Lecture 15: Control of Gene Expression I: Transcriptional Control Flashcards
What is a repressible operon?
When an excess of product leads to a shutdown of the production of enzymes that synthesize that product
In a repressive operon, what does the repressor need in order to bind to the operator and block RNA polymerase from binding?
A co-repressor (in the Trp operon, it’s tryptophan, which is the product)
When is the Trp operon on and when is it off?
On when tryptophan is low (repressors don’t have their co-repressor so can’t bind operator), and off when tryptophan is high (lots of tryptophan to be the co-repressor)
What is signal transduction?
Mechanisms that allow a cell to detect internal or external changes and respond appropriately
Components of the two-component signal transduction system
Sensor and response regulator
Purpose of sensor in two-component signal transduction system
Phosphate transferred to histidine residue in “transmitter domain” as response to environmental change
Purpose of response regulator in two-component signal transduction system
“Receiver domain” that receives phosphate from sensor, becoming active (can either activate or suppress genes)
Sensor in PhoR-PhoB system and how it works
PhoR, a transmembrane protein: inactive when phosphate bound, active when phosphorylated (Pi does not remain bound)
PhoR-PhoB system: what does activated PhoR do?
Phosphorylated PhoB, the response regulator, which then becomes a transcription factor to turn on genes that will help cope with low phosphate levels
What is the preferred carbon source for energy in bacteria like E.coli?
Glucose because it’s easier to use; they only use lactose when glucose is absent
Enzymes required for lactose metabolism in E.coli (lac operon)
Per ease to transport lactose into cell, and β-galactosidase to cleave lactose molecule to yield glucose and galactose
Which protein does lacZ gene make?
β-galactosidase
Which protein does lacY make?
lactose permease
Which protein does lacA make?
a transacetylase
Which protein does lacI make?
Repressors protein that binds to operator on DNA (made all the time, as it turns of the other lac genes since they’re usually not needed)
Control mechanisms for lac operon
lac repressors (negative control) and CAP-cAMP complex (positive control)
Recognition sites of lac repressors (lacI)
Site that recognizes specific operator sequence for lac operon, and an allosteric site that can bind lactose and lactose analogs like IPTG
What happens when lactose or IPTG binds lacI?
Repressor is INACTIVE (off) which means the lac genes are ON
Lac operon: what happens when lactose is absent?
Repressors binds operator region, blocking binding of RNA polymerase
What is an inducer (how does it relate to lac operon)?
Inducers inactivate the repressors, leading to expression of lac genes (in the lac operon, lactose and IPTG are inducers)
Methods of transcriptional control in prokaryotes
Trp Operon, PhoR-PhoB, Lac Operon
Lac operon: what happens when lacI is a constitutively active mutant (I-)?
All lac genes synthesized at all times, whether or not the inducer was present
Lac operon: where is the mutation in mutants that express lac enzymes even when repressor is present?
Mutation is in the operator region, blocking repressor binding (O^c for operator constitutive)
Lac operon: what happens once glucose is gone?
Cells synthesize cAMP, which binds to each subunit of dimerization catabolism activator protein (CAP)
