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Applied and Environmental Microbiology, January 2008, p. 305-308, Vol. 74, No. 1
0099-2240/08/$08.00+0 doi:10.1128/AEM.01793-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.
Cloning of a Novel Gene Encoding β-1,3-Xylosidase from a Marine Bacterium, Vibrio sp. Strain XY-214, and Characterization of the Gene Product
Yoshiaki Umemoto,
Ryosuke Onishi, and
Toshiyoshi Araki*
Graduate School of Bioresources, Mie University, 1577 Kurimamachiya, Tsu, Mie 514-8507, Japan
Received 2 August 2007/
Accepted 16 October 2007

ABSTRACT
The β-1,3-xylosidase gene (
xloA) of
Vibrio sp. strain XY-214
was cloned and expressed in
Escherichia coli. The
xloA gene
consisted of a 1,608-bp nucleotide sequence encoding a protein
of 535 amino acids with a predicted molecular weight of 60,835.
The recombinant β-1,3-xylosidase hydrolyzed β-1,3-xylooligosaccharides
to
D-xylose as a final product.

INTRODUCTION
β-1,3-Xylan is found in the cell walls of red (
Porphyra and
Bangia spp.) and green (
Caulerpa,
Bryopsis, and
Udotea spp.)
algae (
12,
16), whereas β-1,4-xylan is contained in the
cell walls of land plants (
25). The β-1,4-xylanases (1,4-β-
D-xylan
xylanohydrolase; EC 3.2.1.8) and β-1,4-xylosidases (β-
D-xyloside
xylohydrolase; EC 3.2.1.37) required for complete breakdown
of β-1,4-xylan are useful tools for production of
D-xylose,
which is expected to be a source of xylitol and bioethanol (
5,
26). These two enzymes are produced by many microbes, including
aerobic and anaerobic mesophiles and thermophiles, and their
biochemical and molecular characteristics have been widely studied
(
7,
19,
27). However, there have been only a few reports (
1,
2,
4,
9,
29) on β-1,3-xylanases (1,3-β-
D-xylan xylanohydrolase;
EC 3.2.1.32) and no reports on a β-1,3-xylosidase that
is able to hydrolyze β-1,3-linked xylooligosaccharides
to
D-xylose.
Vibrio sp. strain XY-214 is a marine bacterium
that secretes an extracellular β-1,3-xylanase (TxyA) into
the growth medium in the presence of β-1,3-xylan as an
inducer (
3,
4). Recently, we found that the organism produces
an intracellular β-1,3-xylosidase A (XloA).
In this paper, we describe the cloning and sequence analysis of the novel gene encoding XloA and the purification and characterization of recombinant XloA (rXloA) produced by transformed Escherichia coli.

Construction of a probe for cloning of the xloA gene.
The organism was grown in peptone medium (0.5% peptone, 0.1%
yeast extract, 3% NaCl, 0.05% MgSO
4, 0.2% K
2HPO
4, 0.04% KH
2PO
4,
pH 7.6) containing 0.5%
D-xylose for 15 h at 25°C. After
centrifugation, the precipitated cells were resuspended in a
small volume of 50 mM 2-morpholineethanesulfonic acid (MES)
buffer (pH 7.0) and disrupted by sonication. The supernatant
resulting from centrifugation of the cell lysate was purified
by chromatography with DEAE-Toyopearl 650M (Tosoh, Tokyo, Japan),
Ether-Toyopearl 650S (Tosoh, Tokyo, Japan), and MonoQ 5/50 (GE
Healthcare) columns. The final enzyme preparation was ascertained
to be homogeneous by sodium dodecyl sulfate-polyacrylamide gel
electrophoresis (
13) and then transferred by blotting to a polyvinylidene
difluoride membrane (Bio-Rad). The protein band was detected
by staining with Coomassie brilliant blue G-250, and a protein
of approximately 60 kDa was excised from the membrane. The N-terminal
amino acid sequence was determined by automated Edman sequencing
with a Procise 49X-clC protein sequencer and a 140D Micro Gradient
delivery system (Applied Biosystems). A partial internal amino
acid sequence of XloA was also determined by performing the
same procedure on peptides obtained by digestion of the purified
enzyme with a lysyl endopeptidase (Wako Chemicals). Degenerate
primers were designed on the basis of the underlined parts of
the N-terminal amino acid sequences of purified XloA (sequence
TTTIQNPILKGFNPDPSIVR) and a 22-kDa digested peptide (sequence
WLSLSERPGFLRL
KGRHYLY). To construct a probe for the cloning
of the gene for XloA (
xloA), one specifically amplified oligonucleotide
was obtained from a template of genomic DNA of
Vibrio sp. strain
XY-214 by PCR with the degenerate primers. The PCR product ligated
into a pT7Blue vector (Novagen) was composed of a 1,128-bp nucleotide
sequence encoding 376 amino acid residues. The sequences of
the first 20 N-terminal and the last 20 C-terminal amino acids
of the fragment were confirmed to be identical to each 20 N-terminal
amino acid sequence of native XloA and a 22-kDa peptide. Therefore,
the DNA insert in the pT7Blue vector was considered to be derived
from the gene encoding XloA. On the basis of the insert nucleotide
sequence, a second set of primers was designed. The PCR product
(300 bp) that was amplified from the insert nucleotide sequence
with the second set of primers was used as the probe for the
cloning of
xloA after it was labeled with AlkPhos (GE Healthcare).

