Agriculture Reference
In-Depth Information
strains on agar plates, and apparently was produced by most field isolates of
Rhizobium
leguminosarum
[Hirsch, 1979; Wijffelman
et al.
, 1983]. The compound was named
small
bacteriocin due to its low molecular weight, which allowed diffusion through dialysis
membrane tubing. All
small
non-producing
R. leguminosarum
bv.
viciae
strains harbored
highly self-transmissible plasmids that were able to repress
small
production and excretion, as
a producing phenotype was observed in strain derivatives cured of these plasmids, which also
became insensitive to the bacteriocin [Wijffelman
et al.
, 1983]. Therefore, production of
small
, and the presence of genes for repression and sensitivity in self-transmissible plasmids,
were considered a characteristic of the
R. leguminosarum
species [Gray
et al.
, 1996]. More
recently, the structure of
small
bacteriocin made by
R. leguminosarum
bv.
viciae
strain A34
was purified from chloroform extracts by HPLC and NMR spectra, revealing that in fact it
was a QS signal of the AHL type [Schripsema
et al.
, 1996].
In strain A34, the production of 3-hydroxy-C
14:1
-HSL is encoded by the
cinI
gene, and its
expression is regulated by
cinR
, being the cluster of these two genes located chromosomically
[Lithgow
et al.
, 2000]. The processes that are regulated by quorum sensing in rhizobia are
still not known in full, but current knowledge indicates that it is a rather complex system
involving several
luxRI
homologues. Six regulators of the LuxR class have been identified in
strain A34, which regulate gene expression in response to AHLs [Sánchez-Contreras
et al.
,
2007]. These different regulatory systems operate as a network in which there is cross
regulation mediated via different AHLs (Table 2, Figure 6.).
The
cinR
and
cinI
genes encode a LuxR-type regulator and an AHL synthase,
respectively, and are common to all
R. leguminosarum
bv.
viciae
strains analyzed to date,
being always found in the chromosome. In strain A34, CinI produces 3-hydroxy-C
14:1
-HSL,
which positively autoregulates
cinI
expression in a CinR-dependent manner [Lithgow
et al.
,
2000]. CinR and CinI are at the top of a hierarchical cascade that regulates the expression of
at least other three AHL synthases, distributed between the symbiotic plasmid pLR1JI, and a
non-symbiotic megaplasmid [Rodelas
et al.
, 1999, Lithgow
et al.
, 2000, Wilkinson
et al.
,
2002, Wisniewski-Dyé
et al.
, 2002]. One of these pairs of
luxRI
homologues, carried by
pRL1JI, is the
rhiRI
gene cluster, responsible for regulation of expression of the
rhiABC
operon [Rodelas
et al.
, 1999, Lithgow
et al.
, 2000]. The
rhiABC
operon is also localized to
plasmid pRL1JI, positioned adjacent to the nodulation and nitrogen fixation genes, and it is
expressed in a cell-density dependent way in the rhizosphere. Although
rhiR
was one of the
earliest QS-regulators sequenced in bacteria, the role of the cell-density regulated Rhi
proteins remains unclear to date, due to little observable effects on mutants and the absence in
the databases of similar gene products with recognized biochemical functions [Wisniewski
and Downie, 2002]. Mutations of the
rhiA
gene affect nodulation in mutant strains already
impaired for nodulation due to the deletion of the
nodFEL
genes [Cubo
et al.
, 1992]. RhiA
and RhiB appear to be cytoplasmic proteins, while RhiC is a predicted periplasmic protein.
Remarkably, RhiA is a protein specific of
R. leguminosarum
bv.
viciae
(not detectable in
strains of the other two biovars) which is present in large amounts in the cytoplasm of cells in
the rhizosphere, but not after bacteria differentiate to bacteroids inside the legume nodules
[Dibb
et al.
, 1984]. A role of the
rhi
genes linked to the association of
R. leguminosarum
bv.
viciae
with the specific legume host is presumed.
