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(
Keyer & Imlay, 1996
). Later on, binding of other metals, such as Mn, was
shown to modulate the function of these proteins (
Hantke, 1987
;
Lee &
Helmann, 2007
). Furthermore, a range of physiological conditions such as
oxidative (
Nunoshiba et al., 1999
) and acidic stress (
Hall & Foster, 1996
) was
shown to affect Fur regulation.
The molecular mechanism by which iron mediates Fur interaction with
DNA can be concluded from structural analysis of the large family of metal-
dependant transcription regulators. It was concluded that a protein dimer is
required to recognize a palindromic DNA sequence in an iron-dependant
manner (
Bes, Hernandez, Peleato, & Fillat, 2001
). The structural support
for this idea was provided first by crystallization of Fur from
P. aeruginosa
(
Pohl et al., 2003
). The surface of the protein is highly acidic, especially in
the cavity formed at the interface of the two proteins (
Sheikh & Taylor,
2009
). These data, together with biochemical analysis, led to the proposal
that this groove is the DNA-binding site (
Ahmad, Brandsdal, Michaud-
Soret, & Willassen, 2009
). By structural modelling (
Ahmad et al., 2009
) and
crystallographic analysis of the apo and holo form of the iron-dependant
transcription factor PerR from
Bacillus subtilis
(
Jacquamet et al., 2009
), it
was proposed that the regulatory mode for iron sensing is the reorganiza-
tion of the dimeric structure (
Fig. 3.8
). In the absence of the metal, the
structure becomes relaxed, which in turn reduces the binding activity of the
transcriptional regulator. A recent computation study of Fur proteins from
Synechocystis
sp. PCC 6803 suggested that higher multimeric forms of these
regulators should also be taken into account (
Garcin et al., 2012
).
Fe(II)-loaded Fur dimer recognizes a specific DNA sequence anno-
tated as 'Fur box' (e.g.
Hantke, 2001
). The Fur box is a three forward-
forward-reverse tandem hexamer with a recognition unit NAT(A/T)AT
(
Escolar, Perez-Martin, & de Lorenzo, 1998
) and can be found in non-
transcribed or (and) transcribed regions of genes. In the iron-loaded form,
Fur adopts the V-shaped structure, binds to the Fur box leading to a block
RNA polymerase binding (
Faraldo-Gómez & Sansom, 2003
). Thus, activa-
tion of iron starvation-dependant expression is induced by dissociation of
Fur enforcing binding of the RNA polymerase to the according gene. In
turn, the Fur-dependant activation of gene expression under normal iron
levels is, in most cases, known so far indirectly manifested by repression of
the sRNA RyhB, a Fur-repressed repressor (
Massé, Salvail, Desnoyers, &
Arguin, 2007
;
Mellin, Goswami, Grogan,Tjaden, & Genco, 2007
). However, in
some cases, only the iron-free Fur-apo form can bind to promoter regions,
which represses gene expression under low iron concentrations as shown