Cloning of the xloA gene.
To obtain the complete
xloA gene, Southern hybridization with
the probe was carried out. Digestion of
Vibrio sp. strain XY-214
genomic DNA with both XbaI and SpeI gave a 4.2-kbp fragment
that hybridized with the AlkPhos-labeled probe. When XbaI-SpeI
fragments were ligated into pBluescriptII KS(–) (Stratagene)
and transfected into
E. coli DH5

(Stratagene), 1 of the 40 colonies
was selected as positive by direct PCR with the second set of
primers. The cloned plasmid was named
xloA/pBluescript(XbaI-SpeI).
Nucleotide sequence analysis of the positive DNA fragment inserted
into the vector was carried out on a Beckman CEQ2000XL sequencer
(Beckman Coulter) with a GenomeLab DTCS-Quick Start kit. The
nucleotide sequence data were analyzed with GENETYX-WIN computer
software (Software Development, Tokyo, Japan). Homology searches
were carried out with the BLAST program at the National Center
for Biotechnology Information website.
The plasmid xloA/pBluescript(XbaI-SpeI) contained a 4,206-bp DNA fragment including the xloA gene and three possible open reading frames. The xloA gene consisted of a 1,608-bp nucleotide sequence encoding a protein of 535 amino acids with a predicted molecular mass of 60,835 Da. The sequences TTCATA and TACCCT with a 17-bp spacing, having a certain homology to the –35 and –10 promoter consensus sequences of E. coli, were identified upstream of the coding region (21). A possible transcription terminator that consisted of a 36-bp palindrome sequence corresponding to an mRNA hairpin loop, followed by a T-rich portion, was found downstream of the TAA termination codon (23).

Similarity of the deduced amino acid sequence encoded by the xloA gene.
The sequence of the XloA protein was scanned with BLAST, and
the protein showed similarity to the β-xylosidases that
belong to family 43 of glycoside hydrolases (GH43). XloA exhibited
the highest protein similarities to the GH43 β-xylosidases
(identity percentages are in parentheses) from
Bacillus sp.
strain KK-1 (47%) (GenBank accession no. AF045479),
Bacillus halodurans C-125 (47%) (AP001519),
B. subtilis (46%) (U66480),
Selenomonas ruminantium GA192 (45%) (AF040720),
Geobacillus stearothermophilus T-6 (45%) (AY690618),
Clostridium acetobutylicum (44%) (AE007842),
Bacillus pumilus IPO (44%) (X05793), and the
GH43

-
L-arabinofuranosidase from
Chromohalobacter salexigens DSM 3043 (46%) (CP000285). This suggested that XloA should be
classified into the GH43 family, which is a component of the
superfamily clan GH-F together with the GH62 family, by forming
a fivefold β-propeller architecture (
20). Recently, the
catalytic residues of GH43 β-xylosidase (XynB3) from
G. stearothermophilus T-6 were revealed to be Asp15, Glu187, and
Asp128 (
8). From the sequence similarities between XloA and
XynB3, we surmised that the active-site amino acids in XloA
are Asp16, Glu189, and Asp130. Although GH43 glycosidases from
different organisms were found to be either intra- or extracellular
(
15,
17,
24), the
xloA gene was found not to include any recognizable
signal peptides of gram-negative bacteria, as determined by
the SignalP server (
18), which therefore suggested that XloA
is an intracellular enzyme.

