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amino acid monomers of the siderophore backbone. The core module of
an NRPS consists of three domains: the adenylation (A) domain for activa-
tion of the selected amino acid monomer, a peptidyl carrier domain (P) for
transferring the monomers to various catalytic sites and a condensation (C)
domain for forming peptide bonds between the monomers ( Du & Shen,
2001 ; Marahiel, Stachelhaus, & Mootz, 1997 ). In several cases, NRPSs are
accompanied by polyketide synthases (PKSs) that are known to catalyse the
condensation of carboxylate groups ( Staunton & Weissman, 2001 ). The two
main classes differ in the number of protein units; while type I PKSs are
multifunctional enzymes, type II PKSs are made of separate enzymes with
sole functions ( Shen, 2003 ).
The third pathway for siderophore synthesis is NRPS-independant
(NIS), as identified for the synthesis of aerobactin in Escherichia coli and rhi-
zobactin 1021 in Sinorhizobium meliloti ( Challis, 2005 ). Thus, while the other
two synthesis pathways are important for hydroxamate and catecholate-type
siderophores, NIS has only been demonstrated for hydroxamate-type sid-
erophores. For aerobactin, the two synthetases IucD and IucB are known to
catalyse the synthesis of the terminal hydroxamate group important for iron
coordination. The enzymes IucA and IucC are responsible for amid bond
formation that links the carboxylate and diamine units together. The four
synthetases RhbC, RhbD, RhbE and RhbF, participating in the rhizobac-
tin synthesis, show similarities with the IucA/B/C/D enzymes. Addition-
ally, the two PLP-dependant-like enzymes RhbA and RhbB are supposed
to catalyse 1,3-diaminopropane before the hydroxamate group is formed
( Challis, 2005 ). Several genes coding for NRPSs and PKSs are annotated
in the genomes of cyanobacteria ( Silva-Stenico et al., 2011 ), however, the
siderophores produced by these genes could not be identified.
Components of the NIS pathway were identified by a bioinformatic
survey only in the freshwater cyanobacteria Anabaena sp. PCC 7120,
Anabaena variabilis ATC 29413, in the marine cyanobacteria Synechococcus
sp. PCC 7002, Synechococcus sp. and Prochlorococcus sp. while in the same
study, NRPS components were identified in more than 50% of the anal-
ysed species ( Hopkinson & Morel, 2009 ). Whether the absence of NIS or
NRPS components in the genome of the other species indeed indicate
absence of siderophore synthesis machineries or simply a false discovery
rate of the survey remains to be analysed.
The only gene cluster which was further investigated, consists of the
genes spanning all2658 to all2635 (in Anabaena sp. PCC 7120; Jeanjean
et al., 2008 ) including seven NRPSs and two PKSs ( Kaneko et al., 2001 ).
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