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Review
. 2012 Oct;21(10):1403-17.
doi: 10.1002/pro.2132. Epub 2012 Aug 21.

Quorum sensing: how bacteria can coordinate activity and synchronize their response to external signals?

Zhi Li  1 , Satish K Nair
Affiliations
Review

Quorum sensing: how bacteria can coordinate activity and synchronize their response to external signals?

Zhi Li et al. Protein Sci. 2012 Oct.

Abstract

Quorum sensing is used by a large variety of bacteria to regulate gene expression in a cell-density-dependent manner. Bacteria can synchronize population behavior using small molecules called autoinducers that are produced by cognate synthases and recognized by specific receptors. Quorum sensing plays critical roles in regulating diverse cellular functions in bacteria, including bioluminescence, virulence gene expression, biofilm formation, and antibiotic resistance. The best-studied autoinducers are acyl homoserine lactone (AHL) molecules, which are the primary quorum sensing signals used by Gram-negative bacteria. In this review we focus on the AHL-dependent quorum sensing system and highlight recent progress on structural and mechanistic studies of AHL synthases and the corresponding receptors. Crystal structures of LuxI-type AHL synthases provide insights into acyl-substrate specificity, but the current knowledge is still greatly limited. Structural studies of AHL receptors have facilitated a more thorough understanding of signal perception and established the molecular framework for the development of quorum sensing inhibitors.

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Figures

Figure 1
Figure 1
Acyl homoserine lactone (AHL)-dependent quorum sensing system as exemplified by LuxI/R system in V. fischeri.
Figure 2
Figure 2
Structures of acyl-homoserine lactone molecules produced by different bacteria.
Figure 3
Figure 3
Reaction scheme of the synthesis of N-acyl-homoserine lactone catalyzed by AHL synthase.
Figure 4
Figure 4
Sequences and structures of three AHL synthases. (A) Structure-based sequence alignment of the AHL synthases. Identical residues are shaded. Residues deleted or substituted in order to facilitate crystallization are indicated by red crosslines or blue frames. Secondary structures of TofI, LasI, and EsaI are shown below the sequences. (B–D) Three-dimensional structures of TofI, LasI, and EsaI. The stands β4 and β5 are labeled.
Figure 5
Figure 5
Acyl-chain binding in TofI. (A) The secondary structure elements that form the acyl-chain binding pocket. (B) Chemical structures of C8-HSL and J8-C8, the product and inhibitor, respectively, of TofI. (C) Surface representation of the acyl-binding tunnel in TofI. The hydrophobic residues that form the tunnel are shown. (D) Superimposition of the TofI ternary structure and the EsaI structure. Only J8-C8 is shown from the TofI structure. (E) Superimposition of the TofI ternary structure and the LasI structure. Only J8-C8 is shown from the TofI structure.
Figure 6
Figure 6
Binding of 5′-methylthioadenosine (MTA) to TofI. (A) Top view of the binding sites of MTA and J8-C8. (B) The tunnel between J8-C8 and MTA. (C) Close-up view of the MTA binding site.
Figure 7
Figure 7
Structures LuxR-type AHL receptors. (A) Overall structure of TraRAT-AHL-DNA ternary complex. The AHL ligand (3-oxo-C8-HSL) is shown as yellow spheres. NTD: N-terminal domain; CTD: the C-terminal domain. (B–D) Dimerization of the NTD of TraRAT, QscR, and CviR. The residues on each monomer that are within 5 Å of the opposite monomer are shown in green. The bound ligands are shown as yellow spheres. (E) Comparison between the AHL-binding modes of TraRAT and QscR; 3-oxo-C8-HSL bound to TraRAT (gray) is shown in yellow and 3-oxo-C12-HSL bound to QscR (not shown) is shown in pink. (F) Structure-based sequence alignment of the NTD of LuxR receptors. Secondary structure of TraRAT is shown below the sequences.
Figure 8
Figure 8
C-terminal domain of LuxR-type AHL receptors. (A) Structure-based sequence alignment of the C-terminal domains of four LuxR proteins. Secondary structure based on TraRAT is shown below the sequences. (B) The C-terminal domains of TraRAT dimer binding to the tra box. Arg206 and Arg210 of one monomer and their interacting bases are shown as ball-stick model. Strands E and F of the DNA are indicated. (C) TraRNGR binding to antiactivator TraM. (D) CviR binding to a chlorolactone compound (CL, yellow). (E) QscR dimer superimposed with TraRAT-AHL-DNA complex. The C-terminal domains of TraRAT are shown in gray. The N-terminal domains of TraRAT are not shown.
Figure 9
Figure 9
Triphenyl compounds binding to LasR. (A) Chemical structures of the cognate ligand (3-oxo-C12-HSL) of LasR and triphenyl (TP) compounds TP-1, TP-2, and TP-3. (B) Comparison between the binding modes of 3-oxo-C12-HSL (yellow) and TP-1 (pink) to LasR.
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