Expression and purification of rXloA.
For the production of rXloA in
E. coli, the
xloA region was
subcloned into the pET22b(+) vector (Novagen) as follows. The
full-length
xloA gene was amplified from
xloA/pBluescript(XbaI-SpeI)
as the template by PCR with primers 5'-GGAATTC
CATATGACAACTACGATTCAG-3'
and 5'-AA
GCGGCCGCATGCTCTAGGTACTCAAAGT-3', which contained artificial
NdeI and NotI sites (italicized), respectively. The PCR product
was digested with NdeI and NotI and then ligated into a pET22b(+)
vector linearized with the same enzymes to construct
xloA/pET22b.
This plasmid provides rXloA with a six-His tag fused at its
C terminus.
xloA/pET22b was transformed into
E. coli BL21(DE3)
competent cells (Novagen) and used for the production of rXloA.
The absence of undesired mutations in the amplified DNA fragment
was verified by DNA sequencing.
E. coli BL21(DE3) transformants carrying xloA/pET22b were cultivated at 37°C in 800 ml of Luria-Bertani medium in the presence of ampicillin (100 µg ml–1). When the optical density at 600 nm of the culture reached about 0.6, the temperature was changed to 25°C and isopropyl-β-D-galactopyranoside (IPTG) was added to the culture to give a final concentration of 1 mM for induction of gene expression. After an additional incubation of 20 h at 25°C, the cells harvested by centrifugation were suspended in binding buffer (20 mM sodium phosphate, 0.5 M NaCl, 10 mM imidazole, pH 7.4) and disrupted on ice by sonication. The supernatant of the cell lysate was collected by centrifugation and fractionated by a HiTrap chelating HP column (GE Healthcare) precharged with Ni2+. The final preparation of purified rXloA was used in all of the experiments in this study. β-Xylosidase activity was measured by using p-nitrophenyl-β-D-xylopyranoside (PNPX) (Sigma) as a substrate. One unit of β-xylosidase activity was defined as the amount of enzyme releasing 1 µmol p-nitrophenol per min from the substrate. The protein concentration was measured by the method of Lowry et al. (14) with bovine serum albumin as the standard.

Characterization of rXloA.
The final recombinant enzyme was purified 37-fold by using a
HiTrap chelating HP column and ascertained to be a single band
on sodium dodecyl sulfate-polyacrylamide gel electrophoresis.
Its relative molecular mass was estimated to be approximately
60 kDa, which is almost the same size as a monomer of native
XloA from
Vibrio sp. strain XY-214. However, the relative molecular
mass of the native enzyme was estimated to be 251 kDa by gel
filtration chromatography on a Superdex 200 HR 10/30 column
(GE Healthcare) (data not shown). According to the nucleotide
sequence, the relative molecular mass of the XloA monomer should
be 60,835 Da. These results suggest that XloA is a tetramer.
Similarly, the GH43 β-xylosidase from
G. stearothermophilus T-6 (PDB code 2EXH) (
8) has been reported to be a tetramer in
the crystal state, whereas the GH43 β-xylosidases from
B. halodurans C-125 (PDB code 1YRZ) (
22) and
B. pumilus IPO
(
28) were revealed to be a dimer and a trimer, respectively,
by gel filtration chromatography.

