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also appears to be a fairly global transcriptional regulator important for cel-
lular function. However, the interpretation whether these are genes targeted
by the apo-FurA, whether they are targets of FurB, which is downregulated
in the according strain, or whether the observations correspond to pleio-
tropic effects caused by a possible iron limitation as a consequence of the
overexpression of an iron-binding protein remains largely unknown. For
the promoter regions of
isiA
,
schT
,
mreBCD
,
thiC
and
ccmM
, a direct inter-
action of FurA with the promoter region was documented, while for
sodA
,
such interaction could not be documented. Thus, it will be of importance
to explore the transcription level, in general, under different conditions to
fully uncover the functionality of FurA.
4.1.2. FurB and FurC
Initially, it was speculated that FurB might act as iron-response regulator
(Irr)-like protein as FurB is more active in the absence of iron and shows a
putative haeme regulatory motif near its C-terminal sequence (CPVHNN;
Nicolaisen & Schleiff, 2010
). However, recent work proposed two at least
additional functions. It was demonstrated that furB and furC, in contrast to
furA, are induced by oxidative stress enforced by H
2
O
2
treatment (
López-
Gomollón, Sevilla, Bes, Peleato, & Fillat, 2009
). In addition, FurB expression
in
E. coli
results in a reduced sensitivity to oxidative stress and in vitro DNA-
protection studies revealed a protective function of FurB (
López-Gomollón
et al., 2009
). On the basis of this observation, it was proposed that FurB acts
as a DPS protein (
López-Gomollón et al., 2009
). In parallel, FurB, but not
FurA or FurC, was found to specifically bind to promoter regions of genes
in a cluster encoding several metalloproteins, and this interaction was found
to be zinc dependant (
Napolitano et al., 2012
). By
in vitro
binding studies, a
DNA element defined by TGATAATXATTATCA was discovered (
Napol-
itano et al., 2012
). At least 33 putative FurB-binding sites were identified in
the genome of
Anabaena
sp. PCC 7120. The subsequent analysis revealed a
strong regulation of at least seven different operons, namely alr1197-alr1199,
all1751-all1750 and all4729-all4721 coding for metalloproteins, as well
as alr3242-alr3243, alr4028-alr4031, all0833-all8032 and all0830, which
code for an ABC transporter, a periplasmic zinc-binding protein (all0833;
ZnuA) and a TonB-dependent outer membrane transporter (alr3242, HutA;
alr4028-alr4029; BtuB;
Mirus et al., 2009
). Thus, FurB might have overlap-
ping functions in sensing oxidation, iron and zinc. Alternatively, it might be
that FurB is indeed a Zur as recently suggested (
Napolitano et al., 2012
).
The interaction of FurB with the promoter region of
furB
was inhibited
