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Journal of Bacteriology, May 1999, p. 3307-3309, Vol. 181, No. 10
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
SufS Is a NifS-Like Protein, and SufD Is Necessary
for Stability of the [2Fe-2S] FhuF Protein in Escherichia
coli
Silke I.
Patzer and
Klaus
Hantke*
Mikrobiologie II, Universität
Tübingen, Tübingen, Germany
Received 7 December 1998/Accepted 15 March 1999
 |
ABSTRACT |
Escherichia coli fhuF mutants, a
sufS::MudI mutant, and a
sufD::MudI mutant were found to have the same
phenotype: the inability to use ferrioxamine B as an iron source in a
plate assay. In addition, the sufS and sufD
genes were shown to be regulated by the iron-dependent Fur repressor.
Sequence analysis revealed that the sufS open reading frame
corresponds to orf f406. The protein SufS belongs to the family of NifS-like proteins, which supply sulfur for [Fe-S] centers. The protein FhuF contains a [2Fe-2S] center. A mutation in the upstream sufD gene (orf f423) caused the same
phenotype. The T7 expression system and a His tag allow the isolation
in good yield of the FhuF protein from a wild-type strain. In contrast,
overproduction of the protein in a
sufD strain failed.
Radioactive labeling of N-His-FhuF with [35S]methionine
showed that the protein was unstable in the
sufD mutant.
 |
TEXT |
Under low-iron growth conditions,
many bacteria secrete specific chelators called siderophores.
Production of siderophores and transport systems for
Fe3+-loaded siderophores is regulated in gram-negative
bacteria by the repressor protein Fur with Fe2+ as a
corepressor (5). In Escherichia coli, a
fhuF-lacZ operon fusion has been used as a reporter to
study iron regulation (15) since this fusion reacts very
sensitively to slight changes in the iron concentration of the medium.
The only phenotype observed for fhuF mutations is a
diminished ability of the cells to use ferrioxamine B as an iron
source. Ferrioxamine B is a poor iron source for E. coli
K-12 (7, 12) because this strain lacks a specific receptor
for it in the outer membrane; this receptor is found in yersiniae
(2) and other enterobacteria (10). The FhuF
protein has been purified and found to be an unusual [2Fe-2S] protein
(12).
Isolation of sufS mutants and their phenotype.
Mutants with a FhuF-like phenotype were selected with the aim of
further defining the function of FhuF in ferrioxamine B uptake. Iron-regulated lacZ operon fusions were isolated by using
the transposing phage MudI (Ap lac) as previously described
(6, 12). Under low-iron growth conditions, transcription of
the fusion was derepressed. The mutants were tested for the ability to
utilize various siderophores as iron sources in a plate assay. Enterochelin, citrate, ferrichrome, and coprogen stimulated normal growth under iron-limiting conditions. Only ferrioxamine B and rhodotorulic acid caused a reduced growth zone on an iron-limited medium in a filter paper disc assay (Table
1). This phenotype was identical to that
of fhuF mutants (12). Mapping experiments using
P1 (data not shown) indicated that the phage was not located in or near
fhuF at 99 min on the genetic map of E. coli
(3).
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TABLE 1.
Growth response of parent strain H1443 and suf
mutants to iron-loaded siderophores on iron-limiting nutrient broth
dipyridyl platesa
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|
Sequencing of the sufS::lacZ
fusion site and localization on the E. coli genetic
map.
Two independently isolated lacZ operon fusions in
E. coli H1489 and H1490 were studied. To identify the fusion
site, chromosomal DNA of the mutants was digested with
HincII and cloned into the SmaI site of the
vector pSU18 (1). By PCR with a MudI-derived primer
(CACGTACATGCCGCCAAACTCACCA) and the UNI primer
(CGACGTTGTAAAACGACGGCCAGT), 0.95- and 0.9-kb fragments were
obtained from strains H1489 and H1490, respectively. Cloning and DNA
sequencing revealed that the MudI phage in strain H1489 was inserted in
open reading frame orf f406 upstream of bp 2662 (Fig.
1) in E. coli K-12 MG1655
section 153 (of 400) of the complete genome (4). Since
f406 is most likely involved in the mobilization of sulfur
(see below), it will be called sufS. It is located at 38 min
near lpp (structural gene for the murein lipoprotein) on the
genetic map of E. coli (3). The MudI phage of
strain H1490 was inserted into orf f423 (sufD)
upstream of sufS and upstream of bp 3750 in section 153 of
the E. coli K-12 genome (4). sufS and
sufD seem to be part of an operon with six open reading
frames, where each gene following the first open reading frame,
f122 (sufA), has a start codon overlapping the
stop codon of the previous gene (Fig. 1).

