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Journal of Bacteriology, April 2000, p. 1788-1793, Vol. 182, No. 7
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Flavodoxin Mutants of Escherichia
coli K-12
Philippe
Gaudu and
Bernard
Weiss*
Department of Pathology and Laboratory
Medicine, Emory University School of Medicine, Atlanta, Georgia 30322
Received 28 October 1999/Accepted 4 January 2000
 |
ABSTRACT |
The flavodoxins are flavin mononucleotide-containing electron
transferases. Flavodoxin I has been presumed to be the only flavodoxin
of Escherichia coli, and its gene, fldA, is
known to belong to the soxRS (superoxide response)
oxidative stress regulon. An insertion mutation of fldA was
constructed and was lethal under both aerobic and anaerobic conditions;
only cells that also had an intact (fldA+)
allele could carry it. A second flavodoxin, flavodoxin II, was postulated, based on the sequence of its gene, fldB. Unlike
the fldA mutant, an fldB insertion mutant is a
viable prototroph in the presence or absence of oxygen. A
high-copy-number fldB+ plasmid did not
complement the fldA mutation. Therefore, there must be a
vital function for which FldB cannot substitute for flavodoxin I. An
fldB-lacZ fusion was not induced by
H2O2 and is therefore not a member of the
oxyR regulon. However, it displayed a
soxS-dependent induction by paraquat (methyl viologen), and the fldB gene is preceded by two overlapping regions that
resemble known soxS binding sites. The fldB
insertion mutant did not have an increased sensitivity to the effects
of paraquat on either cellular viability or the expression of a
soxS-lacZ fusion. Therefore, fldB is a new
member of the soxRS (superoxide response) regulon, a group
of genes that is induced primarily by univalent oxidants and redox
cycling compounds. However, the reactions in which flavodoxin II
participates and its role during oxidative stress are unknown.
 |
INTRODUCTION |
Flavodoxins are small flavin
mononucleotide (FMN)-containing electron transferases that are found in
bacteria and algae. In Escherichia coli, a flavodoxin is
required for the reductive activation of cobalamin-dependent methionine
synthase (31), for biotin synthesis (34), and for
the anaerobic activation of both ribonucleoside triphosphate reductase
(8, 9) and pyruvate formate-lyase (36) through
the formation of glycyl free radicals at their active centers. In
nitrogen-fixing bacteria, a flavodoxin is the electron donor for the
Fe-containing protein of the nitrogenase complex (16). A
flavodoxin can often substitute for a ferredoxin, a small electron
transfer protein with an iron-sulfur center (reviewed in reference
31). Thus, both a flavodoxin and a ferredoxin are substrates for the NADPH:ferredoxin oxidoreductases of cyanobacteria and of E. coli, for the pyruvate-ferredoxin oxidoreductase
of Clostridium pasteurianum, and for the enzymes of
dissimilatory sulfate reduction. In iron-poor media, where we should
expect a ferredoxin deficiency, the flavodoxins of cyanobacteria and Anacystis nidulans are induced.
Most of the reactions of E. coli flavodoxin were
demonstrated in vitro with purified flavodoxin I, the product of the
cloned fldA gene, which was presumed to be the only
flavodoxin of E. coli. However, a putative second flavodoxin
gene, fldB, was discovered during the sequencing of the
unrelated neighboring xerD gene (GenBank accession no.
AE000373 [F. R. Blattner] and Z48060 [F. Hayes]), which
encodes a site-specific recombinase. The deduced structure of
flavodoxin II (FldB) has 43% identity with E. coli
flavodoxin I (FldA), and it has the characteristic flavodoxin
signature, an FMN binding motif near its N terminus.
This study of the flavodoxins was prompted by our interest in the
soxRS (superoxide response) regulon because it includes the
genes for flavodoxin I (fldA) (49) and for
NADPH:ferredoxin (flavodoxin) oxidoreductase (fpr)
(27). The soxRS regulon (17, 44)
responds to the oxidative stress produced by redox agents that engage
mainly in one-electron transfers, agents such as superoxide, nitric
oxide, and paraquat (methyl viologen). The sensor for the regulon
resides in the [2Fe-2S] centers of the ferredoxin-like SoxR protein.
