Lecture 5 Flashcards
(26 cards)
What is meant by a cis-acting regulatory mutation?
It only affects the expression of a gene or operon that is in the same DNA molecule
What is meant by trans-acting regulatory mutation?
Affects expression of a gene or operon in a different DNA molecule
What does it mean if an operator acts in cis?
It needs to be in the same DNA molecule as the operon to function
Does the repressor move around in the cell?
Yes, it is a diffusible molecule.
It is produced by i+ and can affect other DNA molecules -bind to O+ and repress.
How does regulation by glucose occur?
Occurs via cyclic AMP.
Glucose controls cAMP concentration.
What is the enzyme called that makes cAMP?
adenylate cyclase
When is adenylate cyclase activated?
In the absence of glucose
When there is no glucose…
Enzyme adenylate is activated and cAMP concentration is high.
When glucose is present…
cAMP concentration is low
How does cAMP regulate the operon?
cAMP binds to catabolite activator protein (CAP) which is a dimer- this activates it.
cAMP-CAP activates transcription
Why is activation needed?
The lac promoter is a weak promoter -sequences at -10 and -35 are non standard
Promoter by itself fails to recognise it.
How does the CAP-cAMP actually activate transcription?
The complex binds to DNA next to the -35 region of the promoter. This bends DNA by 90 degrees.
This helps RNA polymerase to bind -increases binding of RNA polymerase to promoter and so activates transcription.
So what are the 2 cases when transcription of the lac operon occurs?
When the CAP-cAMP complex binds to DNA (occurs when glucose is absent)
When the lac repressor does not bind to DNA
The combination occurs when glucose is absent and (allo)lactose is present
Transcription does not occur if glucose is present or allolactose is absent.
What is the lac operon an example of?
Catabolic operon -it breaks something down
What is another example of an operon called?
Biosynthetic operons
Contain genes for enzymes of biosynthetic pathways
Turned on when the supply of the end product is low.
Give an example of a biosynthetic operon?
the trp operon
What genes does the trp operon contain?
Genes required for the biosynthesis of the amino acid tryptophan
In the trp operon what signals repression?
Tryptophan is the signal molecule. When concentration in the cell is high the repressor represses the transcription of the trp operon
What is the mechanism when tryptophan is present?
Tryptophan binds to the tryptophan repressor and causes it to bind to the operator, blocking transcription.
What happens when tryptophan is absent?
The repressor dissociates from the operator, allowing transcription.
What is quorum sensing?
It is a “counting” technique
How bacteria communicate with each other, allows them to see how many there of an organism.
Cell density is measured by quorum sensing
What is the name of the squid that contains symbiotic Vibrio fischeri bacteria?
Euprymna scolopes
How is cell density measured ? For bacteria living in a light organ on the lower surface of the squid.
Measured by quorum sensing under the control of the lux operon
What do bacteria use as signals of cell density?
Molecules called acyl homoserine lactones (AHL)
