Biology Reference
In-Depth Information
B (DFB), but were inhibited by the application of excess chelator. This may
suggest that organic iron is acquired in these
Synechococcus
sp. by means of
an extracellular reductive step, in which the ferric DFB is reduced, Fe(II)
released and Fe′ is transported by the cell (
Lis & Shaked, 2009
). This pos-
sibility is consistent with the Fe(II)s model developed for eukaryotic phy-
toplankton, which predicts that inhibition of Fe uptake by excess ligand
results from competition between the cell and ligand for free Fe(II) formed
at the cell surface (
Shaked et al., 2005
).
Recently, a reductive iron uptake strategy was demonstrated for
Synecho-
cystis
PCC 6803 (
Kranzler et al., 2011
). As previously mentioned, its genome
contains no known siderophore biosynthesis genes (
Ehrenreich et al., 2005
)
although Fe(II) and Fe(III) transporters in the inner membrane were identi-
fied: FutABC and FeoB were identified by
Katoh, Hagino, Grossman et al.
(2001)
as Fe(III) and Fe(II) transporters, respectively. Recently,
Jiang, Lou,
Du, Price, and Qiu (2012)
identified a CDF-type transporter which may
be involved in Fe(III) transport. In order to probe the existence of a reduc-
tive uptake strategy, short-term Fe transport assays were conducted in the
presence of the membrane impermeable, Fe(II)-specific ligand, ferrozine
(Fig.
3
.
1
B). When both Fe′ and ferrisiderophore complexes were applied
as substrates, FZ inhibited iron uptake suggesting that free Fe(II) is formed
before transport (
Kranzler et al., 2011
). Fe′ transport rates revealed a high-
affinity uptake system under Fe-limited conditions with an environmentally
relevant Km in the sub-nanomolar range (
Kranzler et al., 2011
).
In Gram-negative prokaryotes, iron reduction may occur outside of the
cell, on the surface of the outer membrane or in the periplasmic space.
Iron reduction in the bulk medium cannot contribute significantly towards
uptake, given the short residence time of Fe(II) in an oxic environment.
Therefore, biologically relevant iron reduction must take place either on
the surface of the outer membrane or in the periplasm. While Fe transport
processes through the outer membrane can be energetically coupled (
Braun
& Endriss, 2007
;
Mirus et al., 2009
), there is no evidence for the presence
of redox processes in this membrane. Redox reactions do take place in the
periplasm of
Synechocystis
PCC 6803, as evidenced by the ability to form
and break disulfide bonds for example (
Pils & Schmetterer, 2001
;
Rukhman
et al., 2005
;
Singh et al., 2008
). However, the Fe(II) trapping agent used
to assay for reduction (FZ) does not cross the plasma membrane (
Garg,
Rose, Godrant, & Waite, 2007
;
Kustka, Shaked, Milligan, King, & Morel,
2005
;
Shaked et al., 2005
). Its ability to cross the outer membrane of a
cyanobacterial cell is unknown. Fe(II)-FZ
3
complexes are not available for
