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Grossman, & Ogawa, 2001
). However, these results do not exclude the pos-
sible involvement of these as well as other putative transporters in iron
uptake under different growth conditions.
2.4. Siderophore Transport across the Periplasm and the
Plasma Membrane
Energy for the active transport of the ferric iron-siderophore complex is
supplied by a 'Ton' system in the periplasmic membrane. It consists of the
energy transducing unit, TonB, anchored to the periplasmic membrane
(Wang & Newton, 1971) and two stabilizing units: the integral membrane
proteins, ExbB and ExbD, also anchored to the periplasmic membrane. The
interaction between TBDT and the Ton system is accomplished by TonB.
The TBDT contains a TonB-box positioned in front of its N-terminal plug
domain, which is recognized by TonB (
Fig. 3.5
). Siderophore transport is, at
first, blocked by the plug domain that prevents passage of the siderophore
through the TBDT channel. The interaction between TonB and the sidero-
phore-loaded TBDT results in conformational changes in the plug domain,
which induces the transfer of siderophores through the pore of the TBDT.
TonB functions as an energy transducer by coupling the proton-motive
force of the cytoplasmic membrane to the outer membrane transporter (e.g.
Faraldo-Gómez & Sansom, 2003
).
Knowledge about the Exb/TonB system and particularly about its
function in siderophore uptake in cyanobacteria is rather sparse. Four
putative
tonB
genes and three
exbB/D
gene cluster were identified in
the genome of
Anabaena
sp. PCC 7120 (
Stevanovic et al., 2012
). Thus,
the complexity of these systems is lower than that found for TBDTs
(
Stevanovic et al., 2012
). Expression profiling and mutant analysis in
Anabaena
sp. PCC 7120 revealed a possible role of
tonB3
and the
exbB3/
D3
gene cluster in siderophore-dependant iron uptake (Fig.
3
.
4
B). These
genes were upregulated under iron-limiting conditions and the mutants
displayed an enhanced expression of genes coding for siderophore syn-
thesis and secretion. Additionally, the transcript levels of the tonB3 and
exbB3/D3
genes are higher in mutants with reduced siderophore cycling
capacity (
Stevanovic et al., 2012
).
The
exbB2
gene, which is not found in the genomic cluster together
with a gene coding for ExbD2, is only expressed under nitrogen starvation,
while
exbB1/exbD1
expression seems to sense the ratio between iron and
other metals. Among the other three tonB genes the transcription of one is
strongly enhanced when iron is present at semi-toxic concentrations, while
