User talk:Namratagkamat/AI-2 mediated Quorum Sensing

Introduction
Quorum sensing is a process of chemical communication that cells use to assess cell population density and synchronize behavior on a community-wide scale. One form of this communication is mediated by autoinducers. The only quorum-sensing system shared by Gram-positive and Gram-negative bacteria is mediated by autoinducer-2 (AI-2). The AI-2 synthase, LuxS is widely distributed among the bacteria, which suggests the possibility of AI-2 as a universal language for interspecies communication. AI-2 is produced and released during the exponential phase of growth and internalized at the later phases of the cell cycle. Quorum sensing using luxS/AI-2 mediated signalling have shown involvement in bioluminiscence, biofilm formation and regulation of virulence factors in different systems.

History
The involvement of AI-2 was first found in Vibrio harveyi. The molecule was used for interspecies quorum sensing for bioluminescence. V. harveyi was found to have two Quorum sensing systems: system 1 in which the autoinducer (AI-1) is an AHL, and is primarily involved in intraspecies signaling and system 2, in which the autoinducer is a furanosyl borate diester involved in interspecies signaling. Hence the name AI-2. Mechanism of AI-2 production and internalization is well understood in Salmonella typhimurium. S. typhimurium uses this regulatory system named the lsr operon for biofilm formation.

Production of AI-2
LuxS plays a major role in the formation of AI-2. The production of AI-2 consists of at least 3 enzymatic steps, starting from S-adenosylmethionine (SAM). Consumption of SAM as a methyl donor produces S-adenosylhomocysteine (SAH), which is subsequently detoxified by the nucleosidase Pfs to yield adenine and S-ribosylhomocysteine (SRH). SRH is converted to 4,5-dihydroxy-2,3-pentanedione (DPD) and homocysteine. This reaction is catalyzed by LuxS, which acts as the SRH cleavage enzyme. Thus, the enzyme has an important role in the activated methyl cycle, that of recycling S-adenosylhomocysteine (SAH), which is a toxic intermediate, to homocysteine, although an alternative pathway does exist; Because of its structural similarity to SAM, SAH is a potent feedback inhibitor of the SAM-dependent methyltransferases. DPD spontaneously rearranges into AI-2.

AI-2 was initially believed to arise from cyclization of DPD followed by reaction with borate. But it is now known that AI-2 can be indepedent of boron, as in the case of S. typhimurium where AI-2 binding protein LsrB interacts with (2R,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran.

Eukaryotes accomplish the corresponding portion of the activated methyl cycle with a single enzyme(SAH hydrolase) and hence skip the formation of AI-2. Hence there is no AI-2 synthesis analogue in eukaryotes.

Structure of lsr operon
AI-2 internalization depends on an ATP binding cassette(ABC) transporter named the Lsr (LuxS-regulated) transporter. The lsr operon with its seven genes in the organization lsrACDBFGE has its transcription induced by AI-2.
 * lsrB encodes the periplasmic AI-2 binding protein
 * lsrC and lsrD encode the channel proteins
 * lsrA encodes the ATPase that provides energy
 * lsrF is similar to genes encoding aldolases
 * lsrG encodes a protein of unknown function
 * lsrK encodes a kinase that phosphorylates AI-2
 * lsrR encodes a repressor for the operon
 * lsrE resembles genes encoding sugar epimerases (In Salmonella typhimurium)

Regulation
Absence of AI-2

The operon is auto-repressed in the absence of AI-2. The product of lsrR prevents the transcription of operon.

Induction of lsr operon

When AI-2 is present in the medium, there is a basal level of internalization. On import, AI-2 is phosphorylated by the cytoplasmic LsrK kinase. Phosphorylated AI-2 binds to LsrR relieving the repression of the lsr operon. This allows further AI-2 import. Observations of increased lsr operon expression in double mutant studies of lsrFG indicate that the lsrF and lsrG genes are involved in further processing the internalized signal.

Catabolite repression by sugars

In the presence of glucose (or other phosphotransferase system sugars), the lsr operon is not transcribed because of catabolite repression, thereby preventing rapid AI-2 uptake. Hence, until the cell exhausts its sugar reserves, AI-2 is not internalized and it accumulates in cell-free culture fluids. Specifically, cAMP–CRP is shown to bind to a CRP binding site located in the upstream region of the lsr promoter. In the absence of glucose, AI-2 is produced but its presence is transient as a result of rapid internalization by the Lsr transporter.

Other forms of regulation

There are two other forms of regulation observed in the lsr operon: glycerol-3-phosphate(G3P) dehydrogenase-based regulation and dihydroxyacetone phosphate (DHAP)-based regulation. When there is a G3P accumulation in the cell, lsr operon is repressed by prevention of cAMP–CRP-dependent activation. DHAP and its derivatives are found to represses lsr transcription by a cAMP–CRP-independent mechanism. DHAP is observed to function as an anti-inducer of the lsr operon by inhibiting the binding of phospho-AI-2 to LsrR. This allows LsrR to remain in its native repressing state even in the presence of AI-2-P.