Biology Reference
In-Depth Information
A mutant lacking the whole gene cluster showed a growth defect under
iron-limiting conditions and produced only a low level of siderophores sug-
gesting an importance of this cluster for siderophore production. However,
the viability of the mutant as well as the remaining secretion of siderophores
point to the presence of additional cluster(s) involved in siderophore syn-
thesis in this organism (
Jeanjean et al., 2008
). Whether the enzymes encoded
by all2658-all2635 are responsible for the synthesis of several or just one
siderophore could not be determined. The cluster was upregulated under
iron starvation, oxidative stress and copper limitation (
Jeanjean et al., 2008
).
The genes of a second cluster described in
Anabaena
sp. PCC 7120 show
similarities with the rhb genes coding for the NIS pathway of rhizobactin
(
Nicolaisen et al., 2008
). As rhizobactin is a citrate-based siderophore, a pos-
sible role of the encoded enzymes in the synthesis of schizokinen is suggested.
As mentioned previously, genes encoding for the NIS pathway are not wide-
spread among cyanobacteria. Of the analysed bacteria, only
Synechoccocu
s sp.
PCC 7002 and
A. variabilis
contain putative genes encoding for this biosynthe-
sis pathway. Both organisms are known to produce hydroxamate-type sidero-
phores (
Hopkinson & Morel, 2009
;
Ito & Butler, 2005
;
Trick & Kerry, 1992
).
The biosynthesis pathway of the more complex siderophore anachelin
is not solved yet although it could be synthesized synthetically (
Gademann
& Bethuel, 2004b
). Anachelin is considered as a natural hybrid contain-
ing fragments of different biosynthetic origin, a salicylate unit, a mixed
polyketide/amino acid fragment, a tripeptide part and an unusual alkaloid
fragment. The salicylate could emanate out of a PKS-dependant pathway
(
Gademann & Portmann, 2008
).
2.3. Siderophore Export and Uptake Cycles
Siderophore secretion by cyanobacteria is an intriguing process. The only
cyanobacterial siderophore secretion pathway studied in detail, so far, is in
Anabaena
sp. PCC 7120. Export through the inner membrane is suggested to
occur via transporters of the resistance, nodulation and cell division (RND),
the major facilitator (MFS) or the ATP-binding cassette (ABC) superfamilies.
Transport through the outer membrane is mediated by a TolC-type pro-
tein (
Bleuel et al., 2005
;
Miethke & Marahiel, 2007
). In
Anabaena
sp. PCC
7120, the MFS-type plasma membrane protein of SchE was shown to be
involved in the export of schizokinen. A deletion mutant of
schE
showed a
drastic reduction in secretion (
Nicolaisen et al., 2010
). The TolC-like protein
of
Anabaena
sp. PCC 7120, HgdD, seems to be involved in secretion of sid-
erophores through the outer membrane (Fig.
3
.
4
A;
Nicolaisen et al., 2010
).
