Essay Example on Quorum sensing QS is a phenomenon of cell cell communication in many Bacteria









Quorum sensing QS is a phenomenon of cell-cell communication in many bacteria This phenomenon is density-dependent and mediated by molecular signals like N Acyl homoserine lactone AHL in gram-negative bacteria e g proteobacteria and small peptides in Gram-positive bacteria In response to these molecular signals bacteria perform various microbial processes like virulence bioluminescence conjugation etc These AHLs are synthesized by LuxI and bind to ABD domain at N terminus of its cognate pair However in many bacteria they do not exist in their respective genomes and have yet functional LuxR suggesting that this LuxR can exist without its cognate pair These LuxR transcription factors are called as orphan or solo LuxR which can be recognized either by endogenous or exogenous signal molecules Few bacterial LuxR solos were associated with interkingdom signaling where the LuxR is recognized by signal molecules released by plant and these bacteria are termed as plant-associated bacteria PAB 

The LuxR solos found in actinobacteria among other non-proteobacteria are evolutionarily distant from the rest which are evolutionarily highly related to well studied proteobacterial LuxR solos Mycobacteria which belongs to the family of actinobacteria also harbors LuxR solos These LuxR proteins are functional as they are part of two-component systems in mycobacteria RegX3 is one of the well-characterized LuxR family proteins that functions as a two-component response protein along with SenX3 in phosphate transport

Recently it has also been shown to regulate esx 5 operon that is crucial for the secretion of virulent factors by Mycobacterium tuberculosis This review presents the current knowledge of the LuxR solos in different bacterial and plant species and contemplates on the role of LuxR solos of Mycobacterium tuberculosis in Quorum sensing Keywords Quorum sensing N Acyl homoserine lactone LuxI LuxR LuxR solos AHL binding domain ABD INTRODUCTION Quorum sensing is a cell to cell communication through which bacteria talk to each other 

They coordinate their behaviour by using this mechanism in a density-dependent manner where the bacteria secrete chemical signal molecules called sensors When they reach a certain threshold level they initiate gene expression which involves in various biological processes Quorum sensing is involved in various biological processes including virulence bioluminescent conjugation biofilm formation sporulation and growth Quorum sensing pathways has three archetypes Gram-negative bacteria which is a LuxI LuxR type QS system It is mediated through AHL Acyl Homoserine Lactone Gram-positive bacteria in contrast to the gram-positive bacteria these are mediated through short autoinducer peptides AIP A universal signal mediated through Auto Inducers AI1 or AI2 Quorum sensing in gram-negative bacteria 1 Vibrio fischeri it is the first bacteria to discover the QS system and extensive studies were carried out to understand this mechanism In LuxI LuxR circuit LuxI synthesizes the AHL N 3 oxohexanoyl homoserine lactone molecules into extracellular space 

When they reach the threshold concentration they are sensed by the LuxR and regulates the gene expression Here the gene expression denotes bioluminescence the bacteria fluoresce due to the high cell density of the bacteria and helps the bobtail squid to escape from the predators 2 Pseudomonas aeruginosa a pathogenic bacteria has two homologues of LuxI LuxR circuit They are LasI LasR and RhlI RhlR LasI and RhlI both synthesize the AHLs like N 3 oxododecanoyl homoserine lactone and N butryl homoserine lactone respectively These autoinducer molecules bind to their respective cognate pair LasR and RhlR respectively and transcribe the genes to attack the host defence mechanism Quorum sensing in gram positive bacteria 1 Streptococcus pneumoniae has ComD ComE QS circuit ComC synthesizes a peptide called as competence stimulating peptide CSP a precursor peptide This peptide is further processed to form an activated peptide When they accumulate to form a quorum of these activate molecules and activates ComD by phosphorylation A cascade of phosphorylation reactions takes place to activate ComE to develop competency 2 Staphylococcus aureus it also consists similar QS system as Streptococcus pneumoniae responsible for virulence But the precursor peptide is synthesized by AgrD instead of ComC which provides active peptide The processed active peptide then activates the AgrC AgrA QS system Universal Quorum sensing system This type of QS system is seen in both gram positive and as well as gram negative bacteria mediated through Autoinducer molecules AI1 or AI2 Few bacteria like Vibrio harveyi and Myxococcus xanthus exhibit this mechanism 1 

Vibrio harveyi a gram-positive bacteria unlike other gram-positive bacteria it has a QS system mediated by AHL molecules instead of AIPs AI 1 molecules are synthesized by LuxLM and sensed by LuxN whereas AI 2 are sensed by LuxQ 2 Myxococcus Xanthus it requires three proteases AsgA AsgB and AsgC to produce an A signal molecule A signal is sensed by SasS and regulates SasR by autophosphorylation It also requires a negative regulator SasN to be inactivated in order to respond to the A signal LuxI LuxR type Quorum Sensing system It is one of the simplest QS system well understood in Vibrio fischeri LuxI biosynthesized the AHL molecule N 3 Oxohexonyl homoserine lactone These molecules are membrane-permeable When the concentration of bacteria increases the AHL concentration also increases and secreted into the extracellular space Once they reached the high cell density they bind to its cognate pair LuxR The AHL LuxR complex transcribes the downstream genes or genes responsible for various microbial processes There are many homologues of LuxI LuxR QS circuits studied extensively in all types of bacteria Few of them are listed below

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