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Journal of Bacteriology, August 2000, p. 4478-4490, Vol. 182, No. 16
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Phosphate Starvation-Inducible Proteins of
Bacillus subtilis: Proteomics and Transcriptional
Analysis
Haike
Antelmann,
Christian
Scharf, and
Michael
Hecker*
Institut für Mikrobiologie,
Ernst-Moritz-Arndt-Universität, 17487 Greifswald, Germany
Received 2 March 2000/Accepted 17 May 2000
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ABSTRACT |
The phosphate starvation response in Bacillus subtilis
was analyzed using two-dimensional (2D) polyacrylamide gel
electrophoresis of cell extracts and supernatants from
phosphate-starved cells. Most of the phosphate starvation-induced
proteins are under the control of
B, the activity of
which is increased by energy depletion. In order to define the proteins
belonging to the Pho regulon, which is regulated by the two-component
regulatory proteins PhoP and PhoR, the 2D protein pattern of the wild
type was compared with those of a sigB mutant and a
phoR mutant. By matrix-assisted laser desorption ionization-time of flight mass spectrometry, two alkaline phosphatases (APases) (PhoA and PhoB), an APase-alkaline phosphodiesterase (PhoD), a
glycerophosphoryl diester phosphodiesterase (GlpQ), and the lipoprotein
YdhF were identified as very strongly induced PhoPR-dependent proteins
secreted into the extracellular medium. In the cytoplasmic fraction,
PstB1, PstB2, and TuaD were identified as already known PhoPR-dependent
proteins, in addition to PhoB, PhoD, and the previously described PstS.
Transcriptional studies of glpQ and ydhF
confirmed the strong PhoPR dependence. Northern hybridization and
primer extension experiments showed that glpQ is
transcribed monocistronically from a
A promoter which is
overlapped by four putative TT(A/T)ACA-like PhoP binding sites.
Furthermore, ydhF might be cotranscribed with phoB initiating from the phoB promoter. Only a
small group of proteins remained phosphate starvation inducible in both
phoR and sigB mutant and did not form a unique
regulation group. Among these, YfhM and YjbC were controlled by
B-dependent and unknown PhoPR-independent mechanisms.
Furthermore, YtxH and YvyD seemed to be induced after phosphate
starvation in the wild type in a
B-dependent manner and
in the sigB mutant probably via
H. YxiE was
induced by phosphate starvation independently of
B and PhoPR.
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INTRODUCTION |
Phosphate starvation induces the
specific Pho regulon as well as the
B-dependent general
stress regulon in Bacillus subtilis.
B-dependent general stress proteins (Gsps) are thought
to provide nongrowing cells with nonspecific, multiple, and prospective
stress resistance. These proteins seem to be involved in the protection of DNA, membranes, and proteins against oxidative stress and appear to
contribute to the survival of extreme environmental conditions, such as
heat or osmotic stress as well as acid or alkaline shock of starved
cells (3, 11, 14, 15).
The expression of the Pho regulon genes requires the two-component
regulatory proteins PhoP and PhoR and enables cells to use limiting
phosphate resources more efficiently or to make accessible alternative
phosphate sources. These Pho regulon genes include the two major
vegetative alkaline phosphatase (APase) structural genes,
phoA and phoB (5, 18), which account
for 98% of total APase activity; a gene encoding an APase-alkaline
phosphodiesterase (APDase), phoD (9), which has a
putative role in cell wall teichoic acid turnover; the high-affinity
phosphate transport operon, pstSACB1B2 (12, 33);
the tuaABCDEFGH operon, which is responsible for the
synthesis of teichuronic acid, which replaces the teichoic acid in the
cell walls of phosphate-starved cells (23); the teichoic
acid biosynthesis operons, tagAB and tagDEF, whose transcription was shown to be repressed by PhoP and PhoR (22); and the phoPR operon, encoding PhoP and
PhoR (35, 36). The activation or repression of Pho regulon
gene transcription require PhoP-phosphate (PhoP-P), which binds to four
TT(A/T)ACA-like sequences repeated at intervals of 11 bp and separated
by approximately 5 bp in the promoter regions of phoA,
phoB, phoD, pstS, tuaA, and
tagAD (8, 21, 22, 23, 32). Gel retardation assays suggested that all four repeats were required for PhoP-P binding and
transcriptional activation; therefore, this conserved sequence arrangement was termed the core binding region (32). It has been shown that a dimer of PhoP-P is able to bind two consensus repeats
in a stable fashion (8). Interestingly, the stronger Pho
regulon phoA and pstS promoters contain secondary
PhoP binding sites which consist of fewer than four TT(A/T/C)ACA-like
repeats within the coding region and which are required for promoter
activation (24). The phoD promoter was
characterized as the strongest Pho regulon promoter and contains the
core binding region and a 5' secondary binding region which is
important for coordinated PhoP binding to the core binding region
(8). It was hypothesized that PhoP binding to the core and
secondary binding regions results in DNA loop formation to activate
transcription from the stronger Pho regulon promoters (8).
