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for the other two a relation to copper was described as well. However,
these TonB proteins do not contain the histidine in the first transmembrane
domain which is thought to be essential for the function of the TonB in
E. coli
(
Larsen et al., 2007
). Thus, their relation to iron transport remains to
be established.
In general, transport of iron-loaded siderophores across the periplasm
and the inner membrane is mediated by a three-component system. It
consists of a periplasmic binding protein (e.g. FhuD/FutA/FecA), a mem-
brane-embedded permease (e.g. FhuB/FutB/FecCD) and an ATP-binding
protein (e.g. FhuC/FutC/FecB). The FhuD/B/C-system, responsible for
the transport of ferrichrome, is well characterized in
E. coli
(
Krewulak &
Vogel, 2011
).
In
Anabaena
sp. PCC 7120, five gene clusters with similarities to the
Fut-, Fec- and Fhu-system are annotated. Out of these, only one cluster
seems to be related to periplasmic iron-siderophore uptake, namely the
fhu-cluster (Fig.
3
.
4
B). The genes present in the cluster are differentially
regulated. It was reported that the expression of the
fhu
genes is influ-
enced by ExbB3/D3 and TonB3. This notion is based, for example, on
the observation that the
fhu
genes were not expressed in the
exbB3
- and
tonB3
-deficient mutants. Additionally, transcript levels were reduced in
mutants of siderophore secretion and uptake and the
fhuB
mutant showed
an iron starvation phenotype.
While for the two putative fec-gene clusters, no correlation between
expression and presence of iron could be observed, their expression were
found to differ under deviant copper (high copper for gene cluster 1 and
low copper for gene cluster 2) and nitrogen-limiting conditions. Out of
the fut-gene cluster, only
futB
showed altered expression under elevated
copper concentrations (
Stevanovic et al., 2012
). The correlation of the
expression profiles for the different components is shown in
Fig. 3.6
.
Remarkably, besides genes expressed under iron-limiting conditions (
Fig.
3.6
, yellow frame), we found at least five other clusters. The
futB
,
tonB1
and
exbB3
are also induced under high copper conditions (blue frame),
while
fecC1
,
tonB4
, and
exbB1
are induced under high iron and cop-
per (green frame). Strikingly, the transport components
fecC1
,
fecD2
and
fecD3
are induced under high iron and low copper concentrations (grey
frame), and
fecD1
and
exbB2
under limiting nitrogen (black frame). This
type of regulation appears to be unique and at this stage, it is unknown
which function this regulation has.
