Erythrochelin-a hydroxamate-type siderophore predicted from the genome of Saccharopolyspora erythraea

Article abstract of DOI:10.1111/j.1742-4658.2009.07512.x

The class of nonribosomally assembled siderophores encompasses a multitude of structurally diverse natural products. The genome of the erythromycin- producing strain Saccharopolyspora erythraea contains 25 secondary metabolite gene clusters that are mostly considered to be orphan, including two that are responsible for siderophore assembly. In the present study, we report the isolation and structural elucidation of the hydroxamate-type tetrapeptide siderophore erythrochelin, the first nonribosomal peptide synthetase-derived natural product of S. erythraea. In an attempt to substitute the traditional activity assay-guided isolation of novel secondary metabolites, we have employed a dedicated radio-LC-MS methodology to identify nonribosomal peptides of cryptic gene clusters in the industrially relevant strain. This methodology was based on transcriptome data and adenylation domain specificity prediction and resulted in the detection of a radiolabeled ornithine-inheriting hydroxamate-type siderophore. The improvement of siderophore production enabled the elucidation of the overall structure via NMR and MSⁿ analysis and hydrolysate-derivatization for the determination of the amino acid configuration. The sequence of the tetrapeptide siderophore erythrochelin was determined to be d-α-N-acetyl-δ-N-acetyl-δ-N-hydroxyornithine- d-serine-cyclo(l-δ-N-hydroxyornithine-l-δ-N-acetyl-δ-N- hydroxyornithine). The results derived from the structural and functional characterization of erythrochelin enabled the proposal of a biosynthetic pathway. In this model, the tetrapeptide is assembled by the nonribosomal peptide synthetase EtcD, involving unusual initiation-and cyclorelease- mechanisms.

Full text of DOI:10.1111/j.1742-4658.2009.07512.x

Erythrochelin – a hydroxamate-type siderophore predicted
from the genome of Saccharopolyspora erythraea
Lars Robbel, Thomas A. Knappe, Uwe Linne, Xiulan Xie and Mohamed A. Marahiel
Department of Chemistry, Philipps-University Marburg, Germany
Keywords
The class of nonribosomally assembled siderophores encompasses a multi-
tude of structurally diverse natural products. The genome of the erythro-
genome mining; nonribosomal peptide
synthetase; radiolabeling; secondary
mycin-producing strain Saccharopolyspora erythraea contains 25 secondary
metabolite gene clusters that are mostly considered to be orphan, including
two that are responsible for siderophore assembly. In the present study, we
metabolites; siderophore
Correspondence
report the isolation and structural elucidation of the hydroxamate-type
tetrapeptide siderophore erythrochelin, the first nonribosomal peptide syn-
thetase-derived natural product of S. erythraea. In an attempt to substitute
the traditional activity assay-guided isolation of novel secondary metabo-
lites, we have employed a dedicated radio-LC-MS methodology to identify
nonribosomal peptides of cryptic gene clusters in the industrially relevant
strain. This methodology was based on transcriptome data and adenylation
domain specificity prediction and resulted in the detection of a radiolabeled
ornithine-inheriting hydroxamate-type siderophore. The improvement of
siderophore production enabled the elucidation of the overall structure via
NMR and MSn analysis and hydrolysate-derivatization for the determina-
tion of the amino acid configuration. The sequence of the tetrapeptide
siderophore erythrochelin was determined to be d-a-N-acetyl-d-N-acetyl-d-
N-hydroxyornithine-d-serine-cyclo(l-d-N-hydroxyornithine-l-d-N-acetyl-d-
N-hydroxyornithine). The results derived from the structural and functional
characterization of erythrochelin enabled the proposal of a biosynthetic
pathway. In this model, the tetrapeptide is assembled by the nonribosomal
peptide synthetase EtcD, involving unusual initiation- and cyclorelease-
mechanisms.
M. A. Marahiel, Department of Chemistry,
Philipps-University Marburg, D-35043
Marburg, Germany
Fax: +49 (0) 6421 282 2191
Tel: +49 (0) 6421 282 5722
E-mail: marahiel@staff.uni-marburg.de
(Received 4 October 2009, revised 10
November 2009, accepted 23 November
2009)
doi:10.1111/j.1742-4658.2009.07512.x
Introduction
Bacterial growth is strongly influenced by the availabil-
ity of iron as an essential trace element employed as a
cofactor [1]. The fact that the bioavailability of iron is
challenging for most microorganisms because it is
mostly found in the Fe(III) (ferric iron) redox state,
forming insoluble Fe(OH)3 complexes, has led to the
evolutionary development of highly efficient iron
uptake systems. In response to iron starvation, many
microorganisms produce and secrete iron-scavenging
compounds (generally < 1 kDa) termed siderophores,
with a high affinity for ferric iron (Kf = 1022 to
1049
m
)1) [2]. After the extracellular binding of iron,
the siderophores are reimported into the cell after rec-
ognition by specific receptors and iron is released from
the chelator complex and subsequently channelled to
the intracellular targets [3–5]. Siderophores in general
Abbreviations
A, adenylation domain; ac-haOrn, a-N-acetly-d-N-acetyl-d-N-hydroxyornithine; C, condensation domain; CAS, chromazurol S;
DKP, diketopiperazine; E, epimerization domain; FDAA, N-a-(2,4-dinitro-5-fluorophenyl)-L-alaninamide; haOrn, d-N-acetyl-d-N-hydroxyornithine;
HMBC, heteronuclear multiple bond correlation; HSQC, heteronuclear single-quantum correlation; hOrn, d-N-hydroxyornithine;
NRP, nonribosomal peptide; NRPS, nonribosomal peptide synthetase; PCP, peptidyl carrier protein.
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L. Robbel et al.
constitute a class of structurally diverse natural prod-
ucts that are classified into two main groups based on
the mechanism of biosynthesis. Common structural
features of siderophores are catecholate, hydroxamate
or carboxylate functionalities conferring chelating
properties for the octahedral coordination of ferric
iron. Some siderophores are assembled via a template-
directed manner by multimodular nonribosomal pep-
tide synthetases (NRPSs). The class of nonribosomally
assembled siderophores can be exemplified by enterob-
actin 1 (Escherichia coli), coelichelin 2 (Streptomy-
ces coelicolor) and fuscachelin A 3 (Thermobifida fusca
YX) (Fig. 1) [6–8]. The second class is known as
NRPS-independent siderophores and involves a novel
family of synthetases, represented by IucA and IucC,
which are responsible for aerobactin (E. coli K-12) bio-
synthesis [9,10]. Siderophores of NRPS-independent
growing and may result in the synthesis of specific
inhibitors based on the structure of the pathogen
siderophore [16].
