Nacylhomoserine

N-acyl homoserine lactones, commonly abbreviated AHLs, are a class of signaling molecules used by many Gram-negative bacteria to coordinate gene expression in a density-dependent manner (quorum sensing). Each molecule consists of a homoserine lactone ring linked to an N-acyl side chain of varying length and substitution. The acyl chain typically ranges from four to fourteen carbons and may carry substitutions such as a 3-oxo or a 3-hydroxy group.

Biosynthesis occurs through LuxI-family synthases that combine a fatty acid-derived acyl donor with a homoserine lactone moiety, producing R-CO-N-homoserine lactone. Signal perception is mediated by LuxR-type transcription factors that bind the AHL; upon binding, the LuxR-AHL complex regulates target genes, affecting phenomena such as bioluminescence, virulence factor production, biofilm formation, and nutrient acquisition.

Structural diversity and receptor specificity yield species- and strain-specific signaling. Classic examples include 3-oxo-C6-HSL used by Vibrio fischeri, 3-oxo-C12-HSL and C4-HSL used by Pseudomonas aeruginosa, and 3-oxo-C8-HSL in Agrobacterium tumefaciens. AHLs are widespread among Gram-negative bacteria but are not found in Gram-positive species, which often rely on different signaling systems.

Stability and turnover of AHLs are influenced by pH and enzymatic degradation; lactonases and acylases produced by some bacteria can inactivate AHL signals. Because AHLs regulate virulence and biofilm traits, they have become targets for quorum-sensing inhibitors and biosensor applications, with potential uses in agriculture, medicine, and industry.