Department of Life Sciences, Faculty of
Agriculture, Kagawa University, Miki-cho, Kagawa, Japan 761-0795
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INTRODUCTION |
Myxococcus xanthus is a
gram-negative soil bacterium that demonstrates complex social behavior
(5, 31). Upon nutritional starvation, cells undergo a
developmental cycle involving cell-cell interactions. More than
105 cells migrate to an aggregation center and form a
fruiting body, within which cells differentiate into myxospores.
M. xanthus cells coordinate their multicellular behavior
through cell-cell communication by transmission of intercellular signals. The transmission of intercellular signals in M. xanthus has been studied by isolating development-defective
mutants, dividing them into five groups (A to E), and conducting
pairwise mixing tests of the different groups (4, 19, 20,
21). These intercellular signals control the expression of
developmentally regulated genes. M. xanthus possesses a
complex signal transduction system: two-component system (sensor
histidine kinases and response regulators), serine/threonine and
tyrosine kinases, and small GTPase (8, 11, 38). In
bacteria, the most common protein kinases involved in signal
transduction are histidine kinases. Thus far, AsgA, EspA, FrzE, and
SasS in M. xanthus have been identified as histidine kinases
(3, 23, 29, 34). Inouye and colleagues have identified a
large family of serine/threonine kinase genes in M. xanthus
and performed several studies of the functions of the encoded protein
serine/threonine kinases (15, 24, 37, 38).
On the other hand, hybrid sensors containing both a histidine kinase
domain and a receiver domain within the same molecule have also been
detected recently in eukaryotic cells (18). These hybrid
sensors play regulatory roles in eukaryotic cell function. For example,
the predicted product of ETR1 in Arabidopsis
thaliana and SLN1 in Saccharomyces
cerevisiae are involved in regulating the ethylene and
osmotic responses, respectively (12, 27). Here, we
report the molecular cloning of a gene in M. xanthus encoding a protein homologous to members of the hybrid sensor family.
Furthermore, its possible physiological functions were clarified.
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MATERIALS AND METHODS |
Strains, plasmids, and growth conditions.
The type
strain of M. xanthus, IFO13542 (ATCC 25232), was used as the
wild type. M. xanthus strains were grown at 28°C in Casitone-yeast extract (CYE) medium (2), and kanamycin (70 µg/ml) was added when necessary. The plasmids pBluescript II SK(
) (Stratagene, La Jolla, Calif.) and pT7-blue T (Novagen, Madison, Wis.)
were used for cloning. Plasmids were propagated in Escherichia coli NovaBlue (Novagen).
DNA manipulations.
The genomic DNA of M. xanthus
was partially digested with Sau3AI and then ligated into
BamHI-cleaved 
EMBL3 arms. A portion (1 µg) of the
recombinant DNA was packaged into bacteriophage particles in vitro,
using a commercial kit (Stratagene).
For detection of histidine kinases of M. xanthus, two
oligonucleotides (Oli-1 and Oli-2) were synthesized. Oli-1
(5'-GACACXGGXATCGGXATC-3') and Oli-2
(5'-GGXACXGGXCTXGGXCTXTC-3') (X can be either C or G) were
deduced from the conserved regions of the glycine-rich sequences of
histidine kinase domains. The oligonucleotides were labeled with
digoxigenin-11-dUTP using an oligonucleotide tailing kit (Roche
Diagnostics GmbH, Mannheim, Germany). The M. xanthus genomic DNA library was screened by plaque hybridization with a mixture of
probes Oli-1 and Oli-2. Three clones that hybridized to the probes were
selected. Recombinant phage DNAs were isolated from these three clones,
designated as pHK1, pHK2 and pHK3, and digested with PstI,
SalI, and SmaI. The fragments were
electrophoretically separated on agarose gels, transferred onto nylon
membranes, and then hybridized with the probes. Hybridization to
PstI-, SalI- and SmaI-digested pHK1
revealed strong bands (12, 7.0, and 2.5 kb, respectively). The
hybridizing PstI, SalI, and SmaI
fragments were ligated into pBluescript II SK, to generate pHK1-P,
pHK1-Sa, and pHK1-Sm, respectively. The recombinant plasmids were
sequenced using synthetic oligonucleotides. pHK1-Sm was also used for
construction of the mokA mutant.
Construction of the mokA disruption mutant.
