AHL System

• acyl homoserine lactones (AHL) are produced by gram negative bacteria

  
• gram negative bacteria

  • signal synthase produces AHL (H phobic) -> AHL passively diffuses through the 2 membranes and out of cell -> AHL diffuses back in or to neighboring cells -> binds signal receptor (AKA effector) which turns into TF -> turns on target gene

  
• AHL are made by LuxI family signal synthases which consist of 2 parts

  • core molecule -> stays the same
  • acyl R group -> changes, is the molecular signature for each bacterial strain, the specific species language

  
• LuxI (signal synthase) is a cytoplasmic enzyme that catalyzes the formation of AHL by combining fatty acyl chains with homoserine

  
• LuxI enzyme makes AI/C6-HSL (toxic) which secrete from the bacteria to diffuse back in or to neighboring bacteria -> AI accumulates and diffuses into the cell to bind LuxR (response regulator) -> becomes TF and turns on genes in 2 directions for

  • luminescence
  • make more synthase -> positive feedback loop

  
• LuxR TF are cytoplasmic receptors that bind AHL and induce expression of lux genes, also induces own expression to make positive feedback loop

  
• expression of the lux operon is cell density dependent (once bacterial numbers reach quorum), positive feedback loop, digital signalling

  
• the LuxI/LuxR positive feedback system

  • LuxI enzyme makes AHL -> AHL diffuses out of cell and reaches quorum -> AHL diffuses into cell and binds LuxR response regulator -> binds to DNA as TF to turn on genes for lux operon and LuxR genes