In this study, the phosphate starvation response in B. subtilis was analyzed using two-dimensional (2D) protein gel
electrophoresis (proteome analysis) to identify new phosphate
starvation-inducible (Psi) proteins. Because the APases are secreted
into the extracellular space, we also analyzed the 2D pattern of
extracellular proteins (secretome analysis). By comparison of the
B. subtilis wild type with a sigB mutant and a
phoR mutant, the Psi proteins were allocated to the
respective regulons. By matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry, the
glycerophosphoryl diester phosphodiesterase GlpQ and the lipoprotein
YdhF were identified as new members of the Pho regulon.
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MATERIALS AND METHODS |
Bacterial strains, growth conditions, and APase measurement.
The bacterial strains used were B. subtilis 168 (trpC2) (1), B. subtilis ML6
(trpC2
sigB::
HindIII-EcoRV::cat)
(20), and B. subtilis phoR (trpC2
phoR
BalI::tet) (12). B. subtilis strains were cultivated under vigorous agitation at
37°C in a synthetic medium described previously (3).
Starvation for phosphate was provoked by cultivating the bacteria in a
medium containing 0.16 mM KH2PO4.
Units of APase activity were calculated as the amount that hydrolyzed 1 nmol of p-nitrophenyl phosphate in 1 min at 30°C and pH
8.0 in 1 M Tris-Cl. Units per milliliter of culture were defined as 2/3
[(A410 × 352)/reaction time (minutes)]
(27).
Preparation of the cytoplasmic protein fraction.
Cells grown
under phosphate starvation conditions were labeled with 5 µCi of
L-[35S]methionine per ml for 5 min at
different times along the growth curve. The control was labeled at an
optical density at 500 nm (OD500) of 0.4, and the other
samples were taken beginning at the transient phase
(t0) at time intervals of 30 min.
L-[35S]methionine incorporation was stopped
by the addition of chloramphenicol and an excess of cold methionine as
well as by transfer of the culture to ice. Cells were disrupted by
sonication, and crude protein extracts were separated from the cell
waste by centrifugation. The supernatant containing the whole-cell
soluble fraction was separated by analytical 2D polyacrylamide gel
electrophoresis (PAGE).
Preparation of the extracellular protein fraction.
B.
subtilis cells were grown in 1 liter of minimal medium under
phosphate starvation conditions and harvested at an OD500 of 0.4 for the control and 1 h after entry into the stationary phase (OD500 = 0.8) for the other samples. The cells
were harvested by centrifugation for 20 min at 4°C. The extracellular
proteins in the supernatant were precipitated with 10% (wt/vol)
trichloroacetic acid overnight on ice and centrifuged for 2 h. The
resulting protein pellet was washed with 96% ethanol (vol/vol) three
times and dried.
Analytical and preparative 2D PAGE.
Analytical 2D PAGE was
performed using the immobilized pH gradient (IPG) technique described
by Bernhardt et al. (4). The protein samples were separated
using IPG strips (Amersham Pharmacia Biotech, Piscataway, N.J.) in the
pH range of 3 to 10. For identification of the proteins by mass
spectrometry, protein samples of 400 µg were separated by preparative
2D PAGE and the gels were stained with Coomassie blue R-250.
Peptide mass fingerprinting.
