Autoinducer-2 (AI-2), a furanosyl borate diester or tetrahydroxy furan (species dependent), is a member of a family of signaling molecules used in quorum sensing. AI-2 is one of only a few known biomolecules incorporating

boron Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...

. First identified in the

marine bacterium Marine prokaryotes are marine bacteria and marine archaea. They are defined by their habitat as prokaryotes that live in marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. All cellular ...

Vibrio harveyi ''Vibrio harveyi'' is a Gram-negative, bioluminescent, marine bacterium in the genus ''Vibrio''. ''V. harveyi'' is rod-shaped, motile (via polar flagella), facultatively anaerobic, halophilic, and competent for both fermentative and respiratory ...

'', AI-2 is produced and recognized by many

Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...

and

Gram-positive bacteria In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bact ...

. AI-2 arises by the reaction of

4,5-Dihydroxy-2,3-pentanedione 4,5-Dihydroxy-2,3-pentanedione (DPD) is an organic compound that occurs naturally but exists as several related structures. The idealized formula for this species is CH3C(O)C(O)CH(OH)CH2OH, but it is known to exist as several other forms resultin ...

, which is produced enzymatically with boric acid, and is recognized by the two-component sensor kinase LuxPQ in

Vibrionaceae The Vibrionaceae are a family of Pseudomonadota given their own order, Vibrionales. Inhabitants of fresh or salt water, several species are pathogenic, including the type species ''Vibrio cholerae'', which is the agent responsible for cholera. Mo ...

. AI-2 is actively transported by the Lsr ABC-type transporter into the cell in

Enterobacteriaceae Enterobacteriaceae is a large family (biology), family of Gram-negative bacteria. It was first proposed by Rahn in 1936, and now includes over 30 genera and more than 100 species. Its classification above the level of family is still a subject ...

and few other bacterial taxa ('' Pasteurella'', ''

Photorhabdus ''Photorhabdus'' is a genus of bioluminescent, gram-negative bacilli which lives symbiotically within entomopathogenic nematodes, hence the name ''photo'' (which means light producing) and ''rhabdus'' (rod shape). ''Photorhabdus'' is known to be ...

'', ''

Haemophilus ''Haemophilus'' is a genus of Gram-negative, pleomorphic, coccobacilli bacteria belonging to the family Pasteurellaceae. While ''Haemophilus'' bacteria are typically small coccobacilli, they are categorized as pleomorphic bacteria because of ...

'', '' Bacillus''), where it is phosphorylated by LsrK. Then, Phospho-AI-2 binds the transcriptional repressor protein, LsrR, which subsequently is released from the promoter/operator region of the lsr operon – and transcription of the ''lsr'' genes is initiated. AI-2 signalling is also regulated by glucose and cAMP/CRP via the ''lsr'' operon. In the presence of glucose, low levels of cAMP/CRP result in almost no ''lsr'' operon (''lsrABCDFG'') expression. Without glucose, cAMP-CRP is needed to stimulate the ''lsr'' expression, while LsrR represses its expression in the absence of the inducer, phospho-AI-2. As AI-2 accumulates, more AI-2 is taken in via LsrABCD, phosphorylated via LsrK, and the ''lsr'' transcription is de-repressed, enabling even more AI-2 uptake. Doubts have been expressed regarding AI-2's status as a universal signal. Although the ''luxS'' gene, which encodes the protein responsible for AI-2 production is widespread, the latter has mainly a primary metabolic role in the recycling of S-adenosyl-L-methionine, with AI-2 being a by-product of that process. An unequivocally AI-2 related behavior was found to be restricted primarily to organisms bearing known AI-2 receptor genes. Thus, while it is certainly true that some bacteria respond to AI-2, it is doubtful that it is always being produced for purposes of signaling.

References

{{Reflist Signal transduction Cell biology Cell communication Cell signaling Tetrahydroxyborate esters