Biology Reference
In-Depth Information
the
isc
system, is found in the chloroplasts of higher plants, suggesting that
photosynthetic organisms may rely primarily on the
suf
system for assem-
bling iron-sulphur clusters for electron transfer cofactors (
Balasubramanian
et al., 2006
). Most Suf proteins are encoded by the
sufBCDS
operon that
is highly conserved in cyanobacterial genomes and negatively regulated by
the
sufR
gene encoded by the complementary strand (
Wang et al., 2004
),
except in
Prochlorococcus
spp. In addition of SufR regulation, it has been
reported that the activation of the
suf
promoters by a shift to high light
conditions (
Seki et al., 2006
).
SufR belongs to the DeoR family of helix-loop-helix proteins that
contain an N-terminal DNA-binding domain and four highly conserved
cysteine residues near the C-terminus. Active SufR is also an iron-sulphur
protein whose binding affinity depends on the presence and redox state of
its [4Fe-4S
2+1+
] clusters (
Shen et al., 2007
). However, SufR does not present
similar structural or function-related motifs to other redox-sensing regula-
tors such as FNR, SoxR and IscR that also possess Fe-S clusters as sensors.
Footprinting and biochemical assays show that both apo and holo-SufR exist
as dimers and the active form binds to two distinct sequences with different
affinities. The fact that the operator sequences contain two perfect inverted
repeats (CAAC-N
6
-GTTG and TAAAACAAC-N
6
-GTTGTTTTA) sepa-
rated by 26 bp and the finding of SufR tetramers has led to propose that
DNA bending might be involved in SufR regulation (
Shen et al., 2007
).
Furthermore, reverse genetics analysis of
Synechococcus
PCC 7002 high-
lighted the role of SufA and IscA in the modulation of Fe-S cluster homeo-
stasis (
Balasubramanian et al., 2006
). These studies also showed that Nfu is
essential for the Fe-S scaffold in cyanobacteria. Noticeably, Nfu is homolo-
gous to the C-terminus of NifU, a key protein for assembly of the Fe/S
cluster in
Azotobacter vinelandii
(
Fu, Jack, Morgan, Dean, & Johnson, 1994
).
3.3. Manganese Homeostasis in Cyanobacteria: the ManR
and RfrA Regulators
Manganese is particularly important in oxygenic photosynthetic organisms,
playing a critical role in forming a cluster of four atoms on the donor
side of photosystem II (PSII), which participates in catalysing the water-
splitting reaction (
Barber, 2008b
). The assembly of Mn
2+
ions to form the
catalytically active Mn
4
-Ca cluster of the oxygen-evolving complex of the
PSII reaction centre is a light-driven process termed photoactivation, which
occurs during
de novo
formation of PSII as well as during the frequent repair
of PSII in response to photoinhibition (
Aro et al., 2005
;
Barber, 2008a
).