In-gel tryptic digestion was
performed using a peptide-collecting device (29). Peptide
solution (0.5 µl) was prepared with equal volumes of saturated
-cyano-4-hydroxycinnamic acid solution in 50% acetonitrile-0.1%
trifluoroacetic acid (vol/vol) to a sample template for a MALDI-TOF
mass spectrometer (Voyager DE-STR; PerSeptive Biosystems). Peptide mass
fingerprints were analyzed using MS-Fit software
(http://prospector.ucsf.edu.).
Analysis of transcription.
Total RNA of the B. subtilis strains was isolated from cells at different times along
the growth curve under phosphate starvation conditions by the
acid-phenol method of Majumdar et al. (25). The control
sample was taken at an OD500 of 0.4, and the other samples
were taken beginning at t0 at time intervals of
30 min. Northern blot analyses were performed as described previously (39). Hybridization specific for glpQ,
ydhF, yjbC, yfhM, and yxiE
was tested with digoxigenin-labeled RNA probes synthesized in vitro
with T7 RNA polymerase from T7 promoter-containing internal PCR
products of the respective genes using the primers listed in Table
1.
The synthetic oligonucleotide primers complementary to the N
terminus-encoding region of the
glpQ and
yjbC
genes (shown in
Table
1) were 5' end labeled with
[

-
32P]ATP and used as primers for primer extension
analysis as described
previously (
39). The corresponding
promoter region was sequenced
using as a template PCR products
containing the promoter region
of the respective genes and the primers
shown in Table
1 as described
by Sanger et al. (
34).
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RESULTS |
Analysis of the cytoplasmic proteins from
phosphate-starved B. subtilis cells.
Proteome
analysis is an excellent tool for the comprehensive understanding of
gene expression regulation in response to changing environmental
conditions. Previous studies done by Eymann et al. (12) and
based on the carrier ampholyte isoelectric focusing technique showed
that nearly 20 proteins were induced after phosphate starvation. The
induction of nine of these was dependent on the two-component system
PhoPR. Among these proteins, the main protein was identified as PstS,
the binding component of the high-affinity phosphate transport system.
In this study, the IPG technique was used and resulted in improved
resolution and reproducibility of the 2D gels. To allocate the
phosphate starvation-inducible (Psi) proteins to different regulons, we
compared the 2D pattern of the wild type (Fig.
1) with the 2D patterns of the
phoR mutant (Fig. 2) and the
sigB mutant (Fig. 3).

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FIG. 1.
Synthesis of cytoplasmic phosphate starvation-inducible
proteins in the B. subtilis wild type labeled with
L-[35S]methionine before (168 control) and
1 h after entry into the stationary phase provoked by phosphate
starvation (168 1h phosphate starvation). PhoPR-dependent proteins are
indicated by boxes, PhoPR-independent proteins are indicated by broken
underlining, and proteins which are controlled by both B
and other sigma factors are indicated by solid underlining. The
remaining proteins are B-dependent Gsp proteins.
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FIG. 2.
Synthesis of cytoplasmic phosphate starvation-inducible
proteins in the B. subtilis phoR mutant labeled with
L-[35S]methionine before ( phoR
control) and 1 h after entry into the stationary phase provoked by
phosphate starvation ( phoR 1h phosphate starvation). For
details, see the legend to Fig. 1.
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FIG. 3.
Synthesis of cytoplasmic phosphate starvation-inducible
proteins in the B. subtilis sigB mutant labeled with
L[35S]methionine before (ML6 control) and
1 h after entry into the stationary phase provoked by phosphate
starvation (ML6 1h phosphate starvation). For details, see the legend
to Fig. 1.
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Cytoplasmic proteins belonging to the Pho regulon were identified as
the APase PhoB; the APase-APDase PhoD; the phosphate
transport system
components PstS, PstB1, and PstB2; and the UDP-glucose
6-dehydrogenase
TuaD, which is involved in the biosynthesis of
teichuronic acid. These
proteins were strongly induced in the
wild type as well as in the
sigB mutant but failed to be induced
in the
phoR
mutant after phosphate starvation. However, there
was low-level
induction of PstS in the
phoR mutant, a result which
was
also obtained in previous transcriptional studies (
33).