When these centers are oxidized, SoxR becomes a transcriptional
activator of soxS, and the newly synthesized SoxS protein
(itself a transcriptional activator) then induces other genes of the
regulon. In the uninduced cell, SoxR is mainly in its inactive, reduced
form, and because it is auto-oxidizable, it must be continually
reduced. Through separate NADPH-linked reductases, both SoxR
(23) and flavodoxin (11) are in redox equilibrium
with NADP+/NADPH. Depletion of NADPH, e.g., during the
production of superoxide by the redox cycling of paraquat, activates
the regulon. FldA and Fpr may be induced to help restore the redox
balance of the oxidatively stressed cell (27, 28).
In this study, we produce insertion mutations of fldA and
fldB and we generate an fldB-lacZ gene fusion.
These constructs are then used to approach the following questions.
What are the phenotypes of the mutants? Is either gene essential? Do
flavodoxins I and II have the same functions? Is fldB a
member of the soxRS regulon? Does flavodoxin II have a
discernible role in protection against oxidative stress?
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MATERIALS AND METHODS |
Nomenclature.
cat, tet, and bla
refer to plasmid- or transposon-derived DNA segments specifying
resistance to chloramphenicol (Camr), tetracycline
(Tetr), and ampicillin or carbenicillin (Ampr),
respectively.
attP and att
are the
preferred DNA sequences in phage
and E. coli,
respectively, at which site-specific integration of phage
occurs.
Strains and plasmids.
Bacterial strains and plasmids used
are listed in Table 1. Some of the
plasmid constructions are detailed in Fig. 1 and 2. Insertions into
att
were performed as previously described
(18), except that a recA+ strain was
used because it was a better donor for subsequent transductions. The
att
::(fldA+
bla+) element of BW1527 was prepared from plasmid
pWB52 (Table 1), which had been cut with NotI to yield a
bla+-fldA+-
attP fragment. This
DNA was then circularized by ligation and used to transform strain
GC4468(pLDR8) to carbenicillin resistance at 42°C. The cells were
grown in Luria-Bertani (LB) broth at 37°C for 1.5 h to allow
gene expression before selection. pLDR8 specifies a thermoinducible
integrase that mediated the insertion of the bla+-fldA+ segment into the
att
site. BW1527 was Kans, indicating that it
had been cured of the temperature-sensitive helper plasmid. The
att
::(
[soxS'-'lacZ]
bla+) element of BW1157 was constructed
similarly.
RZ5::fldB was produced by growing
RZ5 on a strain carrying plasmid pWB51; the lysate was used to
infect
lac strain GC4468, and the desired lysogens were
selected as red colonies on MacConkey agar (Difco) containing
ampicillin.
Media and growth conditions.
LB media (29) were
used for the routine growth of E. coli. The minimal medium
used for aerobic growth (VB medium) was medium E described by Vogel and
Bonner (43) that was supplemented with 0.4% glucose and 1 µg of thiamine/ml. The minimal medium used under anaerobic conditions
was a glycerol-fumarate medium without Casamino Acids (41).
For anaerobic growth in rich solid media, the cells were suspended in
15 ml of nutrient agar (Difco) containing an E. coli
membrane preparation (EC Oxyrase; Oxyrase, Inc.), overlaid with a
barrier of 2% agar (in H2O), and incubated in air
(1). Alternatively, the cells were grown on the surfaces of
agar plates in an AnaeroPak chamber (Mitsubishi Gas Chemical America,
Inc.) under 80% N2-20% CO2. Growth in broth
under stringent anaerobic conditions was performed in
Na2S-supplemented media (20). Antibiotics were
used at the following concentrations: ampicillin, 100 µg/ml; chloramphenicol, 25 µg/ml; tetracycline, 5 or 15 µg/ml. The lower concentration of tetracycline was required for the selection of single
copies of fldB::tet. Carbenicillin (50 µg/ml) was used for the selection of single copies of bla.
Gene transfers.
Generalized transductions were performed
with bacteriophage P1 dam rev6 (42). Bacterial
transformations were performed as previously described (14,
22). Transfers of fldA and fldB insertion
mutations from plasmid to chromosome (by double crossovers) were
accomplished as previously described (30) with a recBC sbcBC strain (JC7623), which does not support the growth of
ColE1-derived replicons (6).
Computer analysis.
Sequence similarity searches were
performed with the BLAST, version 2.0, program at the National Center
for Biotechnology Information website. FMN binding sites were detected
with the MOTIFS program of the Wisconsin Package, version 9.1 (Genetics Computer Group, Madison, Wis.).
Other methods.
