L16 Virulence: Mechanisms of Gene Regulation I Flashcards
Before ingestion, intestinal pathogens often reside in water - describe the conditions of the water.
- Low temperature
- Low ionic strength
- Low [organic nutrients]
- Neutral pH
After ingestion, intestinal pathogens must adapt to its new environment. Describe this environment.
- Higher temperature
- Higher osmotic strength
- Low pH in the stomach
- High pH, bile salts, lack of oxygen, abundant organic nutrients in SI
- Sequestration of iron by the host
To survive this journey, the pathogen must rapidly express a variety of gene products - what are 5 examples?
- Proteins to tolerate the low stomach pH
- Flagella and chemotaxis proteins for migration to a suitable niche
- Adhesins that permit colonization
- Toxins and invasins to elicit disease
- Iron chelators (siderophores) to scavenge for ironW
What is a virulence factor?
Any bacterial property required for entry, growth, or survival in a host
What are 4 examples of virulence factors?
- Capsule (inhibits complement killing)
- Adhesins (adherence to host cells)
- Acid tolerance factors (adapt to stomach)
- Enzymes (synthesize unavailable nutrients)
What is a virulence gene?
Any gene that encodes a virulence factor
___% of genes in V. cholerae and S. typhimurium encode virulence factors.
5-10
Where are virulence genes often located?
- Mobile genetic elements (plasmids or phages)
2. Pathogenicity islands (large, localized regions of the chromosome)
True or false - pathogenicity islands are often absent in closely related non-pathogenic strains
True
Bacteria organize their genes in ___.
Multicistronic operons
What is a cistron?
A sequence of DNA that encodes a polypeptide
What is an operon?
A unit of transcription that includes more than one cistron
What is multicistronic mRNA?
The mRNA that results from transcription of a multicistronic operon
What are the 4 components of a typical operon?
- Promoter
- Operator
- Cistrons
- Terminator
What is RNAP?
RNA polymerase; a biological machine that transcribes DNA into mRNA
What is the promoter?
The site at which RNAp binds
What is sigma?
A subunit of RNAP that specifically recognizes and binds the promoter
What is the closed complex?
The product of the RNAP/DNA interaction
What is a holoenzyme?
RNAP + sigma factor
What are the steps of transcription initiation?
- Formation of closed complex
- Formation of open complex
- Initiation and elongation
Why do we need an open complex?
RNAP causes the double strand of DNA to open, creating the open complex. Strand separation allows for synthesis of the mRNA strand complementary to the DNA sense strand
When the RNA polymerase is the only thing involved in transcription, this is known as ___.
Basal/constitutive transcription
Describe the relationship between glucose and cAMP in E. Coli and organisms like it.
Inverse (as glucose decreases, cAMP increases and vice versa)
Describe the process of repression, using lac as an example.
In this example, glucose is present and cAMP is low. Transcription of lacR is basally active, leading to the repressor protein. The repressor binds to the operator, which blocks RNAP from binding the promoter sequence.
Cells regulate transcription primarily at its ___. They make the decision to do so by setting the ___ of initiation. This depends upon the ability of ___ to bind the promoter.
Initiation; frequency; RNAP
What are the two types of regulator proteins that control the ability of RNAP to bind the promoter?
- Repressor
2. Inducer
What does the repressor do?
Binds the operator and prohibits binding of RNAP to the promoter
What does the inducer do?
Binds the repressor, changes the conformation so that it can no longer bind DNA
What is an activator?
Interacts with RNAP, increasing the ability of RNAP to bind the promoter
Describe the process of induction, using lac as an example.
In this example, glucose is absent and cAMP is high. Lactose is present. Lactose, the inducer, binds to the repressor, preventing it from binding to the DNA. RNA polymerase binds its activator, cAMP-CRP, which helps it bind to the promoter better. Transcription of lac occurs.
RNAP binds to the ___.
Promoter
The repressor binds to the ___.
Operator
By binding the operator, the repressor reduces ___.
RNAP binding affinity
When the inducer binds the repressor, what happens?
It decreases the repressor binding affinity for the operator.
The activator interacts with RNAP and does what to its binding affinity?
Increases
cAMP is a co-activator - what does it do?
Binds the activator, increasing the activator binding affinity
The probability of transcription initiation increases as the stability of the closed complex ___.
Increases
What increases stability of the closed complex? What decreases it?
Activators; repressors
What is the core enzyme, and what is it comprised of?
RNA polymerase; 2 alpha, 1 beta, 1 beta prime subunits
What is a holoenzyme?
RNA polymerase + sigma
What does sigma do?
Recognizes the promoter
What are the two domains of alpha and what do they do?
NTD and CTD; NTD helps assemble the core enzyme; CTD binds DNA involved in closed complex formation and interacts with sigma in the open complex formation
When CRP is bound by cAMP, what happens?
CRP binds DNA
What are the 5 interactions that enhance transcription initiation?
- Sigma/DNA
- DNA/alpha-CTD
- Sigma/alpha-CTD
- CRP/DNA
- CRP/alpha-CTD
What is a regulon?
A group of operons subject to the control of a common regulator
Describe the general sequence of events in a regulon.
- Cell senses a stimulus
- Sensor signals to activate/deactivate a regulator
- Regulator binds to several operons
- Binding turns some on/some off
- Gene products respond to original stimulus
- Gene products exert feedback control on their own expression