Only
small fractions of the APase PhoB and the APase-APDase PhoD
were found
in the cytoplasm. The majority of these APases-APDases
are
secreted into the
medium.
Besides the proteins belonging to the phosphate-specific Pho regulon,
the
B-dependent general stress proteins (Gsps) are
induced after phosphate
starvation. As shown in Fig.
1, the
B-dependent proteins were induced in the wild type as
well as in
the
phoR mutant. However, we observed that the
induction of the
B-dependent proteins was stronger in
the
phoR mutant than in the
wild type. Most of these Gsp
proteins have been identified in
previous 2D studies (
2,
4,
30) or by other approaches,
such as oligonucleotide hybridization
(
31) and DNA array technology
(A. Petersohn, J. Hoheisel,
and M. Hecker, unpublished data).
The GTP cyclohydrolase MtrA was
identified as a new member of
the
B regulon (Fig.
1).
Furthermore, the proteins with currently unknown
functions

YdaT, YaaQ,
and YsnB

which were identified as
B-dependent proteins
by the last two approaches (
31; Petersohn
et al.,
unpublished), could be localized on the proteome
map.
In the next step, we investigated whether any genes are induced by
phosphate starvation independently of either PhoPR or
B.
Only five proteins induced by phosphate starvation in a
B- or PhoPR-independent manner could be identified; they
were YvyD,
YtxH, YjbC, YfhM, and YxiE. Among these, a few
B-controlled and PhoPR-independent proteins remained
phosphate
starvation inducible even in a
sigB mutant
background, indicating
complex regulation by other sigma factors in
addition to
B. YvyD and YtxH were presumed to be induced
in the wild type via
B and in the
sigB mutant
in a
H-dependent manner after phosphate starvation
because both proteins
were phosphate starvation inducible in the
sigB mutant (Fig.
3).
The Psi protein YjbC, with
still-unknown function, and YfhM, which
is similar to epoxide
hydrolases, appeared to be regulated by
B and
extracytoplasmic function sigma factors (
16,
31;
Petersohn
et al., unpublished). All these proteins belong to the
B regulon, forming a link to other Psi regulons. Only
YxiE and
Psi40 were induced by phosphate starvation independently of
B and PhoPR. The identification of Psi40 by MALDI-TOF
mass spectrometry
failed.
Analysis of the secretome from phosphate-starved B. subtilis cells.
The Pho regulon includes the APases PhoA and
PhoB and the APase-APDase PhoD, which were used as reporter enzymes for
monitoring the phosphate starvation response. Because these enzymes
were expected to be secreted into the medium, we also analyzed the extracellular protein fraction from phosphate-starved cells using 2D
gels (secretome analysis). The supernatant was collected when APase
activity was at the maximum. As shown in Fig.
4, the PhoPR-dependent APase PhoB and the
APase-APDase PhoD could be identified as very strongly induced and most
prominent proteins on the secretome map. Surprisingly, we detected only
a low level of the major APase PhoA, which should account for 75% of
the total APase activity. Furthermore, the phosphate binding protein
PstS was also found in the extracellular fraction. Besides these known
Pho regulon proteins, we also identified a second, very strongly
induced phosphodiesterase, the glycerophosporyl diester
phosphodiesterase GlpQ, involved in the hydrolysis of deacylated
phospholipids. Furthermore, the lipoprotein YdhF, with still-unknown
function, was identified as a new member of the Pho regulon secreted
into the medium. Besides these PhoPR-dependent proteins, a few
extracellular proteins induced by phosphate starvation in a PhoR mutant
could be identified (data not shown). Among these were the serine
protease Vpr, YncM, the pectate lyase Pel, and XkdE, encoded by the
prophage PBSX genome.

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FIG. 4.
Extracellular phosphate starvation-inducible proteins in
the B. subtilis wild type. PhoPR-dependent proteins are
indicated by boxes, and the other labeled proteins are
PhoPR-independent phosphate starvation-inducible proteins. The 2D gels
were stained with Coomassie blue R-250.
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Transcriptional studies of glpQ and ydhF as
new members of the Pho regulon.