PCR and general cloning methods were as
described previously (3). DNA fragments with incompatible
ends were blunted with bacteriophage T4 DNA polymerase before being
joined. Curing of the att
::fldA
element was accomplished by infection with
c+
phage at a multiplicity of 10 (38). PCR amplifications of
the fldA and fldB regions were performed with
Taq DNA polymerase and the following primers:
5'-GAAGAAGTCATCCCAGTCACA-3' and
5'-ACCCCCATTTCAATAAGTTTC-3' for fldA and
5'-TTAGTTTCATCCAGCGCC-3' and 5'-CCATTATGCCTTATTGTGCC-3' for fldB. Treatments of growing cells with paraquat or
H2O2 were as previously described
(13). After 1 h, the cultures were chilled, and 0.1 volume of ethanol (37) was added to each. Assays for
-galactosidase were performed by the method of Miller
(29) on cells that were permeabilized with polymyxin B
(37); specific activity is reported in Miller units.
Catalase assays were performed as previously described (13).
 |
RESULTS |
fldA insertion mutation.
The
fldA::cat mutation was constructed on a
plasmid (Fig. 1). However, we were unable
to transfer the mutation to the chromosome by common methods of
transformation with linear DNA (33, 45). This result
suggested that the mutation might be lethal, in which case the host
chromosome should accept fldA::cat by
substitutive recombination only if the cell has a second copy of
fldA+. We introduced a second copy of
fldA+ as part of a nonreplicating circular
element that integrated into att
(see Materials and
Methods). The cloned DNA was a DraI segment of
pRMEcoR1 (Fig. 1) in which fldA was the only open reading frame (GenBank accession no. AE000372 [F. R. Blattner]). The att
::(fldA+ bla)
element was then transduced into JC7623, a recBC sbcBC
strain that can undergo chromosomal transformation. The resulting
fldA+ diploid, BW1528, could now be transformed
to Camr by a double crossover with plasmid
pPG1::cat (containing
fldA::cat) to yield strain BW1529. The
Camr trait of BW1529 could be crossed out in transductions
with strain CAG18433, which contains a Tn10 near
fldA; the linkage was 50% (24 of 48). Therefore, in strain
BW1529, the insertion mutation is in the normal chromosomal
fldA locus; it is not on a plasmid or in the inserted
att
::fldA+ element.

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FIG. 1.
fldA plasmids. Only the inserted E. coli DNA is shown, together with part of the multiple cloning
site. pRMEcoRI was digested with ClaI and religated, thereby
removing all but one HindIII site and producing plasmid
pPG1. A chloramphenicol resistance (cat) element from
Tn9 was excised from plasmid pMOB02 as a 1.9-kb
FspI fragment and ligated into the HindIII
site of pPG1 to produce pPG1::cat. For clarity,
the DraI sites that were used for subcloning fldA
in pWB52 are shown on only one of the plasmids.
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fldA is a vital gene.
An
fldA::cat insertion mutation was
readily transduced into a strain carrying two copies of
fldA+, but it could not be efficiently
transferred to a wild-type strain under aerobic or anaerobic conditions
(Table 2). This result suggested that the
insertion mutation is lethal. A nearby Tn10 served as a
control for the efficiency of transduction. It was easily transferred
to strains carrying either one or two intact copies of fldA.
If the fldA::cat mutation is lethal,
the number of Tetr recombinants of the wild-type strain
should be reduced by the number that would have coinherited
fldA::cat. Compared to the fldA+ diploid, the wild-type strain had 71%
fewer Tetr recombinants aerobically and 48% fewer
anaerobically (Table 2). These results, too, are consistent with the
inviability of the Camr cotransductants. fldA is
the only open reading frame from its region that is contained within
the DraI fragment that was cloned in the att
element (Fig. 1) (GenBank accession no. AE000372). The fact that this
element complements the lethality of
fldA::cat indicates that the defect is
due solely to the loss of fldA function and not to a polar
effect of the insertion mutation.
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TABLE 2.
P1-mediated transduction of
fldA::cat and a nearby Tn10
into strains carrying either one or two copies
of fldA+
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Four fldA::cat recombinants in an
apparently haploid strain were observed (Table 2). Although this result
might seem to argue against the lethality of the mutation, it was
expected because E. coli acquires tandem duplications of any
chromosomal gene at a frequency of about 1% (2);
substitution of one tandem fldA+ allele by
fldA::cat would leave the second copy
of fldA functional. The presence of intact fldA
genes was confirmed by PCR. Aerobic transduction of the haploid strain
was repeated on a larger scale, and eight Camr recombinants
were analyzed. The PCR primers were complementary to sequences that
flank the cat insertion site. All eight transductants yielded the 248-bp products that were expected from templates containing uninterrupted fldA genes.
