Autoinducers are hormone-like molecules produced by bacteria as signals to sense cell density and activate adaptations, by a process termed quorum sensing. N-(3-oxohexanoyl)-L-homoserine lactone (OHHL) is a molecule produced by a number of Gram-negative bacteria that acts as an autoinducer by binding to transcriptional regulatory proteins and activating them [1,2]. OHHL and related molecules such as N-butanoyl- (BHL), N-hexanoyl- (HHL) and N-oxododecanoyl- (PAI) homoserine lactones are produced by a family of proteins which have a length of about 200 amino acids. The 3D structures of autoinducer synthases (see <PDB:1KZF>) have features essential for phosphopantetheine binding and N-acylation that are similar to that of Gcn5-related N-acetyltransferases (GNAT) (see <PDOC51186>) [3,4].

Some proteins currently known to belong to the autoinducer synthase family:

• Vibrio fischeri luxI. The target of OHHL is the luxR protein that acts in the regulation of bioluminescence genes.
• Aeromonas ahyI and asaI, which synthesize BHL and whereof the targets are ahyR and asaR.
• Agrobacterium tumefaciens traI. The target of OHHL is traR which regulates Ti plasmid transfer.
• Erwinia carotovora carI. The target of OHHL is carR which activates genes involved in the biosynthesis of carbapenem antibiotics.
• Erwinia carotovora expI. The target of OHHL is expR which is involved in virulence (soft rot disease) by activating the genes for plant tissue macerating enzymes.
• Erwinia stewartii esaI. The target of OHHL is esaR which is involved in the regulation of the biosynthesis of EPS virulence factor.
• Pseudomonas aeruginosa lasI, which synthesizes PAI and whose target is lasR which activates the transcription of the elastase gene.
• Pseudomonas aureofaciens phzI, whose target is phzR, the activator of phenazine biosynthetic genes.
• Pseudomonas aeruginosa rhlI (or vsmI), which synthesizes BHL and HHL and whose target is rhlR.
• Serratia liquefaciens swrI, which synthesizes BHL.
• Enterobacter agglomerans eagI.
• Yersinia enterocolitica yenI.

The best conserved region is located in the N-terminal region and can be used as a signature pattern. We also developed a profile which covers the entire autoinducer synthase domain.