To verify the protein data and
explain the regulatory mechanisms of gene expression after phosphate
starvation, we used Northern hybridization and primer extension analyses.
By secretome analysis, GlpQ was identified as a very strongly phosphate
starvation-induced protein. The
glpQ gene is part
of the
glpQT operon, encoding a glycerol 3-phosphate (G3P)
permease
and a glycerophosphoryl diester phosphodiesterase which is
involved
in the hydrolysis of deacylated phospholipids (
10,
28,
37).
Northern hybridization analyses using a
glpQ-specific mRNA probe
showed very strong induction of the
1.1-kb
glpQ-specific transcript
after phosphate starvation
in the wild type; the transcript failed
to be induced in the
phoR mutant (Fig.
5B). This
result indicates
that
glpQ is transcribed monocistronically
after phosphate starvation
from a promoter which is located upstream of
glpQ and which requires
the PhoPR system for transcriptional
activation. We were not able
to detect the 2.4-kb bicistronic
glpTQ transcript shown by Nilsson
et al. (
28). In
primer extension experiments, the 5' end of
the
glpQ-specific message was preceded by the sequence 5'
ACACGC-N
17-TATTAT
3' (Fig.
5C and D). The promoter has a

10 region that is similar
to the consensus sequence for a
A promoter; however, there is no sequence similarity at
the

35
region. Because induction depends on the PhoPR system, we
examined
the promoter region for putative PhoP binding sites. The
putative
"Pho boxes," consisting of the four TT(A/T)ACA-like
direct repeats,
are located from

22 to

60 in the promoter region of
glpQ and
are underlined in Fig.
5.

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FIG. 5.
Physical organization of glpTQ (A),
transcript analyses of glpQ (B and C), and sequence of the
glpQ promoter region (D). For Northern blot (B) and primer
extension (C) experiments, 10 µg of RNA each was isolated from
wild-type B. subtilis 168 and the phoR mutant
before (control) and at different times after (30, 60, 90, 120, and 150 min) entry into the transient phase (t0)
provoked by phosphate starvation. The probable 5' end of the
A-dependent glpQ message is marked by +1 (C
and D). The 10 and 35 promoter sequences of the
A-dependent transcript are indicated by boxes, and the
putative PhoP binding sites are underlined (D). The dideoxy sequencing
ladder (ACGT) (C) extends from the same primer as that used for the
primer extension experiments and is complementary to that determined by
DNA sequencing.
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The second new PhoPR-dependent gene identified was
ydhF. The
ydhF gene is located downstream of the APase gene
phoB (Fig.
6A).
Transcriptional studies using a
ydhF-specific mRNA probe
showed the strong induction after phosphate starvation in the
wild type
of a 2.2-kb transcript which failed to be induced in
the
phoR mutant (Fig.
6B). This result indicates that
ydhF might
be cotranscribed with
phoB initiating
from the
phoB promoter,
which depends on the PhoPR system.

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FIG. 6.
Physical organization of the phoB-ydhF
operon (A) and transcript analysis of ydhF (B). For
Northern blot experiments (B), 10 µg of RNA was isolated from
wild-type B. subtilis 168 and the phoR mutant
before (control) and at different times after (30, 60, 90, 120, and 150 min) entry into the transient phase (t0)
provoked by phosphate starvation.
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Transcriptional studies of yjbC,
yfhM, and yxiE as
non-PhoPR-dependently phosphate starvation-induced
genes.
Besides the specific Pho regulon, there is the large
B-dependent general stress regulon which is induced
after entry into the stationary phase provoked by starvation for
phosphate. However, a small group of proteins is induced independently
of either
B or PhoPR, indicating the presence of other
groups of Psi proteins. yjbC, yfhM, and
yxiE might belong to these groups. Transcriptional studies
were performed to identify the regulatory mechanisms responsible for
B- and PhoPR-independent induction.
The
yjbC-specific transcriptional start points were mapped
in previous studies by Petersohn et al. (
31), suggesting
double
control of
yjbC by
B and probably
W. Our transcriptional data showed strong induction of a
promoter
located upstream from putative P
W after phosphate
starvation (P
1)
(Fig.