The following experiment produced independent evidence for the
lethality of fldA::cat. If
fldA is vital, we should not be able to eliminate the
att
::fldA+ element from
a cell bearing the fldA::cat mutation.
To cure strains of this element, they were infected with
c+ bacteriophage at a high multiplicity.
During lysogenization, the transient production of the phage-encoded
Int and Xis proteins should lead to the excision and subsequent loss of
the nonreplicating att
::(fldA+
bla+) element. The method was the same as that for the
curing of
prophages by superinfection (38). After
infection of BW1528 [fldA+
att
::(fldA+
bla+), 24% (23 of 96) of the tested colonies lost the
att
element (i.e., became Amps). However,
none (0 of 89) of the tested colonies of BW1529
[fldA::cat att
::(fldA+
bla+)] were cured, indicating that a functional
fldA gene is essential for viability.
An fldB insertion mutant is viable.
In plasmid
pPG5 (Fig. 2), a tetracycline resistance
element replaced most of the fldB gene. The mutation was
transferred to the chromosome of strain JC7623 (see Materials and
Methods). All twenty of the Tetr recombinants examined were
Amps, i.e., plasmid free. Four of the transformants were
used as PCR templates in reactions with primers that bracketed the
insertion site (see Materials and Methods). The mutant DNAs generated
the 1.6-kb products expected for the interrupted gene, and they failed to yield the 0.6-kb product that was obtained from
fldB+ chromosomal DNA (results not shown).
Although the recombinants no longer had an intact copy of
fldB, their colony size on LB agar was observed to be equal
to that of their fldB+ parents under both
aerobic and anaerobic conditions. Their growth rate and viability in LB
broth under stringent anaerobic conditions (with Na2S) were
also the same as those of their fldB+ parent.
Therefore, unlike the fldA insertion mutant, the
fldB mutant is viable.

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FIG. 2.
fldB plasmids. Plasmid pPG2, an
fldB+ plasmid with a high copy number (due to a
partial rop deletion), was produced by replacing the
EcoRI-PvuII region of pBR322 with a 3.4-kb
EcoRI-PvuII fragment of Kohara phage 469.
Plasmid pPG5 is a derivative of pPG2 in which most of the
fldB gene was replaced by a 1.6-kb segment of pBR322
containing the tet gene. The HindIII site is
in the ribosome-binding site for fldB. The thin lines
represent pBR322 DNA.
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fldB mutants are still prototrophs.
The
fldB::tet mutation was transduced from
strain BW1485 into strain W3110, an ancestral K-12
(F

) prototroph. Three transductants were
plated on a glycerol-fumarate minimal medium under both aerobic and
anaerobic (80% N2-20% CO2) conditions and on
VB (glucose) minimal medium under aerobic conditions. Their growth was
indistinguishable from that of the fldB+
parental strain grown on the same media.
Multiple copies of fldB cannot substitute for
fldA.
Why is an fldB mutation not lethal whereas
an fldA mutation is lethal? Either flavodoxins I and II are
needed for different reactions or, if they participate in the same
reactions, the activity of FldB may be too low to permit an
fldA mutant to survive. To see if overproduced FldB could
substitute for FldA, we attempted to transduce an
fldA::cat mutation into a strain
carrying pPG2, a high-copy-number fldB plasmid. The plasmid
contains a functional promoter for fldB, which was used to
construct an fldB'-'lacZ gene fusion, as described in the
next section. The results (Table 3) were
similar to those obtained with a wild-type plasmid-free strain (Table
2): whereas a Tn10 marker was readily transferred, the
closely linked fldA::cat mutation was
not. In the control experiment, both markers were transferred
efficiently to a strain carrying two copies of
fldA+. Therefore, even a high gene dose of
fldB cannot prevent the lethality of an fldA
mutation. These results suggest that in E. coli there is at
least one vital reaction that specifically requires FldA.
fldB belongs to the soxRS regulon.
