User talk:ANILKIRANCHOKKALLA/Las-Rhl QS circuit

Introduction
Quorum sensing, a form of bacterial cell communication is defined as the regulation of gene expression as a function of cell density. Regulation through quorum sensing has been observed in many physiological processes like virulence, competence, biofilm formation etc. The minimal requirements for a quorum sensing circuit is a set of an autoinducer(chemical signalling molecules), autoinducer synthase, autoinducer binding regulatory protein. An organism can have multiple sets of functional components. The only difference between the gram positive and gram negative bacterial quorum sensing is that the former uses processed oligo-peptides and the latter uses acylated homoserine lactone as autoinducers. Recent studies revealed the inter and intra species cell communication through quorum sensing. Pseudomonas aeruginosa, an opportunistic gram negative pathogen, was found to employ Las-Rhl quorum sensing circuit which works in tandem for regulating the expression of virulent genes.

Functional Components
Las system Rhl system
 * PAI1 : Autoinducer
 * LasI : PAI1 synthase
 * LasR : PAI1 binding regulatory protein
 * PAI2 : Autoinducer
 * RhlI : PAI2 synthase
 * RhlR : PAI2 binding regulatory protein

Regulatory Mechanism
Initiation

The first step in regulatory mechanism involves the intracellular production of both autoinducers(PAI1,PAI2) by the respective synthases and autoinducers diffusion to the extracellular environment. The extracellular concentration of these autoinducers reaches a critical threshold at high cell density thereby facilitating diffusion into internal environment.

— Preceding unsigned comment added by ANILKIRANCHOKKALLA (talk • contribs) 09:10, 27 September 2013 (UTC)

Regulation of Virulent genes

In the intracellular environment, PAI1 selectively binds to LasR resulting in the formation of LasR-PAI1 protein complex. This complex binds to the promoter sequence upstream of the corresponding target virulent genes which include lasB(encodes elastase), lasA(encodes protease), aprA(alkaline phosphatase) and toxA(encodes exotoxinA) thereby transcriptionally activating their expression.

The formation of RhlR-PAI2 complex and its regulatory activity on the corresponding target virulent genes can be understood in the similar lines of Las system except for the variation in target virulent genes. Target virulent genes of Rhl system include rpoS(encodes stationary sigma factor), rhlAB(encodes rhamnosyltransferase) and lecA(encodes pyocyanin synthesis).

Besides the regulation of virulent genes, these protein complexes activates their respective autoinducer synthases thereby establishing a positive feedback loop.

Las-Rhl system : Tandem functioning

Compared to the primitive LuxI/LuxR system in Vibrio fischeri, the Las-Rhl system is considered to be hierarchical because of tandem functioning. This is attributed to the individual regulatory effects of PAI1 and LasR-PAI1 complex on Rhl system. The Las-PAI1 protein complex is the sole activator of rhlR gene and PAI1 inhibits the binding of PA12 to Rhl protein. Initially, when the free PAI1 concentration is high, the formation of RhlR-PAI2 complex is inhibited. Due to abundant availability of free PAI1, formation of LasR-PAI1 complex is enhanced which inturn activates the Las system target genes and the production of RhlR protein. Gradual decrease in the free PAI1 concentration which is the result of respective protein complex formation leads to the release of inhibitory effect on the PAI2 binding to RhlR protein. Thus the regulatory effect of RhlR-PAI2 complex on its target genes is observed at a significant level during the low concentration of free PAI1(or high concentrations of LasR-PAI1 complex). In this manner, the Las-Rhl system is functioning in tandem and leads to hierarchical regulatory system.

Eukaryotic Analogue
Since quorum sensing involves the cell-cell communication,it was hypothesized that the evolution of bacterial quorum sensing has lead to multicellularity. Recent studies revealed that Temnothorax albipennis(ant), Apis mellifera(Honey bee) use quorum sensing like phenomenon to coordinate their activities. Unlike bacterial quorum sensing these eukaryotic analogues are in no way related to the regulation of gene expression.