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Figure 2. Chemical structure of some AHLs used as QS signals by Gram-negative bacteria (Dong
et al.
2007; Williams
et al.
, 2007).
3.2. Functional Components of the AHL-Mediated QS Regulation
3.2.1. The LuxR and LuxI Protein Families
Bacteria commonly perceive changes in their surrounding environment by two-
component systems, which typically consist of a sensor of the stimuli (which also works
transmitting the signal), and a transcriptional regulator of the sensor's response to the signal.
In the case of the regulation based on self-produced signals, a third component would be
required, which generates the signals. However, the QS systems mediated by AHLs only have
two main regulatory genes, which were firstly analyzed and sequenced in bioluminescent
Vibrio fisheri
, and named
luxR
and
luxI
[Engebrecht
et al.
, 1983]. Analysis of these genes in
V. fischeri
led to the basic model for AHL-dependent QS, which is now the archetype for
other analogous cell-to-cell communication systems (Figure 3). By means of the AHL-based
mechanism, Gram-negative bacteria can efficiently couple gene expression to fluctuations in
cell-population density.
The
lux
genes are organized in two divergent transcriptional units, separated by ca. 155
bp. The first transcriptional unit carries seven genes in the
luxICDABEG
operon under the P
R
promoter. The second unit consists of the regulatory
luxR
gene and the P
L
promoter
[Engebrecht and Silverman, 1984, 1987; Devine
et al.
, 1988]. LuxI is the autoinducer
synthase that generates
N
-(3-oxohexanoyl)-
L
-homoserine lactone (3-oxo-C
6
-HSL), and LuxR
acts as a regulatory protein that binds the autoinducer molecule and activates the transcription
of
luxICDABEG
by facilitating the binding of RNA polymerase to the target promoter
[Stevens
et al.
, 1994]. Hence, LuxR covers both the roles of sensing/transducing the signal
and the regulation of the response to it. LuxR binds to AHL through an amino-terminal
domain, and the carboxy-terminal region of the protein carries an H-T-H motif [González and
Marketon, 2002].
The
lux
genes are thus regulated by LuxR and the autoinducer product generated by LuxI.
At low cell density, transcription of both
luxR
and
luxI
happens at a basal level, enough to
allow a certain concentration of LuxR and LuxI to accumulate in the surround of cells.
