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5. QS Mediated By Autoinducer 2 (AI-2)
AI-2 was originally identified as a QS autoinducer in Vibrio harveyi , which controls
bioluminescence in conjunction with an AHL-based mechanism [Henke and Bassler, 2004].
Interestingly, this regulation shares characteristics from QS-systems of both the Gram-
negative and Gram-positive bacteria, as it combines AHLs and a two-component protein
signal transduction system [Schauder and Bassler, 2001]. The gene responsible for ultimate
production of AI-2 is named luxS , and seems widely distributed as it was found present in
more than 60 bacterial species, including classes β , γ , δ and ε of the Proteobacteria,
Spirochaetes, Firmicutes, Actinobacteria, and other genera [Vendeville et al. , 2005], leading
to the suggestion that AI-2 can serve as one universal language [Sun et al ., 2004].
Chemically, AI-2 is a collection of interconvertible furanone-derived molecules [Chen et
al. , 2002, Waters and Bassler, 2005, Chhabra et al. , 2005]. AI-2 emerges from DPD (4,5-
dihydroxy-2,3-pentanedione), which is a byproduct of the activated methyl cycle (AMC),
responsible for the generation of the methyl-donor S -adenosyl methionine (Figure 4)
[Williams et al. , 2007]. DPD was isolated and purified by Cao and Meighen (1989), who also
elucidated its structure. LuxS, a small metalloenzyme containing Fe 2+, is an S -
ribosylhomocysteinase that cleavages S -ribosylhomocysteine to produce DPD [de
Keersmaecker et al. , 2006].
Figure 4. Pathways for detoxification of SAH and synthesis of DPD [Sun et al. , 2004].
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