QUORUM-SENSING LANGUAGE IN BACTERIA AND ITS RELEVANCE IN THEIR INTERACTIONS WITH PLANTS - Soil Ecology Research Developments - page 94

3. QS Mediated by N -Acyl-Homoserine Lactones (AHLs)
3.1. Structure and Synthesis of AHLs
The QS mechanism was first described in the symbiotic relationship formed by the
bioluminescent bacteria Vibrio fisheri and the squid Eupryma scolopes. E. scolopes appears
luminescent in dark marine environments due to the maintenance of high density V. fisheri
populations in its specialized light organ [Whitehead et al. , 2001].
V. fisheri only initiates bioluminescence at high cell population densities due to the
accumulation of an activator molecule, N -(3-oxohexanoyl)-homoserine lactone (3-oxo-C 6 -
HSL), first isolated and characterized by Eberhard et al. (1981). Further research carried out
in the last two decades with several other bacterial groups demonstrated that AHL-based QS
is widespread amongst Gram-negative bacteria. This mechanism has been studied in depth for
many genera of Proteobacteria, and complex QS systems, involving hierarchical cascades of
regulation and interconnection with other non-AHL based signaling strategies, have been
thoroughly studied in Pseudomonas aeruginosa and Vibrio harveyi (for a review, see de
Kievit and Iglewsky, 2000, Whitehead et al ., 2001, Waters and Bassler, 2005, Williams et al. ,
2007). Gram-negative Proteobacteria that nodulate legume roots also communicate by AHLs,
and the state of the art of QS involvement in these plant-bacterial symbiotic interactions will
be further discussed at the end of this Chapter.
The basic structure of AHLs is a homoserine-lactone (HSL) ring, unsubstituted in the β-
and γ-positions, which is N -acylated with a fatty acyl group at the α-position [Chhabra et al. ,
2005]. The acyl side-chain is of variable nature in length, saturation level and oxidation state.
Some examples of well-characterized AHLs structures are given in Figure 2. The HSL moiety
originates from amino acid metabolism, being shown by in vitro studies using recombinant
proteins that S -adenosyl methionine is the amino acid substrate for its synthesis [Whitehead et
al. , 2001, Chhabra et al. , 2005]. The acyl side-chain is a product of fatty acid metabolism,
mainly derived from cellular pools of the appropriate acyl-ACP (acyl carrier protein) rather
than acyl-CoA [Hanzelka and Greenberg, 1996; Whitehead et al. , 2001]. The length of the
acyl side-chain is commonly 4-12 C, and may be substituted in C-3 with a carbonyl or
hydroxyl group [Hoang et al. , 2002, Watson et al. , 2002]. Some Rhizobium and
Sinorhizobium species make AHLs with a side chain of up to 18 C atoms [Lithgow et al. ,
2000, Marketon et al. , 2002], which may include unsaturated bonds.
It was firstly assumed that AHLs were able to freely diffuse through cellular membranes
[Kaplan and Greenberg, 1985]. This hypothesis is supported by the fact that exogenously
supplied AHL activate QS systems. However, more recent work demonstrated that long-chain
AHLs (>C 8 ) may be actively transported. This is the case of an active efflux pumping system
required for the effective translocation of AHLs produced by Pseudomonas aeruginosa
[Pearson et al. , 1999]. The need of active transport for long-chain AHLs in other bacteria has
not yet been reported though.
Most Gram-negative bacteria produce more than one type of AHL, but also different
organisms can produce the same AHL [Hardman et al. , 1998]. These leads to the cross-
talking phenomenon, whose implications which will be discussed in more detail later in this
Chapter.