Prokaryotic Gene Regulation Flashcards
(32 cards)
Define the operon
a unit of DNA in prokaryotes that controls the expression of a cluster of genes
What does the operon include?
Promoter, Operator, Structural genes, regulatory gene
What is the function of the promoter?
where RNA polymerase binds to start transcription
What is the function of the operator?
a regulatory sequence where repressors/activators may bind
What is the function of the structural genes?
the actual genes that encode proteins
What is the function of the regulatory gene? Give an example of it.
produces a repressor or activator protein
ex: lac operon in E. coli
Define the polycistronic mRNA
Single molecules of mRNA that encode two or more protein-coding sequences, or cistrons
Define the structural gene. Give an example.
actual genes that encode proteins
ex:
In the lac operon, the structural genes are:
- lacZ (β-galactosidase)
- lacY (permease)
- lacA (transacetylase)
How do the operon, polycistronic mRNA, and structural gene relate?
- An operon contains structural genes.
- These structural genes are transcribed together into a polycistronic mRNA.
- That mRNA is translated into multiple proteins, one for each structural gene.
What are the components of the lac operon?
- Structural genes
- Regulatory Elements
- Regulatory Gene
- Inducer
What are the regulatory elements?
- Promoter
- Operator
- CAP binding site
What does the promoter do?
The binding site for RNA polymerase to initiate transcription of the structural genes
What does the operator do?
It is a DNA sequence that serves as the binding site for the lac repressor (LacI), blocking transcription when bound
What is the CAP (CRP) binding site?
A region where the cAMP-CRP complex binds to enhance transcription when glucose levels are low
What is an example of a regulatory gene?
LacI
What does the LacI do?
Encodes the lac repressor protein, which binds to the operator and prevents transcription in the absence of lactose
What does an inducer like allolactose do in the lac operon?
It binds to the lac repressor, causing it to release from the operator and allowing transcription.
What proteins and small molecules regulate the expression of the lac operon?
- LacI (repressor): Blocks transcription by binding to operator
- Allolactose: Induces transcription by inactivating repressor
- CAP (activator): Enhances transcription when bound to cAMP
- cAMP: Forms complex with CAP to promote transcription
What is the relationship of glucose and lactose levels to the expression of genes?
Low glucose + with lactose = strong expression
High glucose + no lactose = no expression
Low glucose + no lactose = no expression
High glucose + with lactose = low expression
How does the Lac repressor (LacI) regulate negative expression in the lac operon?
- Binds to operator to block transcription when lactose is absent
- Inactivated by allolactose (present when lactose is around)
How does the CRP (CAP) regulate positive expression in the lac operon?
- Binds to CAP binding site near promoter when complexed with cAMP
- cAMP levels are high when glucose is low
- cAMP-CRP complex enhances RNA polymerase binding, boosting transcription
What is the function of glucose?
Glucose acts as a repressor of alternative sugar metabolism genes (like the lac operon).
How does the level of glucose affect repression?
High glucose:
Low cAMP → cAMP-CRP complex doesn’t form → no enhancement of lac operon transcription (even if lactose is present).
Low glucose:
High cAMP → cAMP-CRP complex forms → activates transcription of alternative sugar operons (like lac operon if lactose is present).
What is the difference between positive and negative forms of regulation?
Positive regulation increases gene expression, while negative regulation decreases it
- Positive regulation typically involves an activator protein that binds to DNA and promotes transcription
- Negative regulation involves a repressor protein that binds to DNA and inhibits transcription.
