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Introduction
lsr Transport Cassette is a transport system first discovered in marine bacteria,. lsr stands for luxS-regulated. The main function of this transport system is to remove Autoinducer-2 (AI- 2) from the extracellular culture medium and transport it into the cell. This system is found in many species of bacteria including Salmonella typhimurium and Escherichia coli.

LuxS plays an important role in the metabolism of S-adenosylmethionine (SAM), which is subsequently converted to 4,5-dihydroxy-2,3-pentanedione (DPD), the precursor of AI-2.

AI-2 is a quorum sensing signalling molecule which was found to play a crucial role in interspecies communication. It was first discovered in the marine bacterium Vibrio harveyi. As mentioned earlier, it is synthesized by LuxS and accumulates extracellularly.

In Salmonella typhimurium, the AI-2 present outside the cell is imported into the cell by a transporter encoded by the lsr operon. Upstream of the operon, there is a divergently transcribed gene encoding LsrR, which was found to repress the transcription of the lsr operon by directly binding to their promoters.

Structure
The lsr transport system consists of seven genes:

•	lsrA, lsrC' and lsrD' genes encode an ABC transport system that imports AI-2.

•	lsrB gene encodes an AI-2 binding component present in the periplasmic region.

•	lsrF and lsrG genes play an important role in AI-2 modification.

•	lsrE gene encodes an epimerase of uncharacterized function.

The following genes are also present upstream of the divergently transcribed lsr operon:

•	lsrK gene encodes an AI-2 kinase.

•	lsrR gene encodes a transcription factor that represses the divergently transcribed lsr operon

Mechanism of Regulation
The periplasmic binding protein component, LsrB, is required for the transport of AI-2 into the cell. It binds specifically to AI-2.

lsrA, lsrC and lsrD genes encode an ATP Binding Cassette transport system that imports AI-2 into the cell.

Another gene, lsrK, homologous to carbohydrate kinases, phosphorylates AI-2 upon entry into the cell and forms Phospho-AI-2.

lsrF and lsrG modify phospho-AI-2 further. However, the precise modifications are not known.

A regulatory gene, lsrR, present upstream of the lsr operon encodes a protein that represses the lsr operon. Regulation of the lsr operon requires the presence of lsrK, indicating that phospho-AI-2 may be the actual signal that is sensed either directly or indirectly by the repressor LsrR.

Basal expression of the LsrACDB transporter allows some AI-2 to enter the cytoplasm. Once the AI-2 has been phosphorylated, Phospho-AI-2 binds to LsrR and relieves its repression effect on the lsr transporter genes, thus stimulating a positive feedback and enabling additional AI-2 uptake.