Chapter 16 Flashcards
When is the control of gene expression often most strongly controlled?
at the first step: initiation of transcrition
How is the initiation of transcription controlled?
by proteins that bind the DNA in the gene promoter
- RNA polymerase and other helping proteins bind as well
In E. coli, a separate regulatory gene codes for what?
Is it part of the operon it controls?
- a regulatory protein
- no
What is an operator?
- binding site
- when the regulatory protein binds a DNA sequence
Where is the operator located in relation to the promoter?
- near the promoter
- often between the promoter and the first coding region of the operon
What are DNA-binding proteins?
- regulatory proteins
- expressed gene will give rise to a protein
- can bind without unwinding the DNA
Are DNA binding domains of each family homologous or analogous?
What about members of different families?
homologous
- but can evolve to bind different DNA sequences
- only analogous
What are the different types of DNA-binding proteins?
- Helix-turn-helix
- Zinc finger
- Leucine zipper
Define constitutive
- no operators present
- promotor is always available and functions all the time
What are negative inducible operons?
- regulated by repressor protein (the regulatory protein is a repressor)
- inducible: transcription is off unless turned on
Negative inducible operons:
How is transcription turned on?
- by a small molecule (inducer)
- binds to the repressor and turns it off
In inducible operons, the lack of what allows gene to be transcribed?
repressor
What are negative repressible operons?
- regulated by a repressor protein
- repressible: transcription is on unless turned off
Negative repressible operons:
How is transcription turned off?
- by a small molecule (corepressor)
- binds to the repressor and turns it on
- repressor can now bind to the operator and inactivate the gene
in repressible operons, what must be present in order to turn off gene transcription?
active repressor
What is a positive control?
When the regulatory protein that binds is an activator
- increases transcription
What is positive inducible?
- transcription is off
- activator protein is inactive unless binded to an inducer
What is positive repressible?
- Transcription is on
- activator protein is active unless binded to a small molecule that inactivates it
- represses the operon
- activator protein is active unless binded to a small molecule that inactivates it
The Lac operon allows E. coli to use what as a carbon and energy source?
lactose (milk sugar)
Regulation of the lac operon: Is it on or off when there is…
Glucose only?
Glucose and lactose?
Lactose only?
Neither?
- nearly turned off
- nearly turned off
- turned on
- nearly turned off
The lac operon contains how many cistorns ( protein coding regions)?
What are they?
3
- lac Z
- lac Y
- lac A
What is the lac Z cistron?
- codes for (beta)-galactosidase
- breaks lactose into glucose and galactose
- converts some lactose into allolactose
- breaks up into glucose and galactose
What is the lac Y cistron?
- lactose permease
- cell membrane transported for lactose
- even with the lac operon off, there is still enough to allow a low level of lactose in
lac Y cistron:
Even with the lac operon off, there is still enough to allow a low level of lactose in.
Why is this important?
The cell has to be able to detect that lactose is present
What is the lac A cistron?
- transecetylase
- acetylates lactos-related substrates
- not need for lastose metabolism, but gives E. coli an advantage in the presence of non-metabolizable lactose analogs
What are analogs?
a molecule similar to another molecule
Lac operon:
Define when it is positive inducible
CAP activates the operon in the absence of glucose
Lac operon:
Define when it is negative inducible
the lac repressor inactivates the operon in the absence of lactose
Positive inducible control of the lac operon by CAP:
Most cellular organisms can convert ATP into what?
What type of molecule is it?
- cyclic AMP (cAMP)
- signaling molecule
In E. coli, cAMP level are what in relation to glucose levels?
reversely proportional
lac operon/positive inducible:
How is CAP activated?
What is its un-abreviated name?
- cAMP bind to it
- catabolite activator protein
lac operon/positive inducible:
How does the lac operon become active?
- when the active CAP activates it
- binding upstream from the promoter and recruiting RNA polymerase
Lac operon/negative inducible:
The lac operon is coded by what?
- the lac I locus
- has its own constitutive promoter (not part of the lac operon)
Lac operon/negative inducible:
In the absence of lactose, the lac repressor binds to what of the lac operon?
What happens afterwards?
- operator
- blocks RNA polymerase
Where is the operator located in the lac operon?
between the promoter and the first coding region
Lac operon/negative inducible:
binding of the lac repressor to the operator is what type of process?
What does this mean?
- equilibrium process
- the operator isn’t constantly bound
Lac operon/negative inducible:
In the presence of lactose what converts some lactose into allolactose?
What does the allolactose do?
- (beta)-galactosidase
- binds to the lac repressor and prevents it from binding
Lac operon:
Is the lac operon on or off, and is the CAP and lac repressor active or inactive when there is…
Glucose only?
Glucose and lactose?
Lactose only?
Neither?
- lac operon: nearly off, CAP: inactive, Lac repressor: ACTIVE
- lac operon: nearly off, CAP: inactive, Lac repressor: inactive
- lac operon: nearly ON, CAP: ACTIVE, Lac repressor: inactive
- lac operon: nearly off, CAP: ACTIVE, Lac repressor: ACTIVE
Does the cell favor lactose or glucose?
glucose
