Quorum Sensing

Quorum Sensing has been extensively studied in various bacteria systems. Here we will discuss the basic components of Quorum Sensing and briefly explain how it works.

Youtube video showing a simple model of Quorum Sensing

Quorum sensing acts through the synthesis, secretion and receptor binding of signal molecules, usually Acyl Homoserine Lactones (AHLs) in proteobacteria. There are 2 major components in a quorum sensing circuit:

• LuxI protein family acts as the signal synthase which catalyses the synthesis of quorum sensing signal molecule (AHLs, also known as autoinducers)
• LuxR protein family acts as the signal receptor-transcription factor which recognises its cognate ligand at certain critical threshold levels, and the resulting conformational change facilitates the binding to specific DNA sequence to regulate transcriptional activities of certain genes

Fig.1: Diagram depicting a simplified quorum sensing circuit involving LuxI (light blue) and LuxR (dark blue) in Vibrio fischeri. The increase of AHL level stimulates LuxR dimerisation and binding to specific DNA sequence to regulate gene transcription. In this case activated LuxR dimer binds to upstream region of luxI, stimulating transcription of luxICDABE. This results in increased production of LuxI and the Luciferase protein (purple) which generates light through a redox reaction.
[Source: Subrealism Blogspot]

A step-wise description of the quorum sensing circuit involving LuxI and LuxR is as followed:

1. LuxI and LuxR are constantly expressed in basal levels, thus the signal molecule concentration increases with the bacteria population.
2. As signal molecules synthesised by LuxI diffuse freely across bacteria, LuxR senses its concentration by binding reversibly to the signal.
3. When the concentration reaches a critical threshold, LuxR is activated through conformational changes, sometimes oligomerisation, leading to formation of active DNA binding site.
4. Activated LuxR binds to specific DNA sequence and interact with RNA Polymerase to regulate gene transcription.
5. Some target genes of LuxR include luxI and luxR which codes for LuxI and LuxR respectively. This may form a positive feedback loop to further increase LuxI/R production, or a negative feedback loop to reduce amount of gene expression.

Both LuxI and LuxR were discovered initially in Vibrio fischeri. Subsequent research has identified various proteins homologous to LuxI and LuxR, operating in similar quorum sensing mechanism. A few examples of LuxI/R homologues and their cognate AHLs are as followed.

Fig.2 LuxI/R homologue and AHL in different organisms
[Source: Stevens et al, 2010]

In Pseudomonas aeruginosa, 2 complete quorum sensing circuits exist: the LasI/R and the RhlI/R systems. QscR (for Quorum sensing control Repressor), another LuxR homologue, recognises the LasI-synthesised N-3-oxo-dodecanoyl-homoserine lactone (3OC12-HSL) and binds to specific DNA binding sequence (a palindrome of two 7-bp half-sites separated by a 4-bp spacer).

Fig 3: N-3-oxo-dodecanoyl-homoserine lactone
The C12 acyl chain (with carbon atoms numbered in red), 3-oxo group and homoserine lactone (HSL) ring are labelled.

Fuqua (2006) discusses how QscR might actually act as a quorum sensing inhibitor to both LasI/R and RhlI/R systems (see Fig 4). Some experimental evidence supported this theory, but the exact mechanisms of the molecular interactions are yet to be elucidated. Ledgham et al (2003) investigates the in vivo interaction of QscR with LasR and RhlR and observed that QscR-LasR and QscR-RhlR heterodimers were formed in the absence of their cognate HSLs, and dissociate readily upon adding the ligands. QscR knock-out mutants have been shown to overexpress phenazine, a blue pigment, through aberrant activation of two phenazine biosynthesis operons (PhzA-G and PhzA2-G2).

Fig 4: Model of P. aeruginosa quorum-sensing network.
Black arrows indicate direct transcriptional control (positive/negative or as indicated), blue arrows indicate protein-AHL interactions, and red arrows are protein-protein interactions. Solid arrows are well-supported mechanisms of regulation, while dashed arrows are more tentative. Looped arrows indicate positive feedback on cognate AHL synthesis via LasR and RhlR. Underlying circles represent genes under direct transcriptional control of each LuxR-type protein.
[Source: Fuqua, 2007]

Lee et al (2005) found that QscR responds better to 3OC10-HSL (ACL having shorter acyl chain-C10) compared to the LasI synthesised 3OC12-HSL, and this relaxed specificity suggests that QscR could respond to AHL synthesised by other bacterium species, enabling 'eavesdropping' or inter-species communication with other co-habiting bacteria. The structural details of QscR in interaction with 3OC12-HSL is discussed in the 'Structural Insight' tab.

Quorum Sensing and Pathogenicity

P.aeruginosa is an opportunistic pathogen, i.e. it takes advantage of immunosuppressed organisms to establish infection. QS in P.aeruginosa regulates the expression of several virulence factors so regulation of this process is essential for pathogenicity of the organism. This has been confirmed through studies in different animal models (Teresa R. et al, 2000). For example, deficiency of LasR in a bacterial strain had significantly decreased virulence in a mouse suffering from pneumonia. Further research on the key components regulating this response can provide useful insight to its pathogenicity and lead to the development of important therapeutic strategies.