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FIG. 1.
Map of the suf gene cluster at 37.8 min on
the linkage map of E. coli (3). Stop and start
positions and the insertion sites of the two MudI phages are given
according to the base pair numbering of section 153 of the E. coli genome sequence (4). The designations of the open
reading frames in the GenBank database are given in parentheses. The
arrows indicate the direction of transcription.
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To confirm the determined positions of the insertions, the mutants were
transduced with P1 carrying the marker
lpp::Tn
10 by
using the donor strain
WR19-1 (
14). A cotransduction frequency
of 95% was observed
with each inserted phage, as expected from
the short distance between
sufS and
lpp (Fig.
1); these results
support the
sequencing
data.
Regulation of sufD and sufS by iron.
Derepression of lacZ by low iron had been the selection
criterion for the sufD and sufS mutants. This
indicated that the putative operon may be regulated by Fur, and indeed,
inspection of the promoter region revealed a possible Fur binding site
at positions 7305 to 7323 in section 153 of the E. coli K-12
genome (4) (GATAATGATtATCAgTtca; the lowercase
letters differ from the Fur box consensus sequence GATAATGATAATCATTATC).
To test whether the observed regulation is Fur dependent,
fur-28
zbf::Tn
10 was transduced by P1 into strains
H1489 and H1490,
which resulted in strains SIP593 and SIP596,
respectively. The
phenotype of the transductants was tested on
MacConkey lactose
iron plates; indeed, the
fur mutants
formed red colonies, while
their parent strains formed white colonies.
Quantitative determination
of

-galactosidase activities confirmed
these observations. Strains
H1489 and H1490 had

-galactosidase
activities of 13 to 16 U during
exponential growth on TY
(
12) medium, and after 1 h of growth
with 50 µM EDDA
(
7) added, activities of 38 to 46 U were observed.
The Fur
mutants grown on TY medium contained

-galactosidase activities
of 46 to 48 U; addition of 50 µM EDDA slightly enhanced the activities
to
59 to 62
U.
Synthesis of N-His-FhuF in a sufD mutant.
Strain
H1490 sufD::MudI was grown at 42°C. Of 10 temperature-resistant colonies tested, two were ampicillin sensitive,
which indicated deletion of the integrated MudI phage. One of these strains, H5385
sufD, was transformed with plasmids
pGP1-2, which encodes the T7 RNA polymerase (16), and
pKF191, which carries the gene encoding a FhuF protein with a His tag
cloned downstream of the T7
10 promoter region (12).
The His tag allows one-step isolation of the N-His-FhuF protein with
Ni
2+-nitrilotriacetate resin (
12). The membranes
of the ruptured
cells of strain WM1576/pKF191
fhuF+ were sedimented, and the clear brown
supernatant was applied
to an
Ni
2+-nitrilotriacetate-agarose column. At the top of the
gel matrix,
a deep brown band appeared, which migrated through the
column
during elution with 0.5 M imidazole. About 40 mg of protein/g
(wet weight) of cells was isolated. The same procedure was used
with
cells of strain H5385
sufD (pGP1-2 pKF191
fhuF+). However, no brown band appeared, and
only very small amounts
of N-His-FhuF protein could be demonstrated by
sodium dodecyl
sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of
the imidazole
eluate. The amounts were so small that we were unable to
determine
by UV-visible-light spectroscopy whether the eluted FhuF
protein
contained the [2Fe-2S]
center.
To determine whether the N-His-FhuF protein was produced in the
sufD strain, the protein was selectively labeled with
[
35S]methionine (
12,
16). After 10 min,
[
35S]methionine was chased with 50 mM unlabeled
methionine. After
SDS-PAGE, a strongly labeled N-His-FhuF protein band
was observed,
and after a 30-min chase with unlabeled methionine, most
of the
labeled protein band disappeared (Fig.
2); in contrast, the same
protein was
stable in a
suf+ background (Fig.
2).

