User talk:Praphulla249

Agr Operon


The expression of virulence factors and surface proteins in the human pathogen Staphylococcus aureus is regulated by accessory gene regulator(agr). Quorum sensing system plays a vital role in regulating the synthesis of virulence factors in Staphylococcus aureus,which in turn is controlled by the agr operon. The agr system is highly conserved throughout the Staphylococcus family.

The agr locus has two divergent transcription units,RNAII and RNAIII.The RNAII transcript,driven by P2 promoter encodes quorum sensing system and consists of four open reading frames AgrB, AgrC,AgrD and AgrA.Wherein AgrA plays a major role in activating the agr operon. The RNAIII transcript,driven by P3 promoter regulates the expression of other genes related to quorum sensing.It results in increased production of virulence factors and decreased expression of surface proteins.

Not only does the agr operon regulate the expression of virulence factors but also the expression of its own genes.The system has a positive feedback loop which ascertains the expression of targeted genes at a certain threshold level of bacterial cell density.

Structure
The agr operon contains two transcription units RNAII and RNAIII. RNA II, driven by P2 promoter,is a polycistronic mRNA encoding four proteins :

• agrB : modifies and secretes AIP from the cell

• agrD : pro-autoinducing peptide( signalling molecule)

• agrC : membrane bound protein serves as receptor

• agrA : DNA binding response regulator

Significance
Quorum sensing systems in Straphylococcus aureus are key virulence reguators.They depend on small signalling molecules, known as autoinducers, produced and detected by the bacteria.At low cell densities the organism expresses proteins for attachment and colonization,whereas at high cell densities it starts overexpressing the toxins through the expression of agr operon. Based on the concentration of the autoinducing peptides,the agr operon transduces the signal that the required concentration of cell density is attained ,which in turn positively regulates the operon resulting in high expression of virulence factors.

Mechanism
AgrD codes for a pro-peptide; AgrB is an integral membrane protein which modifies the AIPS,resulting in an octapeptide autoinducing peptide (AIP) and secretes it out of the cell. Once the transmembrane receptor domain of AgrC is bound by AIP, it will phosphorylate AgrA which further activates the two promoters P2 and P3. RNA III, driven by P3 promoter, acts as a regulatory factor to activate numerous toxin production genes and encode for β-hemolysin. Activated AgrA increases the expression of RNAII which leads to production of sufficient number of AIPs indicating that the cell density has reached a threshold level.

Regulation


The synthesis of RNAIII is regulated by two Staphylococcal quorum sensing systems SQS 1 and SQS 2. SQS 1 composes a  RNAIII-activating protein (RAP) and a target of RNAIII-activating protein (TRAP) which is membrane associated. As cell density increases, RAP is secreted out. Once a sufficient concentration of RAP is attained, it will induce the histidine phosphorylation of TRAP, leading to the synthesis of SQS 2, which encodes the products of agr system.The activation of agr system leads to production of RNAII. After the activation of agr system, phosphorylation of AgrC is induced by secreted AIP, followed by the production of RNAIII.

Other parallel quorum sensing circuits also regulate the expression of agr locus :
 * Sar : elevates the levels of agr transcription


 * Srr : inhibits the synthesis of RNAIII


 * RIP : blocks RAP


 * Repressor of Toxins (ROT) : prevents binding of RAP to TRAP

Eukaryotic homologue
Molecular mechanisms that drive the quorum signalling systems are present even in the eukaryotes.Some social insects like bees and ants use these systems for communication. Many higher social organisms are believed to use chemical signalling pathways that are quite analogous to the cell to cell communication systems in prokaryotes.