Quorum sensing is used by a large variety of bacteria to regulate gene 
expression in a cell-density-dependent manner. Bacteria can synchronize 
population behavior using small molecules called autoinducers that are produced 
by cognate synthases and recognized by specific receptors. Quorum sensing plays 
critical roles in regulating diverse cellular functions in bacteria, including 
bioluminescence, virulence gene expression, biofilm formation, and antibiotic 
resistance. The best-studied autoinducers are acyl homoserine lactone (AHL) 
molecules, which are the primary quorum sensing signals used by Gram-negative 
bacteria. In this review we focus on the AHL-dependent quorum sensing system and 
highlight recent progress on structural and mechanistic studies of AHL synthases 
and the corresponding receptors. Crystal structures of LuxI-type AHL synthases 
provide insights into acyl-substrate specificity, but the current knowledge is 
still greatly limited. Structural studies of AHL receptors have facilitated a 
more thorough understanding of signal perception and established the molecular 
framework for the development of quorum sensing inhibitors.