A promising approach for the isolation of secondary
metabolites, predicted from genome analysis, results
from feeding experiments of a predicted precursor mole-
cule in an isotopically labeled form to cultures of the tar-
get strains. Direct identification of the incorporated
label either by NMR, if using 15N-enriched precursors,
or by radio-LC-MS, if employing 14C-labeled building
blocks, facilitates the identification of new natural prod-
ucts of the orphan pathway and has successfully been
applied in the discovery of orfamide A [17]. The accu-
rate prediction of adenylation domain specificity was
found to be crucial for successful mining and structural
prediction and is the basis of the methodology applied
in the present study [7,8]. This approach was applied for
the aerobic mesophilic Gram-positive filamentous acti-
nomycete Saccharopolyspora erythraea NRRL 23338,
the producer strain of the macrolide polyketide erythro-
mycin. The recently sequenced and annotated genome
comprises 8.2 mb and contains at least 25 biosynthetic
operons for the production of known or predicted sec-
ondary metabolites, including two gene clusters for the
biosynthesis of siderophores [18,19]. Transcriptome data
for S. erythraea using GeneChip DNA microarrays, col-
lected by Peano et al. [20], indicate an up-regulation of
gene expression associated with siderophore assembly
under specific conditions.
origin encompass desferrioxamine
E (Streptomyces
coelicolor M145), putrebactin (Shewanella putrefaciens)
and further compounds [11,12]. The biosynthetic genes
of these secondary metabolites are usually clustered
within one operon, showing coordinated transcrip-
tional regulation [13].
Extensive bioinformatic analysis of these biosynthet-
ic clusters allowed the prediction of the incorporated
building blocks and the mechanism of iron coordina-
tion [14,15]. This genomics-based characterization of
natural products has been successfully applied in the
discovery of the siderophores coelichelin and fuscach-
elin A. Because siderophores often function as viru-
lence factors in pathogens, the interest in the structural
and functional characterization of these compounds is
In the present study, we report the identification and
isolation of erythrochelin, a hydroxamate-type sidero-
phore produced by the industrially relevant strain
S. erythraea, utilizing a novel radio-LC-MS-guided
genome mining methodology. Structural and func-
tional characterization was carried out relying on
NMR and MSn analysis and derivatization-based
elucidation of the overall stereochemistry. Further-
more, the functional properties of erythrochelin acting
as an iron-chelating compound were investigated. On
the basis of the analysis of the S. erythraea genome,
transcriptome and the structural characterization, an
NRPS-dependent assembly of erythrochelin mediated
by a tetramodular NRPS is proposed.
OH
O
OH
NH
OH
OH
HN
O
O
O
H2N
N
NH2
N
OH O
H
H
O
O
O
O
OH
O
HO
O
OH
O
HO
N
H
NH
N
N
O
O
H
O
H
1
2
HO
OH
O
Enterobactin
Coelichelin
OH
OH
N
OH
OH
HO
O
O
O
O
N
H
H
H
N
H
N
N
O
N
N
N
H
N
H
H
H
O
O
O
O
Results
O
3
HN
HN
NH
Fuscachelin A
The etc gene cluster in S. erythraea
NH2
H2N
NH
Analysis of the sequenced and annotated genome of
S. erythraea led to the discovery of two NRPS-gene
clusters linked to siderophore biosynthesis and trans-
port [18]. One of the two was predicted to encode for
Fig. 1. Representatives of nonribosomally assembled oligopeptide
siderophores: the catecholate siderophore enterobactin 1, the
hydroxamate siderophore coelichelin 2 and the decapeptide fus-
cachelin A 3. The latter two siderophores were discovered via gen-
ome mining methodology.
a
mixed hydroxamate ⁄ catecholate-type siderophore
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L. Robbel et al.
Erythrochelin siderophore characterization
Table 1. Comparison of active-site residues determining the adeny-
lation domain specificity of EtcD with known adenylation domains.
Variations in the residue pattern are highlighted in bold. EntF, ente-
robactin synthetase; CchH, coelichelin synthetase.
(Nrps3), whereas the second operon was envisaged to
encode a tetramodular NRPS putatively capable of
assembling a hydroxamate-type siderophore (Fig. 2).
In this operon, 11 coding sequences are clustered in a
region covering 28.8 kb, with an average GC content
of 71.2%.
A-domain
Active site residues
Substrate
Product
A1
D V W A L G A V N K
D V W T I G A V D K
D V W H F S L V D K
D V W H F S L V D K
D M E N L G L I N K
D M E N L G L I N K
D V F A L G A V N K
D V W T I G A V D K
The NRP synthetase encoded by etcD (sace_3035 ⁄
nrps5) comprises four modules, each containing the
essential condensation (C), adenylation (A) and pept-
idyl carrier protein (PCP) domains. In addition, mod-
ules 1 and 2 contain an epimerization (E) domain
each, which is responsible for stereoconversion of the
accepted l-amino acids to d-isomers, indicating the
presence of two d-configured residues in the assembled
product. The N-terminal region of module 1 shares a
high degree of homology to condensation domains,
suggesting the function of an initiation module mediat-
ing the condensation of an external building block
with the PCP-tethered substrate. Module 4 contains a
C-terminal C-domain instead of a thioesterase domain
commonly responsible for product release through
hydrolytic cleavage or macrocyclization [21]. Upstream
of etcD, a gene with high sequence homology to char-
acterized l-ornithine hydroxylases (etcB) is located. On
the basis of the proposed function of EtcB, the incor-
poration of d-N-hydroxyornithine residues into the
readily assembled oligopeptide was predicted [22]. Fur-
thermore, genes present in the cluster encode for pro-
teins traditionally associated with secondary metabolite
biosynthesis and siderophore transport: a transcrip-
tional regulator (etcA), MbtH-like protein (etcE) and
proteins for siderophore export and uptake (etcCFGK).
A bioinformatic overview of the encoded proteins and
the corresponding functions is provided in Table S1.
