Control of Gene Expression in Prokaryotes Flashcards
Are genes always on or off unless controlled?
On
What dictates levels of gene expression?
Strength of a promoter
What do repressors bind to?
Operators
What does the operator overlap with?
RNA polymerase binding sites
Where do activators bind?
To sites that do not overlap with RNA binding sites
How do activators help increase gene expression?
They help RNA polymerase to bind to the promoter
In the prescence of both glucose and lactose which sugar will a prokaryotic cell use first?
Glucose
What is an operon?
A unit of genomic DNA that contains a cluster of genes under the control of a single promoter which act together as part of a unified pathway
Give an example of how an activator might work
- in the prescence of a ligand the activator is turned on
- this makes protein-protein interactions with RNA polymerase which helps binding
Give an example of how a repressor may work
- if ligand is absent the repressor can bind to the operator
- this means RNA polymerase cannot bind
What negative control mechanism is used by lac operon?
Lac repressor
Which positive control mechanism is the lac operon controlled by?
CAP-cAMP complex
Which enzyme does lac z code for?
β-galactosidase
What does β-galactosidase do?
Cleaves lactose into glucose and galactose
What enzyme does lac y code for?
β-galactoside permease
What does β-galactoside permease do?
It is a membrane bound protein which pumps lactose into the cell
What enzyme does lac A code for?
β-galactoside transacetylase
What does β-galactoside transacetylase do?
- Transfers an acetyl group from acetyl-CoA to β-galactosides
- Activates lac z
What is the structure of the lac repressor?
Tetramer
What is allolactose?
A product of metabolism of lactose
What occurs to the lac repressor in the prescence of allolactose?
- allolactose binds to the lack repressor
- Binding of allolactose prevents the lac repressor from binding to the operator
- RNA polymerase is allowed to bind and transcribe the operon
What chemical does CAP sense?
cAMP
What kind of molecule is CAP?
A protein
What happens to cAMP levels in the prescence of glucose?
They reduce
What happens in the cell when glucose levels reduce?
- Adenylate cyclase is activated
- cAMP levels rise
What kind of promoter is the lac promoter?
A weak promoter
How is the lac operon activated by CAP?
- When glucose levels fall, cAMP levels rise (due to activation of adenylate cyclase)
- cAMP binds to CAP
- CAP can then bind to the operon and recruit RNA polymerase
- produces strong gene expression
What is the dormant state of a bacterium infected with a virus called?
Lysogenic growth
What does the Cro repressor turn off?
Expression of the lambda repressor
Which operator does the lambda repressor have the highest affinity to?
OR1
Which operator does the cro repressor have the highest affinity for?
OR3
What does the lambda repressor turn off?
Expression of all phage genes, including the gene for the Cro repressor
Which operator does the lambda repressor first bind to?
OR1
Which operator is nearest to the promoter for the lambda repressor gene?
OR3
Which operator is nearest to the promoter for the Cro repressor gene?
OR1
What happens when the lambda repressor binds to OR1?
Due to the proximity of OR1 to the promoter for the Cro repressor, it blocks cro expression
What kind of binding occurs between lambda repressor molecules on operators?
Cooperative binding
What occurs when lambda repressor binds to OR2?
- It produces enchanced lambda repressor expression by enhancing RNA polymerase binding
- Now acts as a lambda activator
What occurs when lambda repressor binds to OR3?
It blocks lambda repressor expression by blocking binidng of RNA polymerase
How does the lambda repressor maintain good levels of expression?
It alternates between binding to OR2 and OR3
What prompts breakdown of the lambda repressor?
The cell entering a stressful situtation e.g. DNA damage
What happens when the cell enters a stressful situation?
- The cell produces lots of proteases
- The lambda repressor is broken down by proteolysis
- Cro represor is no longer blocked
What happens when the cro repressor binds to OR3?
- Blocks lambda repressor expression
- Allows for the expression of the remaining 50 phages genes
- Lytic growth occurs
