This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Secondary Metabolites Biosynthesis → Autoinducer Biosynthesis|
Expected Taxonomic Range: Vibrionales
Cell-cell communication in bacteria is accomplished through the exchange of extracellular signalling molecules called autoinducers. This process, termed quorum sensing, allows bacterial populations to coordinate gene expression as a function of cell density. Many processes benefit from community cooperation, including bioluminescence, virulence factor expression, antibiotic production and biofilm development.
Autoinducer systems depend on two main components - the autoinducer and its receptor, which also functions as an autoinducer-dependent transcriptional regulator that binds DNA immediately upstream of the genes that are controlled.
Autoinducer systems have been implicated in regulation of various systems. For example, luxI/luxR of Aliivibrio fischeri regulate bioluminescence, luxM/luxN of Vibrio harveyi regulate bioluminescence and polyhydroxybutyrate biosynthesis, lasI/lasR of Pseudomonas aeruginosa regulates virulence factors, rhlI/rhlR of Pseudomonas aeruginosa regulates rhamnolipid synthesis as well as virulence factors, and traI/TraR of Agrobacterium tumefaciens regulates plasmid conjugal transfer [Fuqua96].
Several types of autoinducers are known. Gram-negative bacteria typically use acyl-homoserine lactones, known as type AI-1, as their autoinducers. The first autoinducer to be discovered was VAI-1, an AI-1 type compound from Aliivibrio fischeri [Eberhard81]. Many variants of AI-1 type inducers exist, differing from each other in the composition of the acyl moiety. In some cases, one organism may produce multiple types of acyl homoserine lactone type autoinducers, each produced by a dedicated synthase. A few examples are HAI-1 from Vibrio harveyi, AAI-1 from Agrobacterium tumefaciens, PAI-1 and PAI-1-2 from Pseudomonas aeruginosa and VAI-1 and VAI-1-2 from Aliivibrio fischeri [Fuqua96].
Another very common family of autoinducers include compounds produced from autoinducer 2, which was first described from the marine bacterium Vibrio harveyi, in which it is used to control luciferase expression [Bassler94, Surette99]. AI-2 type compounds are produced by a remarkably wide variety of Gram-negative and Gram-positive bacteria, leading to the proposal that AI-2 is a 'universal' signal that functions in interspecies cell-to-cell communication [Miller01, Rezzonico08].
About This Pathway
The precursor of AI-2, autoinducer 2, is synthesized by the enzyme S-ribosylhomocysteine lyase, in a reaction that is also a step in the S-adenosyl-L-methionine cycle I pathway (SAM cycle). The enzyme converts S-ribosyl-L-homocysteine to L-homocysteine in a reaction that also produces autoinducer 2. Within the SAM cycle, the main product of the enzyme is the former compound. However, in organisms that produce an AI-2 autoinducer, autoinducer 2 is of major importance as it appears to be the last enzyme-generated precursor for AI-2, and is converted to the functional autoinducer in a series of chemical modifications without the help of any known enzyme.
The exact nature of the chemical transformations depends on the species. In Vibrionales the spontaneous transformations include cyclyzation to (2S,4S)-2-methyl-2,4-dihydroxydihydrofuran-3-one, hydration to (2S,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran, and finally, in the presence of free borate ions, complexation with the later to form the active autoinducer, (2S,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran-borate [Chen02, Rezzonico08]. This particular form of the autoinducer appears to be unique to Vibrionales, which detect it using dedicated AI-2 receptors encoded by the luxP and luxQ genes [Bassler94, Reading06].
Other bacteria that produce AI-2 utilize a different form of the autoinducer. In those organisms autoinducer 2 appears to form a different stereoisomer, namely (2R,4S)-2-methyl-2,4-dihydroxydihydrofuran-3-one, which does not complex borate [Miller04] (see autoinducer AI-2 biosynthesis I). The mechanism that controls which stereoisomer is formed is still not understood.
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Chen02: Chen X, Schauder S, Potier N, Van Dorsselaer A, Pelczer I, Bassler BL, Hughson FM (2002). "Structural identification of a bacterial quorum-sensing signal containing boron." Nature 415(6871);545-9. PMID: 11823863
Eberhard81: Eberhard A, Burlingame AL, Eberhard C, Kenyon GL, Nealson KH, Oppenheimer NJ (1981). "Structural identification of autoinducer of Photobacterium fischeri luciferase." Biochemistry 20(9);2444-9. PMID: 7236614
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