Autoinducer2

Autoinducer-2 (AI-2) is a small signaling molecule used in bacterial quorum sensing to coordinate gene expression based on cell density. It is produced by many bacteria and notable for its proposed role in interspecies communication.

AI-2 is derived from 4,5-dihydroxy-2,3-pentanedione (DPD), the product of the LuxS enzyme acting on S-ribosylhomocysteine. In aquatic environments, DPD can cyclize and form borate esters, yielding a family of AI-2 signals whose precise chemical forms depend on boron availability.

Signal reception varies by species. In Vibrio species such as V. harveyi, AI-2 is detected by the periplasmic binding protein LuxP, which forms a complex with the sensor kinase LuxQ. In Escherichia coli and Salmonella, AI-2 is transported into the cell by the LsrABC transporter and phosphorylated by LsrK, relieving repression by LsrR and altering gene expression.

Biological roles for AI-2 include regulation of biofilm formation, motility, virulence factor expression, and other community behaviors. Because AI-2 can be produced and detected by many taxa, it is considered a universal signaling molecule in bacteria, though responses are species-specific.

History and context: AI-2 was identified in studies of quorum sensing systems and linked to LuxS, an enzyme conserved across bacteria. The concept of universal AI-2 signaling has driven research into interspecies communication and antimicrobial strategies that target quorum sensing.

In research, AI-2 is studied as a potential target for quorum-sensing inhibitors and as a tool for modulating microbial communities in medical, industrial, and environmental settings.