Plasmid pHK1-Sm, which contains the mokA gene on a 2.5-kb
SmaI fragment, was digested with NcoI, and the
ends were blunted with T4 DNA polymerase (Fig.
1). A 1.2-kb DNA fragment containing a
kanamycin resistance (Kmr) gene was amplified by PCR using
TnV as a template and a pair of primers,
5'-GTGCTGACCCCGGGTGAATGTCAG-3' and
5'-ATCGAGCCCGGGGTGGGCGAAGAA-3', containing SmaI
sites (9). The resulting DNA fragment was digested with
SmaI and inserted into the blunted ends of pHK1-Sm. The
disrupted gene constructed as described above was amplified by PCR
using the oligonucleotide 5'-CTGGAGATTCGCTTCACG-3' for the
5' end of the 2.5-kb SmaI fragment of pHK1-Sm and
5'-GACGTGAAGGGACTGCTG-3' for the 3' end of the 2.5-kb
SmaI fragment of pHK1-Sm. The PCR products thus obtained
were introduced into M. xanthus by electroporation.
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FIG. 1.
Restriction map and gene structure of mokA.
The NcoI fragment replaced by a Kmr gene is
indicated by an arrow. TM, transmembrane domain.
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Developmental assays.
To obtain fruiting bodies, M. xanthus wild-type and mutant strains were grown in CYE medium. The
cells were harvested at late log phase, washed with 10 mM Tris-HCl-8
mM MgSO4, pH 7.6 (TM buffer), and resuspended in TM buffer
to a density of 2 × 109 cells/ml. The cells were
spotted onto CF agar plates (10) at 150 µl of cells per
plate, and the plates were incubated at 30°C. Fruiting bodies were
harvested at various times by gentle scraping of the surface of agar
plates and suspended in cold TM buffer.
For glycerol induction of spore formation, cells were grown
vegetatively in CYE medium. The cells were harvested and washed with
1% Casitone-8 mM MgSO4. Glycerol was added to a final
concentration of 0.5 M, and cells were shaken at 30°C for 10 h
(6).
For each of the above assays, undifferentiated cells were killed by
incubation at 60°C for 15 min and by five 30-s treatments with a
Branson sonifier. Spores were counted using a Petroff-Hausser counter.
The viability of spores was also confirmed on CYE agar plates.
Osmotic stress experiments.
M. xanthus wild-type
and mutant strains were grown in CYE medium, and cells were harvested
at a density of 8 × 108 cells/ml. Then aliquots of
9 × 106 cells were inoculated into 3 ml of CYE medium
containing up to 0.25 M NaCl or 0.2 M sucrose. The cultures were
incubated in suspension until the culture without NaCl or sucrose
attained a density of about 109 cells/ml. Growth of each
strain was determined by counting with a hemacytometer.
Transcript analysis.
Total RNA was isolated from M. xanthus at exponential growth phase, stationary phase, and during
development. For reverse transcriptase PCR (RT-PCR), 1 µg of RNA was
used for cDNA synthesis with BcaBEST polymerase as specified
by the manufacturer (Takara Shuzo, Kyoto, Japan). PCR was performed
with Bca-optimized Taq polymerase, 5'
gene-specific primer (5'-GCACGTCGCTCTGGGTGG-3'), and 3'
gene-specific primer (5'-CGCATCAGGCCCGAATAG-3').
Amplification products were visualized in agarose gels after
ethidium bromide staining and were recorded by a LAS-1000 system (Fuji
Film, Tokyo, Japan).
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RESULTS |
Cloning and sequencing of mokA.
Transient protein
phosphorylation is a critical component of many signal transduction
systems. To search for histidine kinase genes in M. xanthus,
we designed oligonucleotides on the basis of the conserved sequences of
histidine kinase domain and attempted to clone the histidine kinase
genes from an M. xanthus genomic library by using the
oligonucleotide probes. One clone that hybridized strongly to the
probes was selected and used for subcloning. A 2.5-kb SmaI
fragment of the clone was hybridized with the probes and used for
sequence analysis. The predicted amino acid sequence of this fragment
revealed the presence of a putative histidine kinase domain. The
nucleotide sequence of the 5.5-kb
SmaI(1)-SmaI(5) fragment of the clone that contained the complete transcription unit
was determined (Fig. 1). The complete transcription unit, predicted by
the Codon-Preference program based on the G-C codon bias of the third
position in this high-G+C-content organism, was predicted to start at
nucleotide 418 and to stop at nucleotide 4572 (14). The
entire sequence of the fragment is shown in Fig. 2; the gene was designated
mokA (M. xanthus osmosensing kinase A) because of
its phenotype, which is described below. Analysis of this sequence
predicted a coding region that encompasses 1,414 amino acids
resulting in a putative protein with an Mr of
153,000.
