Flashcards
Lecture 27 PMB
Cards in this deck (13)
- 1V. fischeribioluminescent 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! - 2bacterial luciferasea 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 - 3acyl 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 - 4How to determine genes for luminescenceTake 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!
- 5lux operon txsame 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 - 6luxL operonsynthesizes AHL, which in turn promotes tx of lux operon (increasing LuxR)
also transcribes Luciferase, which undergoes various rxns to produce light - 7LuxR proteinswhen 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 - 8different species AHLdiffer by acyl chain length and B carbon modification
- 9gram negative vs gram positive quorum sensingonly gram negative are AHL-mediated, all of the molecules are different for gram positive (use AIPs instead)
- 10Quorum sensing in gram positive bacterialong 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
- 11AIP variationscan be linear or cyclized
**structural similarities but different sequences --> cannot be detected by receptors on different strain!** - 12S. aereus virulence gene txAgrD 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
- 13furanone compoundsdisrupt 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