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Flashcards in Regulation of virulence factors Deck (23):

what is the structure of Fimbriae?

1. Thread-like surface structures, 7 nm diameter,
0.5-2 μm long
2. Repeating protein subunits, helical structure, central pore
3. 500 fimbriae per cell, 1000 protein subunits


whats the role of fimbriae?

- Promote specific adhesion via protein adhesins
- Overcome electrostatic repulsion
- Promote adhesion to target carbohydrate receptors


give an example of phase variation in E. coli

- express different fimbriae with different specificities for host cell carbohydrate receptors
- bacterium will not express all fimbrial types at any given time
- ability to express different types of fimbriae allows bacteria to colonise different cell types as infection progresses


what are the characteristics of P-pili of uropathogenic E. coli

• 9 serotypes
• Cause haemagglutination P-group blood
• Ligand on bladder, ureter and renal tissue
• Mutants deficient in P-pili reduced virulence
• Maternal antibodies against P-pili are protective


Signalling Events Associated P-pili (Type I fimbriae) Mediated Adhesion

• Release ceramide TLR4 agonist
• Activate TLR4
• Release pro-inflammatory cytokines (Il-6 and CXCL8)
• Recruite neutrophils (clearance/damage)
• Tissue damage forms intracellular bacterial communities
• Persistent infection


Why regulate expression in pap?

• Large structure requiring multiple resources to synthesize
• Activates innate host immune responses
• Major target of adaptive immune responses


give an example of Stochastic Regulation

Genetic Variation (e.g. mutation)


give an example of Programmed Regulation

transcriptional regulons


describe the mechanisms evolved to respond to environmental signals in the pap operon

- expression regulated by temperature, catabolite repression (nutrient status)
- efficient expression of pap operon requires positive regulators PapB and PapI
- Phase variation depends on activity of Lrp


what is UAS?

Upstream Activator Sequence


whats the purpose of UAS?

contains promoters/binding sites for regulation by temperature, catabolite repression, transcriptional activators PapB/I, and by the global regulator Lrp


how does Temperature regulate pap?

26 C - H-NS binding represses transcription from PB - no fimbriae

37 C - Weak/no H-NS binding activates transcription of papB from PB – fimbriae produced


describe Catabolite repression at the pap operon

• Binding of cAMP with CRP is required for transcription of papB operon
• High levels of intracellular cAMP in poor growth conditions (e.g. in the urinary tract)
• ‘bends’ DNA, makes promoters accessible to RNA polymerase


explain pap gene Regulation by PapB

• Binding of PapB to sites in the UAS is required for efficient regulation of the papB operon
• Three PapB binding sites are occupied sequentially in response to increasing levels of PapB protein


describe the sites of PapB

• site 1: occupied when [PapB] (and [PapI]) low; PB
activated ⇒ increased PapB production.
• site 2: occupied as [PapB] increases, progressively preventing RNA polymerase binding ⇒ decreasedpapBoperonexpression.
• site 3: occupied if cellular [PapB] very high ⇒ switches off papB operon expression (autogenous regn.)


explain Phase variation of E. coli P-fimbriae

At the population level, E. coli P-fimbriae are expressed in response to temperature:
– OFF at 26oC and below
– ON at 37oC
• But even at 37oC, expression of P-fimbriae is subject to phase variation - i.e. switches between OFF and ON (frequency ~10-5)
• Populations are mixed (heterogeneous) with regard to fimbrial expression


how does switching occur?

Switching involves differential methylation of the promoter Dam Methylase I + Dam Methylase II


explain the action of Dam Methylase I

• Methylates the adenine in the sequence 5’GATC
• Required for initiation of chromosomal replication
• Required for recognition of the template strand during mismatch repair


how do nutrients regulate pap?

Binding of Lrp is also required for efficient transcription of the papB operon

Leucine response regulator; ‘global regulator’ of genes involved in nutrient availability – made in starvation conditions


Role of Pap Regulation in Infections?

OUTSIDE HOST- Pap OFF Low temp (H-NS) No adhesion
INSIDE HOST- Pap ON High temp Low catabolites (CRP) Autoregulation (PapB) Adhesion No immune evasion
Pap ON/OFF- High temp Low catabolites Phase variation (PapI,Lrp,Dam) Adhesion Immune evasion


what is Phase variation?

stochastic regulation of gene expression


how is The pap operon regulated?

by environmental conditions through a range of transcriptional regulators


how is the Phase variation of the pap operon mediated?

by differential methylation of the promoter controlled by DNA replication, Dam, PapI/LRP and PapB