7C and
D). Transcription initiating from P
1 was induced
in a
B-independent manner. The transcriptional start point
was preceded
by the sequence 5' GAGCAG-N
17-AAAAAA 3', which
shares no similarities
with consensus sequences from known promoters.
The putative
W-dependent transcript initiating from
P
2 also was induced weakly
after phosphate starvation.
However, we did not find any induction
at the
B promoter
(P
B) after phosphate starvation (data not shown),
indicating
that
B does not contribute in a substantial
way to the induction of
yjbC in response to phosphate
starvation. This result could be
explained by competition between the
other two sigma factors and
B with the RNA polymerase
core enzyme. Northern blot data showed
the induction of a 1.2-kb mRNA,
indicating that
yjbC may be cotranscribed
with the
downstream
yjbD gene (Fig.
7B).

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FIG. 7.
Physical organization of yjbCD (A),
transcript analyses of yjbC (B and C), and sequence of the
yjbC promoter region (D). For Northern blot (B) and primer
extension (C) experiments, 10 µg of RNA each was isolated from
wild-type B. subtilis 168 and the sigB mutant ML6
before (control) and at different times after (30, 60, 90, and 120 min)
entry into the transient phase (t0) provoked by
phosphate starvation. The probable 5' ends of the
yjbC-specific transcripts are marked by +1 (C and D). The
10 and 35 promoter sequences are indicated by boxes (D). The
dideoxy sequencing ladder (ACGT) (C) extends from the same primer as
that used for the primer extension experiments and is complementary to
that determined by DNA sequencing.
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The
yfhM gene was also found during the promoter search for
B-dependent genes (
16); additionally, it was
found as a
B-dependent gene by DNA array technology
(Petersohn et al., unpublished).
Our Northern blot data showed that a
transcript of 1.8 kb was
weakly induced after more than 1 h of
phosphate starvation in
the wild type but was completely absent in the
sigB mutant (Fig.
8B). This
transcript might initiate from the
B promoter located in
front of
yfhK, which was also found as a
B-dependent gene by DNA array technology
(Petersohn et al., unpublished).
This result indicates that
yfhK,
yfhL, and
yfhM may be
cotranscribed
(Fig.
8A). Interestingly, the
B-dependent
transcript was induced severalfold more strongly in
the
phoR mutant than in the wild type. The second transcript,
corresponding to the 1.2-kb mRNA seen on the Northern blot, was
not
induced after phosphate starvation. The third, 0.9-kb transcript
seemed
to be induced in all strains tested, possibly because of
transcription
from another as-yet-unidentified promoter located
in the region
upstream of
yfhM. This phosphate starvation induction
seems
to be independent of either
B or PhoPR.

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FIG. 8.
Physical organization of the yfhKLM
operon (A) and transcript analysis of yfhM (B). For
Northern blot experiments (B), 10 µg of RNA was isolated from
wild-type B. subtilis 168, the sigB mutant ML6,
and the phoR mutant before (control) and at different times
after (30, 60, and 90 min) entry into the transient phase
(t0) provoked by phosphate starvation.
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The
yxiE gene was induced after phosphate starvation
in a
B-independent as well as a PhoPR-independent
manner. Northern blot
analyses revealed the induction of a 0.5-kb
yxiE-specific transcript
in the wild type, the
sigB mutant, and the
phoR mutant, indicating
that
yxiE is transcribed monocistronically (Fig.
9B). The mechanism
leading to the
induction of
yxiE after phosphate starvation is
unknown.

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FIG. 9.
Physical organization of the yxiE gene (A)
and transcript analysis of yxiE (B). For Northern blot
experiments (B), 10 µg of RNA was isolated from wild-type B. subtilis 168, the sigB mutant ML6, and the
phoR mutant before (control) and at different times after
(30, 60, and 90 min) entry into the transient phase
(t0) provoked by phosphate starvation.
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DISCUSSION |
The phosphate starvation response of B. subtilis was
analyzed using the 2D protein gel electrophoresis technique for
comparison of the cytoplasmic and secreted proteins from wild-type
B. subtilis, the phoR mutant, and the
sigB mutant. These analyses showed that the phosphate
stimulon consists of two main regulons, the phosphate starvation-specific Pho regulon and the
B
general stress regulon. However, there appeared to be a small group of phosphate starvation-induced proteins (e.g., YjbC, YfhM, and YxiE) which might be controlled by currently unknown mechanisms.