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FIG. 2.
Nucleotide and deduced amino acid sequences of the
region of the M. xanthus chromosome containing the
mokA gene. The sequences corresponding to probes are
indicated by dotted lines. Probable transmembrane domains of MokA are
indicated by solid lines.
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An open reading frame (ORF) consisting of 384 bp was located 39 nucleotides downstream of the mokA stop codon. However, the predicted ORF gene product showed no extensive homology with response regulators or other proteins in the GenBank database.
Predicted protein structure and functional domains.
To gain
insight into possible functions of the mokA gene product, a
homology search of the MokA amino acid sequence in the protein database
was performed. The MokA product showed sequence similarity throughout
the entire molecule except the amino-terminal region with histidine
protein kinase EspA of M. xanthus (44% identity) (3), putative sensory transduction histidine kinase
Slr2098 of Synechocystis sp. strain PCC6803 (29% identity)
(17), and osmosensing histidine kinase DokA of
Dictyostelium discoideum (27% identity) (30).
Based on sequence homology, the carboxy-terminal part of the MokA
protein consisted of two domains: the histidine kinase and response
regulator domains of bacterial two-component systems. The histidine
kinase domain of MokA (positions 815 to 1061) was most similar to those
of histidine kinases EspA and AsgA of M. xanthus (50 and
37% identities, respectively) (3, 29) and histidine
kinase HstK of Anabaena sp. strain PCC 7120 (33% identity)
(V. Phalip, G. Brandner, and C.-C. Zhang. unpublished data) (Fig. 3A).
The MokA histidine kinase domain contained all of the residues
conserved among bacterial histidine kinases, including the putative
phosphoryl group acceptor His-840. The MokA histidine kinase
domain also contained the conserved ATP-binding regions, NXXXNX25-38DXGXGX9FXPFX6-14GXGLGL,
that are highly conserved in all histidine protein kinases
(28) (Fig. 3A).
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FIG. 3.
(A) Alignment of the deduced histidine protein kinase
domain of MokA with the histidine protein kinase domains of EspA and
AsgA from M. xanthus and HstK from Anabaena sp.
strain PCC 7120. Nucleotide-binding domains are underlined; the dot
represents the position of the putative autophosphorylated histidine
residue. (B) Response regulator domains of EspA and MokA from M. xanthus, Dra0010 from D. radiodurans, and OrfX17
from B. subtilis, aligned on a conserved sequence of 110 amino acids. The dot indicates the putative phosphorylated aspartic
acid residue.
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The regulator domain of MokA (positions 1236 to 1345) was most similar
to those of the histidine protein kinase EspA of M. xanthus (32% identity) (3), putative regulator
protein OrfX17 of Bacillus subtilis (29% identity)
(32), and putative DNA-binding regulator protein Dra0010
of Deinococcus radiodurans (27% identity) (36)
(Fig. 3B). The three residues (Asp-1243, Asp-1287, and Lys-1344) that
are conserved in all of the response regulators were also present in
MokA. Sequence comparisons with MokA and other regulators suggested
that Asp-1287 is a site of phosphorylation.
The amino-terminal half of the MokA protein (positions 1 to 800) showed
no significant similarities to domains found in other histidine protein
kinases. Analysis of the mokA gene product using the
Kyte-Doolittle algorithm (22) suggested that MokA
possesses two potential transmembrane regions (amino acids 10 to 33 and 758 to 778) in its amino-terminal half, which would place the protein
in the cytoplasmic membrane (Fig. 2).
Distribution of mokA on development.