Properties of rXloA.
The optimal pH was observed to be around 7.0 when the enzyme
activity was assayed with 2 mM PNPX at 37°C for 20 min at
various pHs (3.0 to 12.0) in Britton and Robinson's universal
buffer (40 mM phosphoric acid, 40 mM boric acid, 40 mM acetic
acid). The enzyme was stable at pHs 6.0 to 9.0, retaining more
than 80% of the original activity after preincubation in the
buffer at various pHs at 4°C for 12 h. The enzyme was optimally
active at around 35°C and stable at up to 30°C for 20
min. The enzymatic activity was inhibited more than 90% by Ag
+,
Cu
2+, Hg
2+, Mn
2+, Pb
2+, Zn
2+, and
p-chloromercuric benzoic acid.
The
Km and
Vmax values of rXloA for PNPX were determined to
be 0.244 mM and 1.82 µmol min
–1 mg
–1, respectively.
To examine the substrate specificity of rXloA, enzymatic reactions
were performed after incubation with various substrates at 30°C
for 12 h, and hydrolysis products were analyzed by thin-layer
chromatography (TLC) on a silica gel 60-plastic sheet (Merck)
with a solvent of
n-butanol-acetic acid-water (10:5:1 for β-1,3-xylooligosacharides,
2:1:1 for β-1,4-xylooligosacharides). β-1,3-Xylooligosacharides
(xylobiose to xylotetraose) were prepared by fractionation of
the products released from β-1,3-xylan by the β-1,3-xylanase
of
Vibrio sp. strain XY-214 by using a charcoal column and TLC.
β-1,3-Xylan was prepared from a green alga,
Caulerpa racemosa var.
laetevirens according to the method of Iriki et al. (
12).
β-1,4-Xylooligosacharides were purchased from Biocon (Japan).
Oligosaccharides were visualized by spraying the plate with
diphenylamine-aniline-phosphate reagent (
6). rXloA completely
hydrolyzed β-1,3-xylobiose (TX2), β-1,3-xylotriose
(TX3), and β-1,3-xylotetraose (TX4) to xylose as a final
hydrolysis product (Fig.
1A). The enzyme also cleaved β-1,4-xylobiose
(FX2), β-1,4-xylotriose (FX3), and β-1,4-xylotetraose
(FX4) weakly to form a small amount of xylose (Fig.
1B). Thus,
rXloA preferred to hydrolyze β-1,3-xylooligosaccharides
rather than β-1,4-xylooligosaccharides. Although there
have been many reports of β-xylosidases, all of them act
on β-1,4-xylooligosaccharides. Therefore, XloA of
Vibrio sp. strain XY-214 is the first β-1,3-xylosidase described.
β-Xylosidases (EC 3.2.1.37) are classified into several
families (GH3, -39, -43, -52, and -54) by a glycoside hydrolase
classification system based on the amino acid sequences of their
catalytic domains (
http://afmb.cnrs-mrs.fr/CAZY) (
10,
11). The
GH3, -39, -52, and -54 β-xylosidases are reported to be
retaining enzymes and able to perform both hydrolysis and transglycosylation
reactions, whereas GH43 β-xylosidases are inverting enzymes
and able to perform only hydrolysis (
22). In concordance with
this, rXloA was confirmed not to perform transglycosylation
based on the activity pattern of β-1,3-xylooligosaccharides
by TLC (Fig.
1A). This result also supported the hypothesis
that XloA belongs to the GH43 family.
Furthermore, the enzyme did not act on laminarioligosaccharides comprising β-1,3-linked D-glucose units (Biocon), polysaccharides such as β-1,3-xylan and β-1,4-xylan (Sigma), carboxymethyl cellulose (Wako Chemicals), or laminaran (Nacalai Tesque). Examination of the activities of rXloA on various synthetic substrates composed of p-nitrophenyl (PNP) attached to different sugar units (Sigma) showed only β-D-xylopyranosidase activity, and no activities were detected against other PNP substrates like PNP-
-D-xylopyranoside, PNP-
-D-mannopyranoside, PNP-β-D-mannopyranoside, PNP-
-D-galactopyranoside, PNP-
-D-glucopyranoside, PNP-β-D-glucopyranoside, PNP-
-L-fucopyranoside, PNP-
-L-arabinofuranoside, PNP-
-L-arabinopyranoside PNP-β-D-glucuronide, and PNP-β-D-cellobioside.
Recently, a mutant species of Caulerpa taxifolia has continued to multiply abnormally in the Mediterranean; the mutant algae have destroyed the ecosystem of the sea environment, and fisheries have suffered great damage. Because β-1,3-xylosidase can produce D-xylose efficiently from β-1,3-xylan contained in the cell walls of the algae by combining with β-1,3-xylanase, it will play an important role in mitigating the damage caused by the harmful mutant alga.

Nucleotide sequence accession number.
The nucleotide sequence of the
xloA gene described here has
been submitted to the DDBJ, EMBL, and GenBank databases and
assigned accession no. AB300564.

ACKNOWLEDGMENTS
We acknowledge Jinhua Dong for helpful discussions during the
preparation of the manuscript.

FOOTNOTES
* Corresponding author. Mailing address: Graduate School of Bioresources, Mie University, 1577 Kurimamachiya, Tsu, Mie 514-8507, Japan. Phone: 81-59-231-9561. Fax: 81-59-231-9540. E-mail:
araki{at}bio.mie-u.ac.jp 
Published ahead of print on 9 November 2007. 

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Applied and Environmental Microbiology, January 2008, p. 305-308, Vol. 74, No. 1
0099-2240/08/$08.00+0 doi:10.1128/AEM.01793-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.