The Pho regulon genes are well characterized (5, 9, 18, 22, 23,
33, 35, 36), and most of these were identified on the cytoplasmic
as well as the extracellular proteome map. On the cytoplasmic proteome
map, three components of the high-affinity phosphate transport system
could be identified. These included, in addition to the previously
identified phosphate binding protein PstS (12), the ATP
binding proteins PstB1 and PstB2. PstS was the most strongly induced
protein. The low-level induction of PstS observed in the
phoR mutant is consistent with the transcriptional data for
the pstS promoter (33). Because the activated
pstS promoter could be activated only by PhoP-P, it was
proposed that PhoP might be phosphorylated by other histidine kinases
in the phoR mutant (32). The stronger induction
of pstS than of other Pho regulon genes was found to be due
to the secondary PhoP binding region in addition to the core binding
region, which is located within the coding region of pstS
(24). The integral inner membrane proteins PstA and PstC,
which are the other components of the phosphate transport system, could
not be identified, probably because the separation of hydrophobic
membrane proteins by isoelectric focusing is currently very difficult.
The very alkaline isoelectric point as well as the hydrophobicity may
be also the reasons why only the UDP-glucose 6-dehydrogenase TuaD was
localized on the proteome map and the detection of the remaining seven
proteins encoded by the tuaABCDEFGH operon failed. The tua operon encodes proteins involved in
the biosynthesis of teichuronic acids, which replace
phosphate-containing teichoic acids (23). The Pho regulon
further includes the major APase structural genes, phoA and
phoB, which account for 98% of total APase activity
(5, 17, 18, 19). APase specific activity increases 500-fold
after transition into the stationary phase under low-phosphate
conditions. A third APase structural gene, phoD, encodes an
enzyme with both APase and phosphodiesterase activities (9)
and also belongs to the Pho regulon. However, to a minor extent, the
APase PhoB and the APase-APDase PhoD could be also detected in the
cytoplasmic fraction. These proteins might represent the unprocessed
precursors of the APases still containing the signal peptides. On the
secretome map, the APase PhoB and the APase-APDase PhoD belong to the
most prominent extracellular proteins induced after phosphate
starvation. The major APase, PhoA, was identified only as a minor
phosphate starvation-induced protein probably because this enzyme has
high specific activity. It is also possible that the main fraction of
PhoA is lost because of the very alkaline isoelectric point (pI, 9.9),
which is on the border of the IPGs pH range. Furthermore, the
lipoprotein YdhF was also secreted. ydhF was shown to be
cotranscribed with phoB initiating from the PhoPR-dependent
phoB promoter.
Besides these known members of the Pho regulon, we identified GlpQ as
the second very strongly induced phosphodiesterase of B. subtilis which is regulated by the PhoPR system. The
glpTQ operon has been described as a member of
the glycerol utilization (glp) regulon of B. subtilis (28). The glpT gene encodes a G3P permease that is believed to function as an anion antiporter, where G3P
is taken up by the cell in exchange for internal phosphate (26). The glpQ gene encodes a glycerophosphoryl
diester phosphodiesterase that hydrolyzes deacylated phospholipids to
G3P. While the APase-APDase PhoD is believed to cleave the
phosphodiester bonds of teichoic acids (9), GlpQ is involved
in the degradation of phospholipids. It has been shown previously that
the glpTQ operon-specific transcript is induced
threefold by G3P (28). Therefore, it was concluded that
the B. subtilis glpTQ operon is homologous with the
Escherichia coli glpTQ operon, which is also part of
the glycerol regulon. However, in E. coli, a second
transport system recognizing G3P is encoded by the ugp
operon (6). The ugp-dependent
transport system is induced after phosphate starvation in a
PhoBR-dependent manner (6). In this case, G3P can be
used as the sole source of phosphate. The last gene of the
ugp operon, the ugpQ gene, encodes a glycerophosphoryl diester phosphodiesterase which is similar
to B. subtilis GlpQ. Our studies showed that B. subtilis glpQ is part of the Pho regulon and therefore seems
to be regulated in a manner similar to E. coli ugpQ.
