Biology Reference
In-Depth Information
As a consequence of the function of schizokinen in copper detoxification
under high copper concentrations, an alternative transport system is required.
A system for cotransporting iron and copper would be of evolutionary advan-
tage. It would only be active when copper concentrations are high and would
ensure iron and copper uptake in situations where copper is largely stored in
siderophore complexes. Accordingly, iron uptake by
Anabaena
sp. PCC 7120
grown in the absence of iron but in the presence of copper is lower when
compared to cells grown in the absence of both metals (
Nicolaisen et al.,
2010
) which supports copper uptake under iron-limiting conditions (Fig.
3
.
4
C). IacT was proposed as the outer membrane protein involved in this
process. In line, in the
iacT
overexpression mutant, two genes (all7619 and
all7632) coding for homologous to CusB involved in copper homeostasis in
E. coli
(
Rensing & Grass, 2003
) showed reduced expression and the copper
content was increased as compared to wild type (
Nicolaisen et al., 2010
).
Such a model is consistent with the observed differential expression of
the energizing and transport system.
TonB1
and
tonB4
as well as
fecC1
and
fecD1
are expressed under high copper concentrations (
Stevanovic et al.,
2012
). Furthermore, FutA2 in
Synechocystis
sp. PCC 6803 known to bind
to Fe(III) in vitro could be linked to copper binding in vivo (
Badarau et al.,
2008
). The
futA2
mutant accumulated iron and copper in the periplasm, but
no longer in plastocyanin (the major copper protein in the cells) leading to
the suggestion that FutA2 is involved in copper homeostasis in addition to its
function of iron transport (
Waldron et al., 2007
). The relationship between
IacT and the periplasmic components is not yet established, although, addi-
tionally to this TBDT, an involvement of a periplasmic binding protein in
the uptake of both iron and copper was described (
Stevanovic et al., 2012
).
2.6. Reductive Iron Uptake
2.6.1. Non-siderophore-mediated Fe uptake
Siderophore-mediated iron uptake has been studied extensively in Gram-
negative bacteria (Braun & Killmann, 1999;
Braun & Hantke, 2011
;
Sandy &
Butler, 2009
). Similarly, most studies on iron acquisition in cyanobacteria, also
Gram-negative prokaryotes, focused on siderophore biosynthesis and uptake,
as detailed above. However, siderophore-mediated iron uptake cannot account
for iron uptake in all cyanobacteria. Bioinformatic analysis of two molecular
systems implicated in siderophore biosynthesis, NRPSs and modular PKSs,
was performed on both freshwater and marine cyanobacterial genomes
(
Ehrenreich, Waterbury, & Webb, 2005
). While both NRPS and PKS genes
were prevalent among filamentous and heterocystous strains, they were