The amino acid specificity of the synthetase was pre-
dicted by using a methodology comparing active-site
MycC
A2
L-Arg
L-Ser
L-hOrn
L-Arg
Microcystin
Enterobactin
Coelichelin
Microcystin
EntF
A3
CchH-A3
A4
MycC
residues of known NRPS adenylation domains with
the adenylation domains found in EtcD (Table 1) [23–
25]. The first adenylation domain (A1) is predicted to
activate l-arginine but reveals only 70% identity of the
residues determining the specificity to MycC, suggest-
ing the activation of a structurally analogous building
block. MycC itself represents a NRPS-termination
module involved in the assembly of microcystin by
Microcystis aeruginosa PCC7806, predicted to activate
l-arginine [26]. A2 and A3 are predicted to activate
l-serine and l-d-N-hydroxyornithine (l-hOrn), respec-
tively, as found in the assembly of enterobactin and
coelichelin [6,7]. The C-terminal adenylation domain
A4 again is predicted to activate l-arginine, displaying
60% identity to the characterized A-domain of MycC.
Interestingly, A1 and A4 inherit a highly identical
(90%) specificity-determining residue pattern, leading
to the assumption that both activate the same sub-
strate (Table S2A). On the basis of the bioinformatic
analysis of the etc gene cluster, it was predicted that
the assembled tetrapeptide consists of l-hOrn, l-Ser
and two building blocks analogous to l-Arg.
1 kb
Transporter
C
A1
T
E
C
A2
T
E
C
A3
T
NRPS
Monooxygenase
Regulatory proteins
Fig. 2. Schematic overview of the etc gene
cluster. Putative functions of the proteins
encoded within the operon are shown
based on BLAST analysis. Apart from the core
components for siderophore biosynthesis,
genes encoding for exporters and importers
of the siderophore, as well as typical
transcriptional regulators for secondary
metabolism, are found, determining the
boundaries of the cluster.
C
A4
T
C
etcA LysR family transcriptional regulator
etcB Putative peptide monooxygenase
etcC Iron ABC transporter periplasmic-binding protein
etcD Putative non-ribosomal peptide synthetase
etcE MbtH protein
etcF Putative ABC transporter transmembrane component
etcG ABC transporter protein, ATP-binding component
etcH IclR-type transcriptional regulator
etcI CoA-transferase
etcJ Hydroxymethylglutaryl-CoA lyase
etcK Dicarboxylate carrier protein
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L. Robbel et al.
incorporated into the tetrapeptide siderophore, to cul-
tures of S. erythraea. These experiments were carried
out in rich SCM medium, as previously employed in
transcriptome analysis [20]. Extraction of the superna-
tant followed by radio-LC-MS analysis revealed the
radiolabeling of a compound with a measured m ⁄ z of
604.27 [M+H+] (Fig. 3A). The incorporation of
radiolabeled l-Orn was determined to be 2% of the
total amount of radioactivity fed to the cultures
employing the rich SCM medium. In addition, an
extraction of the SCM medium supernatant after
4 days of growth, subsequent preparative HPLC frac-
tionation and chromazurol S (CAS: an indicator of
iron scavenging properties) liquid assay analysis of the
fractions revealed a CAS-reactive compound (Fig. S1)
Identification and isolation of a hydroxamate-type
siderophore via radio-LC-MS
On the basis of the transcriptome data for S. erythraea
NRRL 23338 grown in SCM medium that clearly
show an up-regulated gene expression of the NRPS
encoding etc cluster, which is linked to siderophore
biosynthesis, siderophore production was investigated
throughout several growth phases [20]. Secondary
metabolite identification and isolation is often chal-
lenging as a result of a high medium complexity or
low amounts of the target compounds. To circumvent
these challenges, a radio-LC-MS-guided genome min-
ing approach was applied by feeding the nonproteino-
genic amino acid 14C-l-ornithine, as predicted to be
A
B
Fig. 3. (A) Radio-LC-MS profiles of radiolabeling experiments employing nonproteinogenic 14C-L-Orn. In both cases, the incorporation of the
radiolabel occurred (red trace), displaying a discrete m ⁄ z = 604.27 ([M+H+]) in the extracted ion chromatogram (EIC). (B) ESI-MS analysis of
ferri-erythrochelin; retention time = 13.2 min. Skimmer fragmentation was completely abolished when analyzing ferri-erythrochelin, which is
indicative of a structurally rigid conformation induced by iron chelation.
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L. Robbel et al.
Erythrochelin siderophore characterization
[27]. The coelution of a multitude of compounds in the
CAS assay positive fraction impeded the direct
MS-based detection and isolation of the siderophore.
To reduce media complexity and to facilitate the isola-
tion procedure, a radiolabeling experiment was carried
out in iron-deficient M9-minimal medium. The incor-
poration of the radiolabel increased from 2% to 4%
(Fig. 3B), whereas coeluting compounds were reduced,
as observed in the total ion chromatogram. To isolate
the siderophore in sufficient amounts for NMR struc-
ture elucidation, a large-scale cultivation of S. erythraea
in iron-deficient modified M9 medium was carried out,
giving rise to siderophore production of 10.2 mgÆL)1
culture (Fig. 4). The physiological function of the
siderophore for iron uptake was confirmed by compar-
ing supernatant extractions of S. erythraea cultures
grown in the absence or presence of iron. The presence
of iron in the medium completely supressed siderophore
production (Fig. S2). UV ⁄ visible spectra of ferri-sidero-
phore compared to the unloaded apo-form show the
typical absorption spectrum for hydroxamate-type
siderophores (kmax = 440 nm), furthermore confirming
the iron-chelating function of the product (Fig. S3).
Additionally, the stochiometry of the Fe(III):sidero-
phore-complex was determined to be 1 : 1 by UV ⁄ visi-
ble and MS analysis, indicating the presence of six
Fe(III)-coordinating groups (Fig. 3C).
Fig. 5. The structure of erythrochelin as determined by NMR.
NMR contacts are indicated by arrows. Blue arrows indicate intra-
residue contacts; red arrows indicate long-range inter-residue
contacts. (A) Long-range 1H–13C correlations observed in dimethyl-
sulfoxide (300 K). (B) NOE contacts observed in dimethylsulfoxide
(300 K). Sequential NOE contacts observed between hOrn3 and ha-
Orn4 confirm the presence of a DKP moiety.
(HSQC) spectrum, which verified the presence of four
amino acids in the sequence. TOCSY cross peaks con-
firmed the presence of three ornithines and one serine
in the compound. Two strong singlets at 1.84 and
1.96 p.p.m. for three and six protons, respectively,
revealed the presence of three acetyl groups, of which
two are attached to very similar amino acids in the
Structure elucidation by NMR
The amino acid sequence and the final structure of the
siderophore were determined using NMR methodology
1
(Fig. 5). The H spectrum revealed the presence of four
1
amide protons at 7.96, 7.74, 8.08 and 8.12 p.p.m.
