Biology Reference
In-Depth Information
Additionally, the nickel response
nrs
operon, induced in the presence of
Ni
2+
and Co
2+
, was also identified. This
nrsBACD
operon encodes a puta-
tive membrane-bound protein complex that exports Ni
2+
by a cation/pro-
ton antiport (NrsA and NrsB), an unknown function protein (NrsC) and
a putative permease belonging to the major facilitator superfamily (MSF),
which contains a histidine-rich region involved in Ni
2+
binding (NrsD)
(
García-Dominguez et al., 2000
). Subsequently,
nrsR
and
nrsS
genes were
included in this metal-regulated cluster in
Synechocystis
. NrsR and NrsS
constitute a two-component signalling system involved in Ni
2+
sensing and
in
nrsBACD
operon regulation.
3.6.1.1. Properties of the NrsR/NrsS two-component system
NrsS is a histidine kinase sensor with an N-terminal periplasmic domain
that senses periplasmatic nickel. NrsR belongs to the PhoB/OmpR
response regulator family with an N-terminal phosphorylation domain and
a C-terminal DNA-binding region. It is suggested that the regulator NrsR
binds two direct repeats GA(A/T)TTTCA separated by 3 bp in the inter-
genic region near the −10 box of the two operons, in a similar way to the
founder member of its family PhoB. The proposed mechanism of action is
that NrsS may sense the presence of nickel in the periplasm and transfer a
phosphate to NrsR. Phosphorylated NrsR binds to the
nrsRS-nrsBACD
intergenic region inducing its expression (
Lopez-Maury, Garcia-Domin-
guez, Florencio, & Reyes, 2002
).
3.6.1.2. The InrS repressor
Recently, a new cytosolic-nickel sensor, InrS, has been described. This
protein represses the expression of the permease NrsD, which exports
nickel to the periplasm from a promoter in the
nrsC-nrsD
intergenic
region. In
Synechocystis
, InrS possesses a histidine-rich motif in the N-ter-
minal region, and it responds to nickel and cobalt but not to copper. It
has been proposed that InrS binds two inversed repeats containing several
G/C flanked by an A/T-rich sequence (
Foster, Patterson, Pernil, Hess, &
Robinson, 2012
).
Overall, when Ni
2+
accumulates in the periplasm of
Synechocystis
, the
nrsBACD
operon is expressed under the control of the NrsRS system,
allowing nickel efflux across the outer membrane. Meanwhile, cytosolic
Ni
2+
is sensed by InrS inducing the expression of the NrsD permease that
exports nickel from the cytosol.