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FIG. 2.
Section of an autoradiograph after SDS-PAGE of
[35S]methionine-labeled N-His-FhuF protein. Strains H5385
pGP1-2 (lanes 1 and 2) and WM1576 (lanes 3 to 5) were transformed with
plasmid pKF191, which allowed the exclusive expression of the gene
encoding N-His-FhuF by the T7 RNA polymerase system (12,
16). Ten minutes after addition of [35S]methionine,
whole cells were pelleted by centrifugation and an aliquot was
separated by SDS-PAGE (lanes 1 and 3). For the chase, the remaining
cells were suspended in fresh medium containing 50 mM unlabeled
methionine and incubated for 15 min (lane 4) and 30 min (lanes 2 and 5)
before loading on the polyacrylamide gel.
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Conclusions.
The NifS protein is required for the construction
of the [Fe-S] cluster of the nitrogenase in Azotobacter
vinelandii. Three open reading frames coding for proteins with
similarities to NifS have been identified in the E. coli
K-12 genome (4). One is the NifS-like protein, now called
IscS (17), which has been purified and described as a
pyridoxal phosphate-dependent cysteine desulfurase by Flint
(8) and is encoded at 57 min on the genetic map of E. coli. A second E. coli protein of this family has been characterized as a selenocysteine lyase (called Csd) with cysteine sulfinate desulfinase activity (11). The corresponding gene has been cloned and mapped at 63 min on the E. coli map. The
similarity of NifS-like proteins has been discussed extensively by
Mihara et al. (11), who pointed out that Csd and NifS are
members of two subfamilies. The SufS protein and Csd show 43% amino
acid identity, and SufS and IscS have 22% identity, which reflects the
fact that SufS also belongs to the Csd group of NifS-like proteins. In
addition, it is interesting that SPL1 from yeast, a protein involved in
RNA splicing, also belongs to the group of NifS-like proteins
(similarity to NifS, 40%; similarity to SufS, 21%).
Examination of the genes in the neighborhood of
sufS
revealed an operon structure (Fig.
1). The product of the first gene,
sufA, is similar to IscA from
E. coli and
A. vinelandii (Fig.
3), which
is encoded by the
iscSUA gene cluster (
17). Genes
encoding counterparts of IscSUA have been found near
nifS in
A. vinelandii and also in different bacteria and eukaryotes.
The
function of IscA and IscU is not known, but it is assumed that
the
proteins from these operons synthesize iron-sulfur centers
of various
proteins (
17). An obvious counterpart for IscU or
NifU is
missing in the SufABCDSE cluster. However, SufE and YgdK
from
E. coli have 35% amino acid identity.
ygdK encodes an
open
reading frame directly downstream of
csd, which again
points to
a certain relationship between Csd and SufS.

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FIG. 3.
Comparison of SufA-like proteins IscA from E. coli (8, 17), IscA from A. vinelandii
(17), and YadR from E. coli (4) with
SufA. A star indicates identity, and a caret indicates similarity.
Percent similarity to SufA is given.
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|
The derived gene product of open reading frame
sufC has
clear-cut similarities to the ATPase subunits of ABC transporters.
The
gene products of open reading frames
sufB and
sufD are predicted
in the databases to be the membrane
components of a family of
ABC transporters. However, using various
protein structure prediction
programs, no membrane-spanning helices
were detected for these
proteins and their homologues in chloroplasts.
No published experimental
data which demonstrate the membrane
localization of any protein
of the SufB-related family were
found.
Since the insertion of MudI into
sufS and
sufD
only had an effect on ferrioxamine B utilization, no other vital
functions
seem to depend solely on these genes. In this early stage of
the
investigation, it is not known if the
sufD product is
directly
involved in ferrioxamine utilization or if the Mu insertion
has
a polar effect on
sufSE. However, it is interesting that
these
genes are regulated by Fur and iron, as observed for
fhuF.
The instability of FhuF could have various causes. One may be that FhuF
is part of the Suf complex and is unstable as single
protein. Another
interpretation, which we prefer, is that the
Suf complex is necessary
for the building of the "distorted" [2Fe-2S]
center in FhuF and
that the apo-FhuF protein is unstable in the
cell. However, this
instability must be very peculiar since the
various FhuF Cys

Ser
mutant proteins which did not contain a [2Fe-2S]
center were isolated
in relatively large amounts (
12).
 |
ACKNOWLEDGMENTS |
We thank Christian Bayertz for technical assistance, Volkmar Braun
(Tübingen) and Uwe Stroeher (Tübingen) for reading the manuscript and discussion, and Karen A. Brune (Allensbach) for editing
the manuscript.
This research was supported by the Deutsche Forschungsgemeinschaft,
Schwerpunktprogramm "Molekulare Analyse von Regulationsnetzwerken in
Bakterien" and Sonderforschungsbereich 323.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Mikrobiologie
II, Universität Tübingen, Auf der Morgenstelle 28, D-72076
Tübingen, Germany. Phone: 49-7071-2974645. Fax: 49-7071-294634. E-mail: hantke{at}uni-tuebingen.de.
 |
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Journal of Bacteriology, May 1999, p. 3307-3309, Vol. 181, No. 10
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
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