To study the
function of MokA, we constructed a mokA insertion-deletion
mutant. The 0.3-kb
NcoI(3)-NcoI(4)
fragment containing the conserved ATP-binding regions of histidine
kinase domains was replaced by a 1.2-kb fragment containing the
Kmr gene. Replacement of the wild-type mokA gene
by the defective gene was confirmed by Southern hybridization and PCR
analyses. When cultured in growth medium, CYE, the mutant grew as well
as the wild type. Under these conditions, the two strains were
morphologically identical. However, on the developmental medium, CF
agar, clear differences were observed between the two strains. The
wild-type cells moved to aggregation centers within 32 h and then
formed spherical fruiting bodies by 48 to 72 h on CF agar. Within the fruiting bodies of the wild type, rod-shaped vegetative cells were
converted to spherical myxospores (Fig.
4A). The mutant cells formed fruiting
bodies about 1 day later than the wild-type strain, and unmature spores
were observed within fruiting bodies (Fig. 4A). As a result, the spore
yield of the mutant strain was approximately 50% of that of the
wild-type strain (Fig. 4B). The spores formed in the disruption mutant
were able to germinate like the wild-type myxospores. When induced by
0.5 M glycerol, the mutant cells sporulated at the same rate as
wild-type cells (data not shown).
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FIG. 4.
Spore formation of M. xanthus wild-type and
mokA mutant strains. The cells were developed on CF agar,
and undifferentiated cells were killed by sonication and heating as
described in Materials and Methods. (A) Photographs of wild-type (W)
and mutant (M) spores taken 7 days after the start of development. (B)
Numbers of spores from wild-type (open circles) and mutant (closed
circles) strains plotted relative to the time of the development after
spotting.
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The timing and level of expression of the mokA gene were
determined by RT-PCR analysis (Fig. 5).
The expected 421-bp RT-PCR product was amplified from mRNA of
vegetative cells. The RT-PCR products declined during the early stage
of development and then reached their maximum after 24 h, when
some mounds were formed in the culture. As a control, the expected
product was not amplified without a reverse transcriptase, indicating
that there was no DNA contamination in the mRNA (data not shown).
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FIG. 5.
RT-PCR analysis of expression of the mokA
gene in M. xanthus. Total RNAs prepared from cultures at
exponential growth (E) and stationary (S) phases and during development
at 12 h (D12), 24 h (D24), and 48 h (D48) were used for
RT-PCR analysis. Molecular sizes of DNA fragments are given in bases.
MW, molecular weight marker.
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mokA mutants are osmosensitive.
Since MokA is
structurally similar to osmotic response proteins of Neurospora
crassa Nik1 (1), S. cerevisiae Sln1
(27), and D. discoideum DokA (30),
the wild-type and mokA mutant strains of M. xanthus were cultivated under osmotic stress and examined for
osmotic tolerance (Fig. 6). The wild-type
strain grew on CYE agar containing up to 0.2 M NaCl, while the mutant
showed sparse growth and no growth on CYE medium containing 0.1 and 0.2 M NaCl, respectively.
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FIG. 6.
Comparison of wild-type (W) and mokA mutant
(M) cells grown on CYE agar containing no addition, 0.1 M NaCl, and 0.2 M NaCl. The cells were streaked onto the plates and incubated at 30°C
for 3 days.
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We also monitored the growth response of the mutant in shaking liquid
culture. In the presence of 0.1 M NaCl in CYE medium, mutant cells
showed nearly a 30-fold decrease in growth compared with wild-type
cells (Fig. 7). The mutant also showed a
similar decrease in ability to grow in the presence of 0.03 to 0.1 M
sucrose (data not shown). The mutant was found to be reasonably normal with respect to stresses such as temperature shifts, pH changes, various antibiotics, and ethanol (data not shown).
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FIG. 7.
Growth of M. xanthus wild-type (open circles)
and mokA mutant (closed circles) strains in CYE medium
containing various concentrations of NaCl. Cultures were
inoculated to 3 × 106 cells/ml. Growth of each
strain was determined by counting in a hemacytometer.
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DISCUSSION |
Bacteria have the ability to sense a variety of environmental
changes and respond appropriately. The environmental changes are
transmitted into the cell by signal-transducing proteins. Two-component
signal transduction systems are among the strategies that bacteria
employ to sensor and respond to the environmental changes. The
prototypical two-component system consists of two proteins, a sensor
histidine kinase and a response regulator. A variety of forms of hybrid
sensors containing both transmitter and receiver modules in their
primary amino acid sequences have been found in prokaryotic and
eukaryotic organisms. In addition, serine/threonine and tyrosine
kinases have been found in several differential bacteria
(18). In M. xanthus, serine/threonine kinases
(Pkn1, Pkn2, Pkn5, and Pkn6) were found to regulate fruiting-body formation and to affect sporulation (35, 37).
