Microbiology - Lecture 2 - Bacterial Communication 2 The LuxR-AHL transcriptional co-activator AHL AHL diffuses freely in and out AHL 1 LuxR receptor 1 1 luxR luxl c D A B E P lux operon acyl-ACP 0 SAM 1 HO Luxl autoinducer synthase C N ZI CH3 N-3-oxohexanoyl homoserine lactone (OHHL) > Lux operon: · other bacteria: different luxI homologues synthesise different AHLs AHLs have fatty acyl group (R) linked to modified AA (homoserine lactone) * acyl chain differs > determines characteristics of binding to luxR * varying: chain length, oxidation and saturation * diffuse through membranes some unique to single species * some shared > cross-talk some produce single AHL
* other produce many > multiple signals · AHLs bind cognate receptor = members of luxR protein superfamily · LuxR = transcription factor * When bound to OHHL > activates expression of lux genes > including genes encoding luxI and luxR * Positive feedback loop * Two domains: 1. Ligand binding domain: affinity for AHLs 2. DNA binding domain: " Monomers interact to form an asymmetric homodimer > when bound by AHLs > can bind target DNA + work with RNApol ( ? 70¿ to activate gene expression > AHLs regulate virulence gene expression in pathogenic bacteria: · P.aeruginosa: * CF, superficial burns/wounds, conjunctivitis * Opportunistic * Common cause of nosocomial infections * Upon entry into host > produces battery of secreted virulence factors: " Toxins: exotoxin A, cytotoxin " Enzymes that break down tissues: phospholipase, elastase, proteases · Siderophores: pyoverdin, pyochelin * Colonisation and dissemination facilitated by range of cell-surface associated virulence factors: " Flagella " Type IV pili " Extracellular polysaccharide: including alginate slime > antibiotic resistant multicellular biofilms # 2 AHL QS systems > Lasl-LasR + Rhll-RhIR > regulate co-rodinated expression of many P.a virulence genes " auto-inducer = 3-O-C12-HSL " Lasl = synthase " LasR = TF
3-O-C12-HSL 1 Lasl C4-HSL Rhll lasR las rhIR rhl LasR RhIR LasR RhIR Las regulon Rhl regulon > Other QS signals: Pseudomonas quinolone signal (PQS) > Interplay among Pseudomonas Las, Rhl and PQS systems: · All regulate virulence factors . Positive regulation Lasl + LasR >> lasR and lasl + rhIR and rhll + pqsH and pqsR · Positive regulation RhIR >> rhll · Negative regulation RhIR >> pqsABCD + pqsR · Positive regulation PqsR + PqsABCDH >> pqsABCD + rhIR Cytoplasm IM OM Lasl LasR 30C12HSL 30C12HSL lasR lasl rhIR rhll Virulence factors pqsH pqsR pqsABCD C4HSL RhIR Rhll C4HSL PqsR PQS< Pqs ABCDH PQS