(Fig. S4). Four cross peaks were observed in the
1H–15N heteronuclear single-quantum correlation
sequence. The observed long-range H–13C correlations
showed the two acetyl groups to be connected to
the d-amino group of two d-N-hydroxyornithines,
Erythrochelin
tR = 30.7
O
O
HN
N
(S)
(S)
NH
OH
O
O
N
OH
OH
(R)
HN
(R)
H
N
O
O
OH
O
N
Erythrochelin
10
20
30
40
50
60
Fig. 4. Preparative HPLC profile of a XAD16
resin extraction of iron-depleted M9 minimal
medium of S. erythraea cultures grown for
72 h. The absence of iron gives rise to an
increased siderophore production of up to
10.2 mgÆL)1 culture.
10
20
30
40
50
60
Retention time (min)
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L. Robbel et al.
respectively, whereas the third one is attached to the
a-amino group of one of the d-N-acetyl-d-N-hydroxy-
ornithines (haOrn) resulting in a-N-acetly-d-N-acetyl-
d-N-hydroxyornithine (ac-haOrn) (Fig. 5A). Three
sequential NOE contacts were observed, one revealing
a connection between the terminal ac-haOrn1 and the
Ser2, whereas the other two were for a sequential
connection between a d-N-hydroxyornithine and a
d-N-acetyl-d-N-hydroxyornithine and its reverse, res-
pectively. Such double sequential connections can only
be established through a diketopiperazine (DKP) unit,
which is composed of a hOrn and a haOrn moiety.
Furthermore, a long-range 1H–13C correlation was
detected between the carbonyl carbon of the serine and
the d-CH2 of the hOrn, which constitutes the DKP.
Therefore, putting all these long-range connections
together, we established a structure for the tetrapeptide
siderophore, which is designated erythrochelin (Fig. 5).
observed. This result provided strong evidence for the
presence of the DKP moiety because such fragmenta-
tion behaviour is characteristic for DKP-containing
compounds and has been detected during fragmenta-
tion of an albonoursin intermediate (Fig. S10) [28].
Determination of overall stereochemistry of eryth-
rochelin was carried out utilizing Marfey’s reagent
[29]. Prior to the N-a-(2,4-dinitro-5-fluorophenyl)-
l-alaninamide (FDAA) derivatization of the amino
acids resulting from total hydrolysis of erythrochelin,
the hydrolysate was analyzed via LC-MS to determine
hydrolysate composition, revealing solely the presence
of Ser- and hOrn-residues (Fig. S11). LC-MS analysis
of the derivatized hydrolysate compared to synthetic
standards indicated the presence of d-Ser, l-hOrn and
d-hOrn in a 1 : 2 : 1 ratio (Figs S12 and S13), as
expected from bioinformatic analysis of EtcD. To
determine the connectivity of the amino acids, as well
as their stereoconfiguration, a partial hydrolysis-deriv-
atization approach was carried out. The C-terminal
hOrn-hOrn-dipeptide was isolated, hydrolytically
cleaved and derivatized (Fig. S14). Solely the presence
of l-hOrn residues was observed, confirming the
stereochemistry to be in full agreement with the pro-
posed biosynthetic model (Fig. S15).
1
The assigned H, 13C and 15N chemical shifts are listed
in Tables S3–7. The observed NOE contacts and the
long-range 1H-13C correlations verified the structure
and are listed in listed in Tables S5 and S6. On the
basis of the results obtained by NMR, the determined
sequence for the peptide is ac-haOrn1-Ser2-cyclo
(hOrn3-haOrn4). The corresponding DQF-COSY,
1H–15N HSQC, heteronuclear multiple bond correla-
tion (HMBC) and ROESY spectra of erythrochelin are
shown in Figures S5–S9.
Discussion
The advance in sequencing technologies, ranging from
whole genome shotgun sequencing to high-throughput
pyrosequencing, has proliferated over 500 sequenced
and annotated microbial genomes, revealing a multi-
tude of gene clusters related to natural product biosyn-
thesis [30,31]. The isolation of the corresponding
products of these cryptic clusters is often challenging
as a result of either a low rate of production or
unknown conditions for secondary metabolite biosyn-
thesis. In addition, bioactivity-guided natural product
isolation is often impeded by unpredictable biological
activities of the target compounds and a lack of appro-
priate screening methods. To circumvent the problem
of a low rate of biosynthesis and unknown biological
activity, we describe a genome mining approach rely-
ing on bioinformatic genome analysis and transcrip-
tome data combined with radiolabeled precursor
feeding studies for NRPS-derived natural products.
In this methodology, transcriptome analysis provides
information on the growth conditions leading to
gene cluster expression, whereas A-domain specificity
prediction defines the radiolabeled precursor.
MS analysis of erythrochelin and determination
of overall stereochemistry
On the basis of the observed NMR spectra, the pres-
ence and connectivity of d-N-acetyl-d-N-hydroxyorni-
thine, d-N-hydroxyornithine and serine in the sequence
was determined. Erythrochelin itself shows an exact
m⁄ z of 604.2938 ([M+H+]; calculated 604.2937) and a
molecular formula of C24H41N7O11 and a m ⁄ z of
657.2056 ([M+H+]; calculated 657.2051) as ferri-ery-
throchelin. To confirm the structural assignment
obtained by NMR, MS3 fragmentation studies were
conducted (Fig. 6). An intense fragment with an m ⁄ z
of 390.1979 ([M+H+]; calculated 390.1983) corre-
sponded to the C-terminal tripeptide comprised of ser-
ine and the DKP moiety built up by hOrn and haOrn
residues (Fig. 6A). The loss of the N-terminal serine
residue gave rise to a dipeptidyl DKP fragment with a
m⁄ z of 303.1662 ([M+H+]; calculated 303.1663). This
fragment was furthermore subjected to MS3 fragmen-
tation (Fig. 6B). The resulting fragments revealed the
presence of hydroxylated and acetylated ornithine resi-
dues. In addition, an intense fragment with an m ⁄ z of
145.0869 ([M+H+]; calculated 145.0971) was
Initial detection of erythrochelin was performed by
cultivation of S. erythraea in a complex SCM medium
utilizing a radio-LC-MS methodology, and confirmed
668
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L. Robbel et al.
