Biology Reference
In-Depth Information
2003
). In cyanobacteria, a variety of enzymes crucial for the organization
of fully functional photosystems contain these clusters. Such iron-sulphur
proteins also include some components of the respiratory electron transport
complexes and enzymes involved in the central metabolism.
3.2.1. The iron sulphurcluster (isc) system
Cyanobacteria contain two main iron-sulphur assembly systems denoted
as
suf
(sulphur utilization factor) and
isc
(iron sulphur cluster) (
Takahashi
& Tokumoto, 2002
;
Wollenberg, Berndt, Bill, Schwenn, & Seidler, 2003
).
Biosynthesis and assembly of iron-sulphur proteins is a highly regulated
process. In
E. coli
, the
isc
operon is under the transcriptional control of the
IscR repressor, which is encoded as part of the
iscRSUA
locus and auto-
regulates its own expression as well as that of
iscSUA
(
Schwartz, Djaman,
Imlay, & Kiley, 2000
). IscR is a member of the Rrf2 family (PF02082)
of transcriptional regulators and contains a winged HTH DNA-binding
domain. The active form of IscR presents an unstable [2Fe-2S] cluster that
is coordinated by three conserved cysteines and a glutamic residue. Under
unfavourable conditions, IscR loses its cluster and becomes inactive in the
apo form, allowing the full expression of the
isc
machinery. Thus, the regu-
latory role of IscR relies upon its iron-sulphur cluster and senses redox
changes in the cell for the optimal assembly of Fe-S clusters. Cyanobacterial
isc
genes are scattered throughout the genome, and some are present in mul-
tiple copies. IscR homologues identified in cyanobacterial genomes display
high sequence similarity at the N terminus in the helix-loop-helix, DNA-
binding region. However, the homology at the C-terminus is relatively low.
Cyanobacterial IscR homologues lack the 16-17 amino acids present at
the C-terminus in IscR proteins from other microorganisms. Furthermore,
the cysteine residues conserved in heterotrophic bacteria are missing in
cyanobacterial IscR homologues, making the presence of an iron-sulphur
cluster unlikely. Therefore, it has been proposed that the cyanobacterial IscR
homologues might sense changes in iron status via interactions with other
sensor proteins containing iron-sulphur clusters, such as IscA and SufA
(
Wu, 2008
).
3.2.2. The sulphur utilization factor (suf) system
The
suf
system in cyanobacteria is considerably more important than the
isc
system. Results from reverse genetics studies performed on
Synechococcus
PCC 7002 indicate that many genes belonging to the
suf
regulon are essen-
tial in cyanobacteria (
Wang et al., 2004
). Furthermore, the
suf
system, but not