However, glpQ is transcribed monocistronically after
phosphate starvation from a promoter located in the region upstream of
glpQ. It could be assumed that the G3P permease GlpT is not
needed after phosphate starvation because the G3P produced by GlpQ is a
substrate for the APases which liberate the inorganic phosphate.
E. coli seems to possess two systems involved in the
transport of G3P, the glpT-mediated system (as part of the
glp regulon) and the ugp-dependent transport
system (as part of the Pho regulon). In contrast, the B. subtilis
glpTQ operon is part of the glp regulon and, in
addition, the glpQ gene is part of the Pho regulon
controlling its expression independently of glpT.
Besides these phosphate starvation-specific proteins, the
B regulon is induced after phosphate starvation.
B-dependent general stress proteins are expected to
provide nonspecific, multiple, and prospective stress resistance to
nongrowing B. subtilis cells in anticipation of future
stress (15). Our studies have shown that a mutation in the
phoR gene not only abolishes the transcription of Pho
regulon genes but also causes the superinduction of
B-dependent proteins (Fig. 2). This finding was also
shown on the transcriptional level for the yfhM gene, which
was much more strongly induced in the phoR mutant than in
the wild type. Previous studies of Farewell et al. (13)
showed competition between
70 and
S for
limiting amounts of RNA polymerase during the stationary phase in
E. coli. Thus, we suggest that the superinduction of
B-dependent proteins in a phoR mutant after
entry into the stationary phase provoked by starvation for phosphate is
caused by an increased amount of
B bound to the RNA
polymerase in the absence of the competing PhoP activation of
A-dependent genes. Another explanation might be that in
the absence of PhoP, the expression of genes that would overcome the
phosphate starvation condition is impaired, resulting in increased
expression of
B-dependent proteins.
Previous studies of Eymann et al. (12) have shown that there
is a third group of phosphate starvation-inducible proteins. We also
identified a few proteins which remained phosphate starvation inducible
in a phoR mutant as well in a sigB mutant. These
proteins can be allocated to two subgroups of phosphate
starvation-inducible proteins. The first subgroup includes YtxH,
YvyD, YjbC, and YfhM, which are regulated by
B and
by additional mechanisms leading to
B-independent
induction by phosphate starvation. ytxH and yvyD are presumed to be induced at the
H promoter in response
to phosphate starvation if
B is not available, but
experimental evidence for this suggestion is still lacking (7,
38). YjbC and YfhM are regulated by
B and probably
by the ECF sigma factor
W, as shown in other studies
(16, 31; Petersohn et al., unpublished). The
yjbC and yfhM genes form a link between the
B regulon and the
H or
W
regulon (7, 16). The mechanisms causing induction of the
B-dependent genes yjbC and yfhM
after phosphate starvation are totally unknown. The second subgroup is
regulated completely independently of
B. yxiE
was allocated to this subgroup. Transcriptional analyses raised the question of which mechanisms other than PhoPR or
B are involved in the phosphate deficiency response of
B. subtilis. However, genes such as yjbC,
yfhM, or yxiE form only a minor part of the
phosphate stimulon. It is obvious that the most important regulons
involved in the phosphate starvation response are the specific Pho
regulon and the
B-dependent general stress regulon.
However, this study is not yet a full description of the phosphate
stimulon because integral membrane proteins still escaped our proteome
approach. The usage of DNA array techniques is a convenient strategy
for finding the still-missing genes of the phosphate stimulon.
 |
ACKNOWLEDGMENTS |
We thank G. Mittenhuber for critical reading of the manuscript.
This work was supported by grants from the DFG, the Fonds der
Chemischen Industrie, the European Commission (BIO4-CT95-0278), and the
Kultusministerium Mecklenburg-Vorpommern to M.H.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Institut
für Mikrobiologie, Ernst-Moritz-Arndt-Universität,
17487 Greifswald, Germany. Phone: 49 (3834) 864200. Fax: 49 (3834) 864202. E-mail:
hecker{at}microbio7.biologie.uni-greifswald.de.
 |
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