Biology Reference
In-Depth Information
between oxidative stress and iron starvation was described in a number of
studies (
Houot et al., 2007
; Li et al., 2004;
Shcolnick et al., 2009
). The data
presented in these studies suggest that intracellular Fe status is a major deter-
minant of oxidative stress, possibly through Fenton-type reactions with free
iron. The response seems to be co-ordinated by the function of PerR-type
regulators and involves the activity of DPS-type proteins, probably as iron
scavengers reducing the risks of intracellular Fenton reactions.
Several transcriptomic studies were performed on the transition from
replete to limiting iron conditions (
Shcolnick et al., 2009
;
Singh, McIntyre,
& Sherman, 2003
in
Synechocystis
sp.
PCC
6803;
Nodop et al., 2008
in
Synechococcus elongatus
PCC 7942). Common to all these studies is a con-
certed decrease in photosynthesis and respiration genes. The
isiAB
operon
is the major upregulated operon. Both effects fit well with the physiological
data. Additional upregulated genes include components of the iron trans-
port system (FuhA and FutA2, for example,
Shcolnick et al., 2009
) and of
the iron sulfur cluster assembly system (sufC, sufD, sufS;
Nodop et al., 2008
).
In
S. elongatus
PCC 7942, specific transcriptional responses in
idi
genes
were observed. The
idiA
gene, the transcription factor coding
idiB
, and the
iron-deficiency-induced gene C,
idiC
, were drastically induced (
Nodop
et al., 2008
). The function of some of the induced genes in iron-deficiency
response has been studied and will be discussed below. However, it is impor-
tant to note that many unknown and hypothetical proteins were affected by
this treatment, suggesting a more extensive and complicated response than
we can currently decipher.
4.3.1. The IsiA Protein
IsiA is present in the genome of many cyanobacterial species of all five cya-
nobacterial branches (
Geiss et al., 2001
;
Nicolaisen & Schleiff, 2010
). The
expression of
isiA
is induced by iron starvation (
Geiss et al., 2001
;
Kouril
et al., 2005
;
Latifi et al., 2005
). Initially, it was discussed that its expression
is the result of Fur-mediated derepression of transcription (
Ghassemian &
Straus, 1996
). Interestingly,
isiA
expression is significantly upregulated in
the furA overexpression strain (González et al., 2010). This might be related
to the observed enhanced expression of
isiA
in response to oxidative stress
(
Michel & Pistorius, 2004
) and thus, the induction of
isiA
expression by
other stresses might reflect a function in photosystem protection in general.
isiA
codes for the CP43′ protein, which forms a chlorophyll-protein
complex. Originally, it was identified as the protein responsible for the
blue shift in the chlorophyll peak and increased 682-nm fluorescence in