An
fldB'-'lacZ protein fusion was constructed on a plasmid
(pWB51; Table 1) and transferred by recombination to
RZ5. The
RZ5::fldB' prophage contained most of
fldB and 1.7 kb of the upstream chromosomal region including
the divergently transcribed xerD gene. The cloned sequence
was preceded by transcriptional terminators from vector pRS414. The
construction fused the promoter, ribosome binding site, and first 356 nucleotides (nt) of fldB to a 5'-truncated lacZ
gene. It is unlikely that the cloned portion of fldB
contained a promoter for a downstream gene because in the chromosome
the next two open reading frames are transcribed in the opposite
direction (GenBank accession no. AE000373). The expression of the
fldB'-'lacZ fusion was determined by measuring the
-galactosidase activity in a lac deletion mutant (GC4468) containing the prophage. It was not significantly affected (
20%) by
anaerobic growth in liquid media. Treatment with 1 mM
H2O2 resulted in an induction of catalase
activity (7.5-fold) but not of fldB expression (
20%).
Therefore, fldB does not belong to the OxyR regulon.
However, fldB expression was induced fivefold by paraquat
(methyl viologen), and the induction was blocked by a mutation in the
soxS gene (Fig. 3). The
unresponsiveness of the soxS mutant was not caused by a
general inhibition of protein synthesis: the cell mass increased
7.5-fold during the 2-h treatment. These results indicate that
fldB is a member of the soxRS regulon.

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FIG. 3.
SoxS-dependent induction of an
fldB'-lacZ fusion by paraquat. At zero time, paraquat was
added to aerated log-phase cultures to a final concentration of 0.2 mg/ml. Samples were removed periodically, and the specific activity of
-galactosidase was measured in Miller units (29). The
strains used were GC4468( RZ5::fldB) and its
soxS3::Tn10 derivative, BW1535.
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soxS expression is not affected by fldB.
If
FldB is required to maintain the normal redox balance of the cell, then
an fldB mutant might constitutively overexpress soxS, the transcription of which is activated by the
oxidized form of SoxR (19, 21). It might at least sensitize
soxS to induction by a redox cycling agent such as paraquat,
which depletes the cell of reducing equivalents while forming
superoxide. To test this hypothesis, the
fldB::tet insertion/deletion mutation was transduced into a soxS'-'lacZ fusion strain. The
specific activities of
-galactosidase in the
fldB+ and fldB mutant strains (BW1157
and BW1534) were the same (86 U). Inducibility by paraquat was tested
at concentrations of from 0.01 to 1.0 mg/ml. Similar levels of
induction were observed in both strains. For example, at 0.01 mg/ml,
which produced about 40% of maximum induction, values for the wild
type and mutant were 2,626 and 2,418 U, respectively.
The gene dose of fldB does not affect paraquat
sensitivity.
The soxRS regulon protects the cell
against univalent redox cycling compounds. Thus, soxS
mutants display an increased sensitivity to killing by paraquat
(47). This is also a property of an fpr mutant
(7) which lacks a soxS-regulatable
NADPH:ferredoxin (flavodoxin) reductase. It was therefore reasonable to
suspect that the copy number of fldB might affect paraquat
sensitivity. The gradient plate technique (15) was used to
test strain W3110 (fldB+) together with three of
its derivatives: BW1531 (fldB::tet), W3110(pPG2[fldB+]), and
W3110(pPG5[fldB::tet]). The LB
agar contained a gradient of from 0 to 120 µg of paraquat/ml and, for
the plasmid-bearing strains, a uniform concentration of ampicillin. All
four strains showed the same degree of sensitivity: 60 to 70 mm of
growth along the gradient.
 |
DISCUSSION |
fldA is 348 nt upstream of fur,
which encodes an iron-responsive regulatory protein. The two genes form
an operon belonging to the soxRS regulon, and fur
is also transcribed independently from an OxyR-regulated promoter
(49). In our experiments, the fldA::cat mutation was complemented by
an att
element that contained fldA. Therefore,
the lethality of fldA::cat must be
directly due to a loss of fldA function. However, we have
not excluded the formal possibility that the lethal effect is due to
the loss of fldA combined with the noninducibility of
fur.