Erythrochelin siderophore characterization
A
B
Fig. 6. MS ⁄ MS fragmentation studies of
erythrochelin. (A) MS2 fragmentation of the
title compound. (B) MS3 fragmentation
pattern of the C-terminal DKP moiety m ⁄ z =
303.1662 ([M+H+]). Calculated and
observed m ⁄ z values for the fragments are
given.
the DNA microarray gene expression profiles obtained
for S. erythraea [20]. Feeding of the nonproteinogenic
amino acid 14C-l-Orn prior to expression of the etc
gene cluster gave rise to radiolabeled erythrochelin,
which could be clearly identified on an analytical scale.
The sensitivity of radioactivity detection and sophisti-
cated analytical separation proved to be advantageous
in this approach. The iron-chelating properties of the
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L. Robbel et al.
radiolabeled compound were confirmed by CAS assay-
guided fractionation of medium-scale fermentation
extractions. A comparison of the masses found in the
CAS-reactive fraction and the m ⁄ z of the labeled prod-
uct revealed erythrochelin to be an ornithine inheriting
siderophore. Due to media complexity and coeluting
impurities, which prevented rapid MS-based single
compound identification, this radio-LC-MS methodol-
erophore upon iron binding. Erythrochelin shows an
absorption spectrum typical of ferri-hydroxamate sid-
erophores with kmax = 440 nm.
Erythrochelin shares a high degree of structural sim-
ilarity to the angiotensin-converting enzyme inhibitor
and siderophore foroxymithine isolated from cultures
of Streptomyces nitrosporeus (Fig. S17) [32–34]. In con-
trast to erythrochelin, the d-amino groups of ac-hOrn1
and hOrn4 are formylated, suggesting that a formyl-
transferase is involved in biosynthesis, analagous to
coelichelin assembly [7]. In an attempt to chemically
obtain foroxymithine, a total synthesis was established
by Dolence and Miller [35] that resulted in a com-
pound exhibiting the same NMR spectroscopic
properties as the isolated natural product. All residues
within the peptide chain showed an l-configuration.
This stereochemistry differs from erythrochelin, in
which two residues show a d-configured stereocenter,
thus suggesting a similar NRPS-based assembly of for-
oxymithine by a synthetase lacking all E-domains. The
lack of sequence information for the S. nitrosporeus
genome impeded the identification of a biosynthetic
machinery governing foroxymithine assembly. Future
work will focus on the investigation of erythrochelin-
mediated angiotensin-converting enzyme inhibition,
aiming to assign a bioactivity going beyond iron
chelation.
ogy was utilized to identify
a minimal medium
enabling erythrochelin production. Cultivation of
S. erythraea under iron-depleted conditions induced
the production of erythrochelin compared to iron-rich
media cultivations. Interestingly, the amount of 14C-l-
Orn incorporation was increased from 2% to 4%
(based on the total amount of radioactivity fed) when
switching to minimal media. It is likely that the decel-
erated growth in iron-depleted minimal media com-
bined with an increase in siderophore production leads
to the increased incorporation of 14C-l-Orn into the
main secondary metabolite erythrochelin. In conclu-
sion, the described approach, solely based on
A-domain specificity prediction and the available tran-
scriptome data, can be applied for the initial detection
and isolation of NRPs [20]. Furthermore, this
approach substitutes the CAS assay-guided fraction-
ation and enabled the scale-down of NRP discovery
from a preparative to analytical scale. In addition, this
approach can be utilized to substitute the detection
and isolation of NRPs based on their biological activ-
ity, which is often challenging to predict. The utiliza-
tion of radiolabeled proteinogenic amino acids, which
can be channelled to ribosomal synthesis of peptides,
remains to be elucidated.
After having identified the CAS-reactive and 14C-l-Orn
incorporating erythyrochelin, a large-scale isolation was
conducted affording 10 mgÆL)1 erythrochelin. The over-
all structure of erythrochelin was determined by NMR
and MS analysis as well as hydrolysate derivatization
for determination of amino acid configuration. The
peptide sequence is composed of d-ac-haOrn1-d-Ser2-
cyclo(l-hOrn3-l-haOrn4). Erythrochelin represents a
hydroxamate-type tetrapeptide siderophore containing
three ornithine residues, of which two are d-N acetylated
and d-N hydroxylated. In addition, the N-terminal a-
amino group of haOrn1 is acetylated. A local symmetry
in erythrochelin is attained by a DKP structure consist-
ing of two cyclodimerized l-Orn residues. The mode of
Fe(III) chelation by erythrochelin remains to be eluci-
dated, although we postulate an iron-binding mode
analogous to gallium-binding by coelichelin (Fig. S16).
MS analysis of ferri-erythrochelin reveals an abolished
skimmer fragmentation compared to erythrochelin,
being indicative of an induced rigidification of the sid-
On the basis of the results obtained in the present
study, a model for erythrochelin biosynthesis by the
tetramodular NRPS EtcD in combination with EtcB
and an acetyltransferase was established (Fig. 7). In
contrast to the second NRPS gene cluster associated
with siderophore production (nrps3), which putatively
encodes for a catecholate-type compound, the etc
gene cluster is congruent with the structure of eryth-
rochelin (Fig. S18). The domain organization and the
predicted substrate specificities of the A-domains do
not reflect in the structure of erythrochelin and
exclude its biosynthesis by Nrps3. The extraction of
culture supernatants of S. erythraea, cell pellets and
lysed cells with a variety of organic solvents did not
lead to the identification of the second siderophore
(data not shown). We therefore assume that either
the extraction conditions were inadequate for the iso-
lation of the natural product, or that the gene clus-
ter is silent under the conditions employed. The
irrevocable evidence for EtcD-mediated erythrochelin
assembly would result from targeted gene deletion of
etcD followed by LC-MS analysis of culture superna-
tants. Erythrochelin biosynthesis by EtcD follows a
linear enzymatic logic, in which the number of
A-domains located within the template directly corre-
lates with the number of amino acids found in the
670
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L. Robbel et al.
Erythrochelin siderophore characterization
Fig. 7. Proposed biosynthesis of erythrochelin by the tetramodular nonribosomal peptide synthetase EtcD. d-N-hydroxylation of L-ornithine is
putatively mediated by the peptide monooxygenase EtcB. d-N-acetylation of L-hydroxyornithine is putatively carried out by an external N-ace-
tyltransferase not encoded in the etc gene cluster. The N-terminal C-domain of the NRPS catalyzes the a-N-acetylation of haOrn1 in cis.
