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(
Shcolnick & Keren, 2006
). In fact, several members of the FurA regulon
(
Table 4.4
) participate in oxidative stress defence, while some Slr1738 tar-
gets in
Synechocystis
, such as
isiA
,
idiA
and
mrgA
are metal-regulated genes
(
Li et al., 2004
). Previous research on the role of IdiA and MrgA show that
these proteins play a pivotal role in the coordination of iron homeosta-
sis and the oxidative stress response (
Michel & Pistorius, 2004
). Moreover,
phenotypic analysis of
Synechococcus
mutants lacking IscA and SufA also
point to these proteins as important players in the concerted modulation of
iron homeostasis and the sensing of redox stress, expanding cyanobacterial
strategies to deal with adverse conditions (
Balasubramanian, Shen, Bryant,
& Golbeck, 2006
). Noticeable among these is the response of
Synechocystis
to Cd stress consisting of an integrated reprogramming of the metabolism
under the control of the Fur member Slr1738 (PerR) (
Houot et al., 2007
).
Under Cd stress, metal homeostasis (especially Fe and Zn) and high-light
tolerance are disturbed, as well as the functionality of the SUF machinery
involved in the synthesis and repair of iron-sulphur centres. Slr1738 targets,
such as the
isiAB
operon, are induced by Cd and H
2
O
2
and, noteworthy,
the addition of iron in the medium increases the cell tolerance to both chal-
lenges. Transcriptomic and spectroscopic analyses also indicate that cyano-
bacteria challenged with H
2
O
2
or Cd use different strategies for supplying
Fe atoms to Fe-requiring metalloenzymes and the SUF machinery. Since
Cd regulates the Zn-controlled genes
znuA
and
ziaA
, these authors suggest
that this pollutant might be transported via Zn-transport systems. Consid-
ering that
znuA
has been identified as a member of the FurB/Zur regulon
in
Anabaena
(
Napolitano et al., 2012
), these results suggest that Slr1738
(PerR) and the Zur orthologue Sll1937 might operate in
Synechocystis
as
common elements of a regulatory network controlling the stress (likely
oxidative stress) generated by Cd. In addition, the control of
ziaA
by SmtB
proteins (
Fig. 4.6
) highlights a potential functional interaction between Fur
and SmtB regulators.
3.1.5.2. Role of FurA in the modulation of nitrogen metabolism
The iron pool in nitrogen-fixing cyanobacteria must fulfil the nitrogenase
complex requirements. This metalloenzyme contains three different types
of Fe-S clusters (
Burgess & Lowe, 1996
), including the iron-molybdenum
cofactor (FeMo-co) at the active site, which contains seven atoms of iron.
As expected, nitrogen fixation in
Anabaena
decreases under low iron condi-
tions (
Sandmann, Peleato, Fillat, Lázaro, & Gómez-Moreno, 1990
). At the
molecular level, transcription of the
nifHDK
operon, encoding nitrogenase,
