Agriculture Reference
In-Depth Information
Table 2. AHL production in
Rhizobium leguminosarum
Genes for AHL
production/regulation
Location
AHLs produced
Phenotype regulated
References
rhiR
/
rhiI
symbiotic
plasmid
C
6
-HSL
Nodulation efficiency
Cubo
et al
. (1992),
C
7
-HSL
Rodelas
et al
. (1999)
C
8
-HSL
cinR
/
cinI
chromosome
3-hydroxy-C
14:1
-
HSL
Growth inhibition
Lithgow
et al.
(2000)
raiR
/
raiI
non-symbiotic
plasmid
3-hydroxy-C
8
-HSL
Unknown
Wisniewski-Dyé
et al.
(2002)
C
6
-HSL
C
7
-HSL
C
8
-HSL
traR
/
traI
symbiotic
plasmid
3-oxo-C
8
-HSL
a
Transfer of the
symbiotic plasmid
Wilkinson
et al.
(2002),
C
8
-HSL
a
Danino
et al
. (2003)
expR
chromosome Unknown Unknown Sánchez-Contreras
et al.
(2007)
a
: AHLs identified only on the basis of their migratory behaviour in thin layer chromatography (TLC).
The second QS system localized to pRL1JI is
traRI
, which regulates expression of the
tra-trb
operon, responsible for conjugal transfer of pRL1JI, in analogous ways to the
Agrobacterium tumefaciens
model
of
Ti-plasmid transfer [Wilkinson
et al.
, 2002, Sánchez-
Contreras
et al.
, 2007]. In the
A. tumefaciens
model, the
traR
gene is transcribed in response
to activation by opines made by the host plant, and then TraR binds to AHL made by TraI,
inducing the plasmid transfer operon in a cell density dependent manner. A negative
regulatory protein (TraM) inhibits premature induction of plasmid transfer when the TraR
levels are basal [White and Winans, 2007].
Adjacent to the
tra-trb
operon in pRL1JI lies another
luxR
-type transcriptional regulator
named
bisR
, which controls
traR
expression and also acts as a QS relay to regulate recipient
induced transcription of pRL1JI [Danino
et al.
, 2003].
traR
expression is activated by BisR
specifically in response to 3-hydroxy-C
14:1
-HSL. In pRL1JI-carrying cells, BisR represses
cinI
transcription (and hence production of 3-hydroxy-C
14:1
-HSL), keeping levels of TraR low
and transcription of
tra-trb
inhibited by TraM. But when pRL1JI-carrying cells are part of a
mixed rhizobial population, BisR can respond to external 3-hydroxy-C
14:1
-HSL made by
potential plasmid recipient cells, then activating the genes for conjugal transfer. The BisR-
mediated regulatory mechanism results in very high plasmid transfer rates [Danino
et al.
,
2003]. This recipient-dependent regulation of the transference of symbiotic plasmids in
R.
leguminosarum
seems to date an exclusive feature of pRL1JI, as in other
R. leguminosarum
bv.
viciae
strains
traRI
are not found on the symbiotic plasmid [Sánchez-Contreras
et al.
,
2007].
