Agriculture Reference
In-Depth Information
transcription of
luxR,
self-limiting the autoinduction of bioluminescence [Whitehead
et al
.,
2001].
In most of the bacterial species analyzed to date, AHLs are synthesized by LuxI homologues
and interact with LuxR-type transcriptional activators. Systems related to LuxR/LuxI have been
identified as cell-density dependent regulators of many bacterial functions, often related to the
relationships of bacteria with eukaryotic organisms, in both symbiotic and pathogenic
interactions. LuxR and LuxI homologues constitute evolutively conserved families of regulatory
proteins, although amino acid alignments of sequences disclose a low level of sequence
similarity amongst members of each family of proteins [Whitehead
et al.,
2001]. Phylogenetic
analysis of the sequences of known LuxR and LuxI homologues reveals an early origin of these
regulators during the evolution of Gram-negative bacteria, as well as the probable co-evolution of
both types of genes as part of regulatory cassettes. In most cases, horizontal transfer of the
luxRI
genes is suggested, as several species carry multiple pairs of homologues of often poorly related
sequences, which in addition are frequently harbored in conjugative plasmids [Gray and Garey,
2001; Wisniewski and Downie, 2002].
3.2.2. AHL Synthases Unrelated To LuxI: The LuxM/Ains and HDTS Families
LuxM/AinS family synthases have been only found to date in
Vibrio
species [Milton,
2006]. It was first revealed in
Vibrio harveyi
that the synthesis of 3-hydroxy-C
4
-HSL was
directed by LuxM, an autoinducer synthase lacking similarity to LuxI, although it catalyzes
identical biochemical reactions to generate a specific AHL [Bassler
et al.
, 1993, Whitehead
et
al.
, 2001]. Other synthases of the LuxM family have been found in
Vibrio fisheri
(AinS) and
V. anguillarum
(VanM) [Gilson
et al.
, 1995, Milton
et al.
, 2001]. A third type of AHL
synthase named HdtS has been detected in
Pseudomonas fluorescens
[Laue
et al.
, 2000].
4. QS Mediated by Autoinducing Oligopeptides (AIPs)
In contrast to Gram-negatives, autoinducing oligopeptides (AIPs) are the principal QS
signals known in Gram-positive genera such as
Lactococcus
,
Streptococcus
,
Staphylococcus
and
Bacillus.
AIPs range from 5-34 amino acids, usually arose after processing of a precursor
peptide, and are often post-translationally modified [Camilli and Bassler, 2006, Williams
et
al.
, 2007]. Mature peptides are secreted into the medium using one associated ATP-Binding
Cassette (ABC) transporter complex [Donabedian, 2003]. At high cell densities, the
accumulated peptides reach a critical threshold concentration and are recognized by a two-
component signaling system. Signal transduction is relayed into the cell by phosporylation,
resulting in an altered gene expression [Kleerebezem
et al
., 1997]. AIPs were shown to be
highly specific to their cognate sensor kinase, thus allowing only the specific recognition of
one species [Havarstein
et al.
, 1997].
According to their structure, AIPs are divided in 3 families [Williams
et al.
, 2007]. One
of the earliest known systems was that of
Streptococcus pneumoniae
. It was observed that at
high cell densities
S. pneumoniae
was capable of competence [Tomasz and Hotchkiss, 1964,
Tomasz, 1965, Tomasz and Mosser, 1966]. Further investigation elucidated an operon
containing the
comCDE
genes, named after the observed phenomenon. The precursor of the
autoinducing peptide is the product of the
comC
gene, whereas the two-component system is
encoded by the
comD
and
comE
genes, respectively [Pestova
et al.
, 1996]. The autoinducing