In this study, we isolated and sequenced an M. xanthus mokA
gene which encodes a member of the hybrid histidine kinase family. The
amino-terminal domain of the mokA product did not display any significant homology to other proteins, while the protein is
predicted to contain two transmembrane regions in the amino-terminal domain. In general, the hydrophobic domains are each predicted to cross
the cytoplasmic membrane, and the hydrophilic region is predicted to be
a periplasmic input domain. The carboxy-terminal domain of MokA
contained both transmitter and receiver modules. The structural
characteristics of MokA are similar to those of E. coli RcsC
(33), N. crassa Nik1 (1), yeast
CaSln1 (25) and Sln1 (27), and D. discoideum DokA (30). The hybrid sensors Nik1,
CaSln1, Sln1, and DokA of eukaryotes act as osmosensor proteins. The
mutation of M. xanthus mokA caused a remarkable reduction in
the growth of cells at high osmolarity (0.1 to 0.2 M NaCl or 0.1 M
sucrose). The mutant responded normally against other stresses. These
data indicate that MokA acts as an osmosensor protein in M. xanthus. With repetitive subculture in CYE medium, some
mokA mutants gained osmotic tolerance (data not shown).
These observations imply that M. xanthus harbors other genes
(possibly encoding sensor kinases) that compensate for mokA
gene dysfunction to adapt to high osmolarity. In E. coli,
hybrid sensors ArcB and BarA can complement a defect in the osmotic
signal transduction caused by an osmotic sensory kinase
(envZ) mutation (13, 26).
The histidine kinases AsgA, EspA, FrzE, and SasS of M. xanthus were found to regulate fruiting-body formation (3,
23, 29, 34). AsgA is an unusual member of the hybrid histidine kinase family in that it has a receiver domain in the amino terminus and a kinase domain in the carboxy terminus and does not contain an
input domain. AsgA is essential for fruiting-body formation and is
thought to interact with other signaling proteins that have input
functions (29). SasS is predicted to be the sensor protein
in a two-component system that integrates information required for
early developmental gene expression (34). In
contrast, the hybrid histidine kinase, EspA, is thought to
function as an inhibitor of sporulation during early development
(3). The results of this study, indicated that MokA also
participates in M. xanthus development. The mokA
mutant cells developed more slowly than the wild-type strain and formed
many unusual spores under starvation conditions. After 7 days of
development, the mutant formed half as many spores as the wild-type
strain. These observations suggest that MokA is required for M. xanthus development but that development can be achieved by cross
talk among sensor and regulator proteins, compensating for the absence
of MokA. On the other hand, mutant cells deficient in osmotic tolerance
may be unable to form normal spores completely.
We constructed another insertion mutant in which the Kmr
gene was inserted into the StuI(3) site of
mokA (Fig. 1) between the transmitter and receiver domains
of MokA. The insertion mutant behaved in the same way as the
mokA insertion-deletion mutant, indicating that the receiver
domain may be required for the normal function of MokA. One ORF that
could be part of an operon with mokA was present downstream
of mokA. The ORF is the last gene of the operon and encoded
128 amino acids. Its amino acid sequence showed no homology with
response regulators or other proteins. These data indicate that the
phenotype of insertion mutants would not be due to polar effects on the
transcription of downstream genes.
The eukaryotic slime mold D. discoideum has a developmental
cycle that resembles that of M. xanthus (16).
The development of D. discoideum is regulated by a signal
transduction system consisting of protein serine/threonine kinases, G
proteins, and cyclic AMP signaling (7). DokA of D. discoideum is part of the osmotic response system and also play
key roles in fruiting-body formation. In N. crassa, the
hybrid sensor Nik1 is required for the ability of cells to grow under
conditions of high osmotic stress and for hyphal development. The
observation that mokA mutants are deficient in normal
development and osmotic tolerance demonstrated that MokA may have
functions similar to those of hybrid sensors (DokA and Nik1) of
eukaryotic microorganisms. We are currently attempting to detect other
components of the signal transduction pathway involving MokA.
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Abstract
Introduction
