressor protein binds to operator in lac operon to prevent transcription

Question 6

Lactose regulation when lactose is present

Answer 6

• Lactose binds the repressor
• Inducer-repressir complex can’t bond to operator
• RNA polymerase can bind productivity + lac operon is transcribed

Question 6

Cis-acting elements

Answer 6

On the same DNA molecule as the gene the control
- Repressors and activators

Question 7

Trans-acting factors

Answer 7

Can interact with multiple genes / DNA sites on diff. DNA molecules
- RNA polymerase, repressors + activators

Question 8

Positive control of the lac operon

Answer 8

• In response to glucose starvation
• Activator is the cAMP receptor protein (CRP)
• Low levels of glucose result in synthesis + build up of cyclic adenosine monophosphate - a signalling molecule

Question 9

Glucose level regulate cAMP levels

Answer 9

• High glucose: ATP is not coverted to cAMP
• Low glucose: ATP is converted to cAMP - this then binds with CRP to form an CRP-cAMP complex - signals glucose starvation

Question 10

The cAMP-CRP complex mechanism of action

Answer 10

• Activates lac operon
• Binds to lac promoter upstream from RNA polymerase
• Then increases rate of transcription intiation by helping RNA-P bind to the promotor

Question 11

Summary - When is lac operon transcribed?

Answer 11

• Glucose but no lactose = repressor blocks transcription
• Glucose + lactose = repressor doesn’t bind but lack of cAMP-CRP means low transcription rate
• No glucose + lactose = repressor doesn’t bond & cAMP-CRP is present to activate transcription