Cyclorelease of the assembled tetrapeptide mediated by the C-terminal C-domain of EtcD results in the formation of a DKP moiety.
product. Initiation of erythrochelin assembly requires
d-N-hydroxylation of l-Orn by the flavin-dependent
monooxygenase EtcB, analogous to the CchB-
catalyzed oxygenation of l-Orn during coelichelin
biosynthesis [22]. l-hOrn itself represents a branching
point in erythrochelin synthesis. This building block
is either directly recognized by A3 or further modi-
fied by means of d-N-acetylation. In this model, ace-
tyltransferase-catalyzed acetylation of l-hOrn gives
rise to l-haOrn, which is recognized by A1 and A4,
and is activated and covalently tethered to the 4¢-
Ppant cofactors of the corresponding PCPs as ami-
noacyl thioester. We propose that acetyltransferases
of the IucB- or VbsA-type, as involved in ornithine
acetylation in aerobactin and vicibactin biosynthesis,
are associated with l-haOrn synthesis [10,36]. These
results are consistent with the bioinformatic analysis
of EtcD adenylation domain specificity, resulting in
the less accurate prediction of l-Arg as substrate for
both A1 and A4. Differences in the specificity-deter-
mining residue pattern are likely to be the result
of minimal structural differences between l-Arg and
l-haOrn (Fig. S1B). When comparing the active site
residues of A1 and A4, a high degree of identity
(90%) is found, indicating l-haOrn as the common
substrate. This model would exclude the online d-N-
hydroxylation and d-N-acetylation of the NRPS-
bound substrates as seen in the hydroxylation of
PCP-bound Glu in kutzneride biosynthesis [37]. Prior
to incorporation of haOrn1 into the growing peptide
chain, the a-N-acetylation is likely to be carried out
by the C1-domain located at the N-terminus
of EtcD, recognizing acetyl-CoA as the substrate.
A similar mechanism was shown to be adopted in
the initiation reaction during surfactin biosynthesis,
with b-hydroxymyristoyl-CoA being the substrate
for NRPS-catalyzed acyl transfer [38]. Epimerization
of the a-stereocenters of l-ac-haOrn1 and l-Ser is
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671

Erythrochelin siderophore characterization
L. Robbel et al.
extract, 0.1 gÆL)1 CaCl2) or iron-deficient M9 medium
(2 gÆL)1 glucose, 6.78 gÆL)1 Na2HPO4, 3 gÆL)1 KH2PO4,
0.5 gÆL)1 NaCl, 1.2 gÆL)1 NH4Cl, 120 mgÆL)1 MgSO4,
14.7 gÆL)1 CaCl2, 0.1 gÆL)1 glycerol, 50 lgÆL)1 biotin,
200 lgÆL)1 thiamin). After 48 h of growth, 5 lCi of l-orni-
thine (Hartmann Analytic, Braunschweig, Germany) was
added. The supernatants were extracted with XAD16 resin
after an additional 2 days of growth. The dried eluate was
dissolved in 10% methanol and analyzed on a Nucleodur
mediated by the E-domains located in modules 1
and 2, being in full agreement with the experimental
determination of overall stereochemistry. The C-
domain catalyzed condensation of the four unique
building blocks follows a linear NRPS assembly line
logic. In the first step, the C2 domain catalyzes the
nucleophilic attack of the Ser1 a-amino group onto
the PCP1-bound ac-haOrn1 resulting in
a PCP2-
bound dipeptide. C3-catalyzed isopeptide bond for-
mation between the d-amino group of l-hOrn3 and
the PCP2-bound d-ac-haOrn1-d-Ser2 dipeptide results
in the translocation of the tripeptide to PCP3. A
nucleophilic attack of the l-haOrn4 a-amino group
onto the PCP3-bound tripeptide thioester functional-
ity results in the fully assembled tetrapeptide consist-
ing of d-ac-haOrn1-d-Ser2-l-hOrn3-l-haOrn4. The
release of the assembled NRP is generally mediated
by C-terminal thioesterase or reductase domains
located in the termination module of the NRPS
assembly line [21,39]. In contrast, we propose that
the cyclorelease of erythrochelin through DKP for-
mation is carried out by the C-terminal C5-domain,
catalyzing the intramolecular nucleophilic attack of
the L-hOrn3 a-amino group onto l-haOrn4. Taking
into account that the synthetases involved in the bio-
synthesis of the DKP-inheriting toxins thaxtomin
and fumitremorgin also contain a C-terminal conden-
sation domain, this C-domain catalyzed cyclorelease
appears to be feasible [40,41]. Apo-erythrochelin is
then exported into the extracellular space to scavenge
iron. The import of ferri-erythrochelin is likely to be
mediated by the FeuA homolog EtcC, which is
responsible for periplasmic binding [4]. In combina-
tion with EtcF, the ABC-transporter transmembrane
component and EtcG, the corresponding ATP-bind-
ing component, ferri-erythrochelin, is actively reim-
ported into the cell [42].
C (ec) column 125 · 2 mm (Macherey & Nagel, Duren,
¨
18
Germany) combined with an Agilent 1100 HPLC system
(Agilent, Waldbronn, Germany), connected to a FlowStar
LB513 radioactivity flow-through detector (Berthold, Bad
Wildbad, Germany) equipped with a YG-40-U5M solid
microbore cell and a QStar Pulsar i (Applied Biosystems,
Foster City, CA, USA), utilizing the solvent gradient:
water ⁄ 0.05% formic acid (solvent A) and methanol ⁄ 0.05%
formic acid (solvent B) at a flow rate of 0.3 mLÆmin)1: lin-
ear increase from 0% B to 50% within 20 min followed by
a linear increase to 95% B in 5 min, holding B for an
additional 5 min. This gradient was also used to analyze
comparative extractions of S. erythraea cultures and eryth-
rochelin and ferri-erythrochelin.
