Lecture 7 - Virulence Gene Regulation Flashcards
Why regulate virulence genes?
It is very rare for pathogens to move directly from a specific host environment to another host environment.
Must be able to adapt to an intermediate environment.
Phase variation
When a gene is either turned on or off
Example of phase variation
Opa protein in Neisseria Gonorrhoeae
(CTCTT)n repeat between start of gene and variable regions of gene
DEpending on number of (CTCTT) repeats, different V regions of gene are in frame or out of frame
This allows the bacterium to express different adhesins
Example of regulation at level of DNA
Slipped-strand misrepair during DNA replication
Example of regulation at the level of transcription
Repressor protein binding to operator region
Example of regulation at the level of translation
Operon encoding several genes on one mRNA
Basic idea behind transcriptional regulation
Regulating function of RNA polymerase enzyme
Which part of gene does RNA polymerase bind to?
Promoter
Operon
Several genes encoded in a single mRNA transcript
Regulon
Several promotors controlled by the same regulator
Example of regulon
Listeria monocytogenes Prf regulation
Listeria monocytogenes Prf regulation
1)
2)
1) prfA binds to prfA gene regulator, upregulates prfA expression
2) prfA also binds to many other promoters, one of which encodes a virulence gene
Lac operon role
Expression of b-galactosidase under low glucose
Gene encoding b-galactosidase
LacZ
Lac operon structure (upstream to downstream) 1) 2) 3) 4) 5) 6) 7)
1) LacI gene encodes repressor protein
2) cAMP activator protein binding site
3) Promoter region
4) Operator region
5) LacZ
6) LacY
7) LacA
Lac operon function
1)
2)
3)
1) When lactose is absent, repressor protein binds operator and prevents RNA polymerase function
2) When glucose is present cAMP activator protein leaves cAMP activator protein binding site and RNA polymerase functions at a far lower rate
3) When lactose is present and glucose is absent repressor protein is absent and cAMP activator protein is present. Translation of operon occurs
DtxR role
Dimerises with Fe2+ to inhibit diphtheria toxin expression
Bacterium causing diphtheria
Corynybacterium diphtheriae
What causes expression of diphtheria toxin?
Low Fe2+
Where does RNA polymerase stop transcribing in bacteria?
When it reaches a transcription terminator
NOT when it reaches a stop codon
How does DtxR prevent toxin expression?
Dimerises with Fe2+, binds to operator region of tox gene
Which bacteria are two component systems common in?
Gram negative
Basic structure of a two component system
1)
2)
3)
1) Sensor (S) protein is in the periplasm/external environment, attached to a regulator (R) protein in the cytoplasm
2) Activation of S results in dimerisation, cross-phosphorylation of R.
3) R phosphorylates a second protein in the cytoplasm which dimerises, activates/represses a gene
Example of a two component system 1) 2) 3) 4)
1) Sensor is activated
2) Histidine on R is cross phosphorylated
3) Phosphoryl transfer to aspartic acid on response regulator
4) Response regulator dimerises, activates gene
Bordatella pertussis bvg regulation
1)
2)
3)
1) S region detects stimulus
2) On R region, histidine, then aspartic acid, then histidine is phosphorylated
3) bvgA protein is phosphorylated, activates genes
Ways to regulate transcription 1) 2) 3) 4)
1) Two component systems
2) Different sigma factors
3) Repressor proteins
4) DNA topography
A way that bacteria can alter DNA topography
Histone-like proteins
EG: H-NS
What does H-NS stand for?
Histone-like nucleoid-structuring protein
Function of H-NS
1)
2)
3)
1) H-NS binding sites exist in genes
2) Two bound copies of H-NS bind together
3) RNA polymerase is prevented from transcription
Basic idea of quorum sensing
Bacteria release autoinducers
When autoinducer levels become high enough, autoinducers reenter bacteria and stimulate response
Can unrelated bacteria participate in quorum sensing?
Yes
Common autoinducer in Gram negative bacteria
Acyl homoserine lactone
AI-1
Why are autoinducers different for Gram positive and negative bacteria?
Gram negative autoinducers need to be more lipophilic as they need to pass through two membranes
AI-1 mode of action 1) 2) 3) 4) 5)
1) LuxI (AI synthase) makes AI-1
2) AI-1 released from bacterium
3) When [AI-1] is high enough, reenters cell
4) Binds LuxR response regulator
5) LuxR/AI-1 complex binds to gene
Vibrio fischeri quorum sensing in squid
1)
2)
3)
1) Baby squid secretes gel which attracts bacteria
2) When V. fischeri have high enough numbers in squid, quorum sense and express luciferase gene
3) Bacteria emit light which squid uses to hunt
Autoinducers used by Vibrio fischeri
AI-1 and AI-2
Squid colonised by Vibrio fischeri
Bobtail squid
Which bacterium is prf present in?
Listeria monocytogenes
Which bacterium is opa protein present in?
Neisseria gonorrhoeae
WHich bacterium is bvg present in?
Bordatella pertussis
Which bacterium is DtxR present in?
Corynybacterium diphtheriae
