Lecture 27 PMB

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Quorum Sensing
Candice Young
Flashcards by Candice Young, updated more than 1 year ago
Candice Young
Created by Candice Young over 6 years ago
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V. fischeri bioluminescent bacterium in symbiosis with E. scolopes squid colonizes juvenile squid, which is VERY species specific --> develops light organ squid hides during day, forages at night light from V. fischeri in light organ hide squid's shadow and hide it from predators!
bacterial luciferase a mixed function oxidase, composed of alpha and beta subunits uses three different reactions to make light antibiotics that block tx or tn stop luminescence --> genes encoding luciferase are transcribed differently in different parts of the growth curve
acyl homoserine lactone (AHL) autoinducer of luminescence activity acyl group varies by species is recycled back in and out of cell --> when conc. high enough, triggers expression of luminescence genes
How to determine genes for luminescence Take sample of v. fischeri --> make genomic DNA library --> transform e. coli while selecting for presence of plasmid --> isolate plasmid with luminescence genes by screening colonies for light production in the dark!
lux operon tx same gene cluster confers both making luminescence and sensing it: strong positive feedback loop!!! LuxI synthesizes AHL, which is really hydrophobic --> diffuses freely outside and accumulates --> at a threshold concentration, AHL diffuses back inside and binds to activator LuxR --> lux operon tx increased
luxL operon synthesizes AHL, which in turn promotes tx of lux operon (increasing LuxR) also transcribes Luciferase, which undergoes various rxns to produce light
LuxR proteins when bound to AHL, fold correctly and dimerize --> bind upstream of lux genes to activate tx when not bound to AHL, are in an inactive monomer conformation and are rather unstable --> tx off
different species AHL differ by acyl chain length and B carbon modification
gram negative vs gram positive quorum sensing only gram negative are AHL-mediated, all of the molecules are different for gram positive (use AIPs instead)
Quorum sensing in gram positive bacteria long precursor peptide is translated, during export it is processed to mature signaling form (AIPs) --> sensed by a histidine kinase, after a certain threshold the HK activity changes --> population dependent effects mediated by downstream regulators
AIP variations can be linear or cyclized **structural similarities but different sequences --> cannot be detected by receptors on different strain!**
S. aereus virulence gene tx AgrD precursor processed by AgrB into short cyclic AIP, secreted outside --> AgrC HK senses AIP outside, changes activity at high levels --> change activates AgrC, which phosphorylates --> AgrC-P activates tx of agr operon (positive feedback) and tx of virulence genes
furanone compounds disrupt quorum sensing by affecting biofilms and reducing virulence of some species halogenated versions of these bind to LuxR homologs --> block tx of lux operon has rather unpredictable effects since virulence can occur at high or low cell density
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