Biology Reference
In-Depth Information
outer membrane proteins bind to typical siderophores. Two are homologues
of the vitamin B12 transporter BtuB characterized in
E. coli
, another two
are homologues of the haeme transporter HutA characterized in
Vibrio chol-
era
(
Mirus et al., 2009
). Haeme and Vitamin B12 belong to the group of
porphyrins that are able to chelate iron or cobalt, respectively. Interestingly,
one of the BtuB and one of the HutA-type transporters are under the con-
trol of FurB (
Napolitano et al., 2012
), which might hint at a different metal
specificity.
To date, only one cyanobacterial TonB-dependant receptor is known
to transport a particular siderophore. This schizokinen transporter, SchT, is
encoded by
alr0379
in
Anabaena
sp. PCC 7120 (
Nicolaisen et al., 2008
). It
is homologous to the hydroxyl-carboxylate siderophore transporter IutA,
characterized as the ferric aerobactin transporter in
E. coli
(
Mirus et al.,
2009
). While the mutation in
schT
drastically reduces the uptake of schizo-
kinen (
Nicolaisen et al., 2008
), two additional transporters of the IutA type
are encoded in the genome (alr2209 and alr2581;
Mirus et al., 2009
). The
function of these two genes has to be explored.
It can be proposed that several of the identified FhuA,ViuA, HutA and
BtuB-type TBDTs of
Anabaena
sp. PCC 7120 are involved in transport
of siderophores as this cyanobacterium contains multiple gene clusters
encoding for proteins involved in siderophore synthesis (
Jeanjean et al.,
2008
;
Nicolaisen et al., 2008
) and a second siderophore of a yet unknown
nature (
Nicolaisen et al., 2010
). Furthermore, it has been shown that cya-
nobacteria are capable of taking up siderophores from different bacterial
domains and even from fungi. For example,
Anabaena
sp. PCC 7120 and
Synechocystis
sp. PCC 6803 can utilize ferric aerobactin originating from
E. coli
(
Goldman et al., 1983
;
Kranzler et al., 2011
). In addition, the dif-
ferences observed in the expression pattern of the genes encoding for the
22 TBDTs present in
Anabaena
sp. PCC 7120 under iron-, copper- and
nitrogen-limiting conditions (
Mirus et al., 2009
) and the dependence of
other bacterial TBDTs on metals such as nickel and cobalt (
Schauer et al.,
2008
) allows us to hypothesize on additional metal-dependant iron-
siderophore uptake systems able to take up siderophores complexed to
metals other than iron.
In
Synechocystis
sp. PCC 6803, the four genes sll1206 (IutA-type),
sll1406, sll1409 and slr1490 (all FhuA-type), coding for putative siderophore
transporters in the outer membrane, were investigated. Mutants thereof
did not show a growth phenotype in Fe-depleted media. Furthermore, no
reduction in Fe(II) or Fe(III) uptake activity was observed (
Katoh, Hagino,
