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(
Hernández, López-Gomollón et al., 2006
) and its expression is slightly
induced under oxidative stress (
López-Gomollón et al., 2009
), FurA could
also act as an oxidative stress-responsive regulator, similar to other members
of the Fur family like PerR of gram-positive bacteria (
Herbig & Helmann,
2001
;
Ricci, Janulczyk, & Bjorck, 2002
).
Overall, the variety of FurA-regulated genes described so far (
Table 4.4
),
including siderophore outer membrane transporters, bacterial actins, photo-
system II reaction centre proteins, CO
2
concentrating mechanism proteins
and peroxiredoxins, provides evidence that FurA functions as a global tran-
scriptional regulator in
Anabaena
sp., supporting its role in major cyanobac-
terial processes.
3.1.4. Genetic regulation of cyanobacterial Fur proteins
Most Fur proteins studied to date show moderate autoregulation. FurA from
Anabaena
binds to its own promoter with an estimated Kd of 0.49 ± 1 nM,
the presence of Mn
2+
and a reducing environment being the optimal con-
ditions for in vitro FurA-P
furA
interaction (
Hernández, López-Gomollón
et al., 2006
). In vivo assays aimed at understanding the relationship between
the regulation and the functions of this master protein unveil a rather com-
plex model. A slight increase in the expression of FurA has been detected
under iron limitation (
Hernández et al., 2002
). Since iron deprivation leads
to oxidative stress, the increase in FurA expression could be explained as a
response to the rise in the level of ROS detected in the cell under iron defi-
ciency (
Latifi, Jeanjean, Lemeille, Havaux, & Zhang, 2005
). This hypothesis
is consistent with the observation that oxidants trigger
furA
transcription
(
López-Gomollón et al., 2009
). This increase in FurA expression might be
used by the cyanobacteria to downregulate iron uptake in order to arrest catal-
ysis of the Fenton reaction. Alternatively, a nonregulatory role directly involv-
ing FurA in ROS quenching has been proposed. Work intended to establish a
plausible mechanism of FurA acting as a redox protein based on its two CXXC
redox motifs is underway (Botello-Morte et al., our unpublished results).
Northern blot analysis of
furA
under nitrogen stepdown in
Anabaena
PCC 7120 and the
ntcA
deletion mutant evidenced that the nitrogen
status modulates FurA expression and that NtcA is involved in this pro-
cess (
López-Gomollón, Hernández, Wolk et al., 2007
). In order to know
whether the increase in
furA
transcription was a general response in the
cyanobacterial filament, constructs of the promoters from each
fur
para-
logue leading the expression of GFP were used to identify a strong induc-
tion of
furA
in heterocysts.