Isolation of erythrochelin from SCM medium
S. erythraea NRRL 23338, maintained on SCM agar slants,
was used to inoculate 30 mL of SCM liquid culture. The
cells were grown for 4 days at 30 ꢀC and 250 r.p.m. and
subsequently used to inoculate 1 L of SCM medium. The
cells were grown for 5 days at 30 ꢀC. The production phase
of the strain was monitored via LC-MS and the CAS assay
[27]. The culture supernatant was extracted with XAD16
resin (4.0 gÆL)1). The resin was collected by filtration,
washed twice with water and the absorbed compounds were
eluted with methanol. The eluate was evaporated to dry-
ness, dissolved in 10% acetonitrile and applied onto a
RP-HPLC preparative Nucleodur C18(ec) 250 · 21 mm col-
umn combined with an Agilent 1100 HPLC system. Elution
was performed by application of the solvent gradient of
water ⁄ 0.05% formic acid (solvent A) and methanol ⁄ 0.05%
formic acid (solvent B) at a flow rate of 16 mLÆmin)1: lin-
ear increase from 0% B to 50% within 50 min followed by
a linear increase to 95% B in 5 min, holding B for an addi-
tional 5 min. The wavelengths chosen for detection were
215 and 280 nm, respectively. Siderophore containing frac-
tions were confirmed by using the CAS liquid assay and
subjected to LC-MS analysis.
Materials and methods
Strains and general methods
S. erythraea NRRL 23338 was obtained from the ARS
(Agricultural Research Service, Peoria, IL, USA) Culture
Collection. Chemicals were obtained from commercial
sources and were used without further purification, unless
noted otherwise.
Large-scale purification of erythrochelin from M9
medium
Radio-LC-MS-guided genome mining
Radiolabeling studies were performed by cultivating
S. erythraea in 100 mL of SCM medium (10 gÆL)1 soluble
starch, 20 gÆL)1 soytone, 10.5 gÆL)1 Mops, 1.5 gÆL)1 yeast
S. erythraea, maintained on SCM agar slants, was used to
inoculate 30 mL of SCM liquid culture. The cells were
672
FEBS Journal 277 (2010) 663–676 ª 2009 The Authors Journal compilation ª 2009 FEBS

L. Robbel et al.
Erythrochelin siderophore characterization
grown for 4 days at 30 ꢀC and 250 r.p.m. The cells were
exchanged from the SCM medium into the iron-deficient
M9 medium by repeated centrifugation and resuspension
of the cells in the target medium. Subsequently, the cells
were used to inoculate 5 L of iron-deficient M9 medium
in PC-flasks until D600 of 0.01 was reached. After 4 days
of cultivation, the cells were harvested by centrifugation
at 6084 g and 4 ꢀC for 30 min. The supernatant was sep-
arated from the cell pellet and incubated with XAD16
resin (4.0 gÆL)1). The resin was collected by filtration,
washed twice with water and the absorbed compounds
were eluted with methanol. The eluate was evaporated to
dryness, dissolved in 10% acetonitrile and applied onto a
RP-HPLC preparative Nucleodur C18(ec) 250 · 21 mm
column combined with an Agilent 1100 HPLC system.
Elution was performed by application of the solvent
gradient of water ⁄ 0.05% formic acid (solvent A) and
methanol ⁄ 0.05% formic acid (solvent B) at a flow rate of
16 mLÆmin)1: linear increase from 0% B to 50% within
50 min followed by a linear increase to 95% B in 5 min,
with sensitivity enhancement [44,45]. The TOCSY spec-
trum was recorded with mixing time of 200 ms, whereas
NOESY and ROESY spectra were taken at 150 and
300 ms mixing times. The 1D spectra were acquired with
65 536 data points, whereas 2D spectra were collected
using 4096 points in the F2 dimension and 512 increments
in the F1 dimension. For 2D spectra, 16–32 transients were
1
used. The relaxation delay was 2.5 s. Chemical shifts of H
and 13C were referenced to the solvent signals, whereas
that of 15N was referenced to the urea signal, externally.
The spectra were processed using topspin, version 2.1
(Bruker).
Amino acid analysis by FDAA (Marfey’s reagent)
derivatization
Five hundred micrograms of erythrochelin were completely
hydrolyzed by the addition of 400 lL of 6 m HCl and incu-
bation at 110 ꢀC for 24 h. The solution was lyophilized and
the remaining residue dissolved in 10 lL of 1 m NaHCO3.
One hundred and seventy microliters of 1% FDAA (Sigma-
Aldrich, Munich, Germany) in acetone were added and the
solution was heated at 37 ꢀC for 1 h. The derivatization
reaction was terminated by the addition of 20 lL of 1 m
HCl. After lyophilization, the derivatized amino acids were
resolubilized by the addition of 1 : 1 water : acetonitrile
solution and 0.1% trifluoroacetic acid to obtain a final vol-
ume of 400 lL. Products of derivatization were analyzed by
RP-LC-MS on a Synergi Fusion-RP 80 250 · 2.0 mm col-
umn (Phenomenex, Aschaffenburg, Germany) utilizing the
solvent gradient: 0–30 min, 0–30% buffer A (10 mm ammo-
nium formate, 1% methanol, 5% acetonitrile, pH 5.2) into
buffer B (10 mm ammonium formate,1% methanol,60%
acetonitrile, pH 5.2) followed by a linear increase to 95%
buffer B in 2 min and holding 95% buffer B for an addi-
tional 5 min. The wavelength chosen for detection was
340 nm and the flow rate was 0.3 mLÆmin)1. [29]. Ten
microliters of sample was added to 90 lL of water prior to
the injection of 10 lL.
holding
B for an additional 5 min. The wavelengths
chosen for detection were 215 and 280 nm, respectively.
Siderophore containing fractions were confirmed by using
the CAS assay. Positive fractions were lyophilized and
subjected to further analysis. The retention time of eryth-
rochelin was 30.7 min.
MS analysis
The MS characterization of erythrochelin was performed
with an LTQ-FT instrument (Thermo Fisher Scientific,
Langenselbold, Germany) connected to a microbore Agilent
1100 HPLC system. Apo- and holo-erythrochelin were ana-
lyzed on a Nucleodur C18(ec) 125 · 2 mm column utilizing
the solvent gradient: 0–30 min, 0–100% acetonitrile into
water, both supplemented with 0.1% trifluoroacetic acid.
The column temperature was 45 ꢀC and the flow rate was
0.3 mLÆmin)1. Collision induced dissociation fragmentation
studies within the linear ion trap were carried out using
online LC-MS.
