Subsystem: Autoinducer 2 (AI-2) transport and processing (lsrACDBFGE operon)

This subsystem's description is:

Quorum sensing is a process of chemical communication that bacteria use to assess cell population density and synchronize behavior on a community-wide scale. Communication is mediated by signal molecules called autoinducers. The LuxS autoinducer synthase produces 4,5-dihydroxy-2,3-pentanedione (DPD), the precursor to a set of interconverting molecules that are generically called autoinducer-2 (AI-2). In enteric bacteria, AI-2 production induces the assembly of a transport apparatus (called the LuxS regulated (Lsr) transporter) that internalizes endogenously produced AI-2 as well as AI-2 produced by other bacterial species.
AI-2 internalization is proposed to be a mechanism enteric bacteria employ to interfere with the signaling capabilities of neighboring species of bacteria.
Following internalization, the kinase LsrK phosphorylates carbon-5 of the open form of DPD. Phosphorylated DPD (P-DPD) binds specifically to the repressor of the lsr operon, LsrR, consistent with P-DPD being the inducer of the lsr operon. Subsequently, LsrG catalyzes the cleavage of P-DPD producing 2-phosphoglycolic acid. This series of chemical events is proposed to enable enteric bacteria to respond to the presence of competitor bacteria by sequestering and destroying AI-2, thereby eliminating the competitors' intercellular communication capabilities.

In S. enterica serovar Typhimurium the rapid disappearance of AI-2 is a consequence of its import by an ABC transporter designated the Lsr transporter (luxS regulated). The lsr operon contains seven genes, lsrACDBFGE, and its transcription is activated by AI-2. The first four genes, lsrACDB, encode components of the AI-2 tr. Adjacent to, but transcribed divergently from the lsr operon is lsrR, which encodes a repressor of lsr transcription, and lsrK, which encodes a kinase that phosphorylates intracellular AI-2 following import. Phosphorylation of internalized AI-2 is required for induction of transcription of the lsr operon, suggesting that phospho-AI-2 is the inducer of this system. It is postulated that phospho-AI-2 binds to the LsrR repressor and inactivates it and that this results in derepression of lsr transcription

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Literature ReferencesQuorum sensing: the many languages of bacteria. Reading NC FEMS microbiology letters 2006 Jan16451172
Phosphorylation and processing of the quorum-sensing molecule autoinducer-2 in enteric bacteria. Xavier KB ACS chemical biology 2007 Feb 2017274596
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Quorum sensing is a process of chemical communication that bacteria use to assess cell population density and synchronize behavior on a community-wide scale. Communication is mediated by signal molecules called autoinducers. The LuxS autoinducer synthase produces 4,5-dihydroxy-2,3-pentanedione (DPD), the precursor to a set of interconverting molecules that are generically called autoinducer-2 (AI-2). In enteric bacteria, AI-2 production induces the assembly of a transport apparatus (called the LuxS regulated (Lsr) transporter) that internalizes endogenously produced AI-2 as well as AI-2 produced by other bacterial species.
AI-2 internalization is proposed to be a mechanism enteric bacteria employ to interfere with the signaling capabilities of neighboring species of bacteria.
Following internalization, the kinase LsrK phosphorylates carbon-5 of the open form of DPD. Phosphorylated DPD (P-DPD) binds specifically to the repressor of the lsr operon, LsrR, consistent with P-DPD being the inducer of the lsr operon. Subsequently, LsrG catalyzes the cleavage of P-DPD producing 2-phosphoglycolic acid. This series of chemical events is proposed to enable enteric bacteria to respond to the presence of competitor bacteria by sequestering and destroying AI-2, thereby eliminating the competitors' intercellular communication capabilities.

In S. enterica serovar Typhimurium the rapid disappearance of AI-2 is a consequence of its import by an ABC transporter designated the Lsr transporter (luxS regulated). The lsr operon contains seven genes, lsrACDBFGE, and its transcription is activated by AI-2. The first four genes, lsrACDB, encode components of the AI-2 tr. Adjacent to, but transcribed divergently from the lsr operon is lsrR, which encodes a repressor of lsr transcription, and lsrK, which encodes a kinase that phosphorylates intracellular AI-2 following import. Phosphorylation of internalized AI-2 is required for induction of transcription of the lsr operon, suggesting that phospho-AI-2 is the inducer of this system. It is postulated that phospho-AI-2 binds to the LsrR repressor and inactivates it and that this results in derepression of lsr transcription
AI-2 was initially identified for its control of the expression of bioluminescence in the marine bacterium Vibrio harveyi. Genetic and biochemical analyses of mutants defective in AI-2 production showed that AI-2 is made from S-adenosylmethionine, which is used as a methyl donor in a variety of cellular processes which yield S-adenosylhomocysteine. S-Adenosylhomocysteine is subsequently metabolized to adenine and S-ribosylhomocysteine. S-Ribosylhomocysteine is the substrate for LuxS, which cleaves it to produce homocysteine and 4,5-dihydroxy-2,3-pentanedione (DPD). DPD cyclizes spontaneously and undergoes further rearrangements to form AI-2. The structure of AI-2 bound to the V. harveyi AI-2 binding protein LuxP was determined, and the results showed that V. harveyi AI-2 is a furanosyl borate diester, indicating that borate adds to the hydrated cyclized DPD molecule.

In V. harveyi, AI-2 is bound by LuxP. This signals LuxQ to become a phosphatase, leading to LuxU and LuxO dephosphorylation, which allows LuxR expression and activation of the luciferase operon. (B) Uptake of AI-2 by the Lsr ABC transporter system in E. coli and Salmonella. The lsrACDBFGE genes are transcribed as an operon, while lsrK and lsrR are transcribed divergently. Once AI-2 is bound, it is transported into the cell through the Lsr ABC transporter, phosphorylated by LsrK, and is then thought to interact with LsrR and relieve repression of the lsr operon

=====REFERENCES:==========

1. Nicola C. Reading & Vanessa Sperandio. (2006) Quorum sensing: the many languages of bacteria. FEMS Microbiology Letters 254:1, 111. PMID: 16451172

2. Xavier KB, Miller ST, Lu W, Kim JH, Rabinowitz J, Pelczer I, Semmelhack MF, Bassler BL. PhosphorylatXavier KB, Bassler BL. Phosphorylation and processing of the quorum-sensing molecule autoinducer-2 in enteric bacteria. ACS Chem Biol. 2007 Feb 20;2(2):128-36. PMID: 17274596

3. Xavier KB, Bassler BL. Regulation of uptake and processing of the quorum-sensing autoinducer AI-2 in Escherichia coli. J Bacteriol. 2005 Jan;187(1):238-48. PMID: 15601708.

4. Walters, M., Sircili, M. P., Sperandio, V. (2006). AI-3 Synthesis Is Not Dependent on luxS in Escherichia coli.. J. Bacteriol. 188: 5668-5681