In in vitro reactions, the anaerobic ribonucleoside triphosphate
reductase (encoded by nrdD and nrdG) requires
FldA; ferredoxin cannot be substituted for it (9). It should
be an essential enzyme for anaerobic growth. However, nrdD
and nrdG mutants fail to grow only under the most stringent
anaerobic conditions, in a low-redox-potential broth medium containing
Na2S (20). These were conditions that we could
not apply to our plating experiments (Tables 2 and 3). The
nrd mutants grew well on solid media in oxygen-depleted
chambers. Therefore, the lethality of an
fldA::cat mutation under our plating
conditions cannot be attributed to its effect on anaerobic nucleotide
reduction alone; there must be at least one other essential pathway
requiring FldA. Similarly, inviability could not be explained by the
requirement of pyruvate formate-lyase for flavodoxin (36). A
mutant lacking the lyase gene grows anaerobically, displaying only a
mild requirement for acetate (35). The results of anaerobic
transductions, similar to those in Table 2, were not significantly
altered by the addition of 5 mM acetate to the medium (results not shown).
Although there is as yet no known phenotype associated with an
fldB mutation, there is strong evidence that the wild-type allele is a functional gene in E. coli. Our
fldB'-'lacZ fusion depended for its expression on both the
fldB promoter and fldB ribosome binding site, and
the hybrid protein was expressed constitutively at a high level. In
addition, its expression was regulated by SoxS (Fig. 3). Even if the
gene product itself is not functional in E. coli, it is
likely to be closely derived from one that is functional in another
organism. Salmonella enterica serovar Typhimurium, a close
relative of E. coli, also has an fldB gene next
to xerD (BLAST program, version 2.0).
Apart from S. enterica serovar Typhimurium FldB, there are
no known or predicted proteins that have a degree of identity with E. coli FldB that exceeds that of FldA (43%). There are,
however, known or predicted proteins from other organisms that are
about 39 to 49% homologous to both FldA and FldB of E. coli. They include putative flavodoxins from Anabaena
sp., Synechoccus sp., Klebsiella aerogenes,
Azotobactor sp., and Haemophilus influenzae. In
the paralogs, the regions of sequence similarities appear to be
distributed throughout the length of the polypeptides. In summary, the
available evidence based on current sequencing data does not indicate
that FldB is a member of a distinct subclass of flavodoxins found among distantly related bacteria.
The xerD-fldB intergenic region (Fig.
4) contains sequences that are similar to
those of known regulatory regions. Two sources using different
algorithms predicted an fldB promoter with the same
transcriptional start site (Fig. 4) (M. G. Reese, Neural network
promoter prediction tool,
http://www.fruitfly.org/seq_tools/promoter.html [revision date,
18 December 1999; last date accessed, 29 December 1999]; GenBank
accession no. AE000373) predict an fldB promoter with the
same transcriptional start site (Fig. 4). The putative
10 hexamer
5'-TACACT-3' is preceded by a 5'-TGN-3' sequence characteristic of
"extended
10" regions that are found in promoters lacking recognizable
35 hexamers (5). Near the
35 region, on the antisense strand, are two overlapping sequences that resemble a
SoxS-binding site, or "soxbox" (26, 46). Although they
might regulate the xerD gene, they appear to be too close to
it, and there is no apparent reason why xerD should belong
to an oxidative stress regulon. Physical and genetic studies are needed
to confirm that these are indeed regulatory sequences.

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FIG. 4.
Putative regulatory sequences upstream of
fldB. The sequence shown begins at nt 3037752 of the
E. coli genome (GenBank accession no. AE000373). The 10
hexamer of the putative 70 promoter of fldB
is lightly underlined, as are the bases complementary to the end of 16S
rRNA in the putative Shine-Dalgarno sequence (SD). The solid bars
underline nucleotides that match the consensus soxbox (SoxS binding
site) sequence,
5'-AX2GCA(C/T)X2(T/A)2(G/A)XCAAAX3(A/T)(A/T)-3'
(46). +1, predicted transcriptional start site for
fldB.
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The inclusion of fldB together with fldA and
fpr in the soxRS regulon underscores the
importance of flavodoxins in this global response to oxidative stress.
The soxRS regulon is induced by the depletion of reducing
equivalents through univalent redox reactions. The induced flavodoxins
together with their reductase may facilitate the restoration of the
redox equilibrium that must accompany recovery from oxidative stress.
 |
ACKNOWLEDGMENTS |
We acknowledge the valuable technical assistance of Fred Kung and
Jason Kron.
This work was supported by National Science Foundation Research Grant
MCB-9996231.
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FOOTNOTES |
*
Corresponding author. Mailing address: Glenn Memorial
Building, 69 Butler St., S.E., Atlanta, GA 30303. Phone: (404)
616-0602. Fax: (404) 616-7455. E-mail: bweiss2{at}emory.edu.
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Journal of Bacteriology, April 2000, p. 1788-1793, Vol. 182, No. 7
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