To determine the stereochemistry of the present amino
acids, amino acid standards (d ⁄ l-Ser and l-hOrn) were pre-
pared to compare retention times and MS spectra, as well
as to perform coelution experiments. The FDAA-deriva-
tized amino acids were synthesized by incubation of 25 lL
of 50 mm amino acid in water, 50 lL of 1% FDAA in ace-
tone and 10 lL of 1 m NaHCO3 at 37 ꢀC for 1 h. The solu-
tion was lyophilized, and the dried products resolubilized in
1 : 1 water : acetonitrile solution and 0.1% trifluoroacetic
acid to obtain 200 lL. l-hOrn was synthesized chemically
according to an established protocol [46]. Coelution experi-
ments were conducted by mixing 10 lL of derivatized ery-
throchelin hydrolysate with 1 lL of derivatized d-Ser
amino acid standard and 3 lL of derivatized l-hOrn stan-
dard. RP-LC-MS analysis was performed as described
above.
NMR structure elucidation
Approximately 16 mg of the title compound was dissolved
in 0.7 mL of dimethylsulfoxide-d6. Measurements were car-
ried out on a AV600 (Bruker, Madison, WI, USA) spec-
trometer with an inverse broadband probe installed with
z-gradient. The 1D spectra 1H and 13C; the homonuclear
2D spectra DQF-COSY, TOCSY, NOESY and ROESY;
the 1H–3C HSQC and HMBC; and the1H–15N HSQC
spectra were recorded at room temperature using standard
pulse software [43]. The phase-sensitive HMBC spectrum
focused on the carbonyl region with high resolution in the
13C dimension was recorded by using pulse software with
a semi-selective 13C pulse built into an HMBC experiment
FEBS Journal 277 (2010) 663–676 ª 2009 The Authors Journal compilation ª 2009 FEBS
673

Erythrochelin siderophore characterization
L. Robbel et al.
5 Andrews SC, Robinson AK & Rodriguez-Quinones F
(2003) Bacterial iron homeostasis. FEMS Microbiol Rev
27, 215–237.
Determination of amino acid connectivity via
partial hydrolysis of erythrochelin
Three milligrams of erythrochelin were partially hydrolyzed
in 200 lL of 6 m HCl at 110 ꢀC for 20 min. The result-
ing solution was lyophilized and resolubilized in 1 : 1
water : acetonitrile solution and 0.1% trifluoroacetic acid
to a final volume of 200 lL and analyzed via an LTQ-FT
instrument to a microbore Agilent 1100 HPLC system.
6 Gehring AM, Bradley KA & Walsh CT (1997) Enterob-
actin biosynthesis in Escherichia coli: isochorismate
lyase (EntB) is a bifunctional enzyme that is phospho-
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Products were analyzed on
a
Nucleodur C18(ec)
7 Lautru S, Deeth RJ, Bailey LM & Challis GL (2005)
Discovery of a new peptide natural product by Strepto-
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265–269.
125 · 2 mm column, utilizing the solvent gradient: 0–30
min, 0–100% acetonitrile into water, both supplemented
with 0.1% trifluoroacetic acid followed by a linear increase
to 95% acetonitrile in 5 min and holding 95% acetonitrile
for an additional 5 min. The column temperature was
8 Dimise EJ, Widboom PF & Bruner SD (2008)
Structure elucidation and biosynthesis of fuscachelins,
peptide siderophores from the moderate thermophile
Thermobifida fusca. Proc Natl Acad Sci USA 105,
15311–15316.
45 ꢀC and the flow rate was 0.3 mLÆmin)1
. Collision
induced dissociation fragmentation studies within the linear
ion trap were carried out using online LC-MS. The target
fragment was isolated from the mixture with an Agilent
1100 HPLC system connected to an AnalytFC fraction col-
lector (Agilent) on a Hypercarb 100 · 2.1 mm column
(Thermo, Waltham, MA, USA) utilizing the solvent gradi-
ent: 0–30 min, 0–70% acetonitrile into water, both supple-
mented with 20 mm nonafluoro-1-pentanoic acid, each
followed by a linear increase to 95% acetonitrile in 2 min
and holding 95% acetonitrile for an additional 5 min. The
wavelength chosen for detection was 215 nm, with a col-
9 Kadi N, Oves-Costales D, Barona-Gomez F & Challis
GL (2007) A new family of ATP-dependent
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.
Product containing fractions were identified by RP-LC-MS.
Positive fractions were lyophilized, hydrolyzed and deriva-
tized with FDAA as described above. Analysis of the deriv-
atized amino acids was performed by RP-LC-MS.
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Acknowledgements
We would like to thank Antje Schafer and Anke
¨
Botthof for their excellent support during this project.
We gratefully acknowledge financial support from the
Deutsche Forschungsgemeinschaft (M.A.M.).
14 Challis GL (2008) Mining microbial genomes for new
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Fig. S13. LC-MS trace of FDAA-derivatized hydro-
lysate.
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fragment.
Fig. S15. LC-MS trace of FDAA-derivatized C-termi-
nal fragment.
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Fig. S16. Proposed Fe(III)-binding modes.
Fig. S17. Structural comparison of erythrochelin and
foroxymithine.
Fig. S18. Schematic overview of Nrps3.
Table S1. Bioinformatic overview of etc gene cluster.
Table S2. (A) Comparison of A1 and A4. (B) Structures
of l-Arg and l-haOrn.
Supporting information
The following supplementary material is available:
Fig. S1. HPLC-profile of SCM medium extraction.
Fig. S2. LC-MS traces of comparative extractions.
Fig. S3. UV ⁄ visible absorption spectra of erythro-
chelin.
1
Table S3. H chemical shifts.
Table S4. 13C chemical shifts.
Table S5. 15N chemical shifts.
Table S6. Observed NOE contacts.
1
Table S7. Long-range H-13C correlations.
1
Fig. S4. 1D H-NMR spectrum of erythrochelin.
This supplementary material can be found in the
online version of this article.
Fig. S5. DQF-COSY spectrum of erythrochelin.
Fig. S6. 1H-15N HSQC spectrum of erythrochelin.
Fig. S7. HMBC spectrum of erythrochelin; amide
protons.
Please note: As a service to our authors and readers,
this journal provides supporting information supplied
by the authors. Such materials are peer-reviewed and
may be re-organized for online delivery, but are not
copy-edited or typeset. Technical support issues arising
from supporting information (other than missing files)
should be addressed to the authors.
Fig. S8. HMBC spectrum of erythrochelin; side chain
protons.
Fig. S9. ROESY spectrum of erythrochelin.
Fig. S10. Fragmentation pattern of C-terminal fragment.
Fig. S11. LC-MS analysis of erythrochelin hydrolysate.
Fig. S12. LC-MS trace of FDAA-derivatized standards.
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