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Applied and Environmental Microbiology, June 2006, p. 4455-4460, Vol. 72, No. 6
0099-2240/06/$08.00+0 doi:10.1128/AEM.02095-05
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Identification of a DNA Methyltransferase Gene Carried on a Pathogenicity Island-Like Element (VPAI) in Vibrio parahaemolyticus and Its Prevalence among Clinical and Environmental Isolates
Hui-zhen Wang,
Minnie M. L. Wong,
Desmond O'Toole,
Mandy M. H. Mak,
Rudolf S. S. Wu, and
Richard Y. C. Kong*
Department of Biology and Chemistry and Research Centre for Coastal Pollution and Conservation, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong Special Administrative Region, People's Republic of China
Received 6 September 2005/
Accepted 29 March 2006

ABSTRACT
In this study we identified a putative virulence-associated
DNA methyltransferase (MTase) gene carried on a novel 22.79-kb
pathogenicity island-like element (VPAI) in
V. parahaemolyticus.
The
V. parahaemolyticus MTase gene was shown by PCR to be prevalent
(>98%) in pandemic thermostable direct hemolysin gene-positive
isolates, which suggests that VPAI may confer unique virulence
traits to pandemic strains of
V. parahaemolyticus.

INTRODUCTION
Vibrio parahaemolyticus, which is widely distributed in coastal
and marine waters, has been the major cause of seafood-borne
gastroenteritis in areas where raw or uncooked seafood is consumed
(
10). Although most
V. parahaemolyticus strains are harmless
to humans and animals,
V. parahaemolyticus infections are often
associated with strains that produce the thermostable direct
hemolysin (TDH) and/or TDH-related hemolysin (TRH) (
10,
22,
24). While sporadic cases of
V. parahaemolyticus infections
prior to 1996 were caused mainly by diverse serotypes, several
major outbreaks of gastroenteritis that were reported since
1996 in many parts of the world, including the United States
(
2), Europe (
19), Asia (
5,
6,
22), and Africa (
1), were associated
with the emergence and pandemic spread of
V. parahaemolyticus O3:K6 strains. Moreover, an increasing number of
V. parahaemolyticus outbreaks have been linked to a number of clonal derivatives
of O3:K6, including the O4:K68, O1:K25, and O1:KUT serovars
(
6,
8), which together constitute the so-called "pandemic group."
These serovars can be distinguished from nonpandemic strains
by the presence of the TDH gene (but not the TRH gene) (
22)
and by molecular techniques such as pulsed-field gel electrophoresis
(
7), arbitrarily primed PCR and group-specific PCR of the
toxRS gene (
20),
orf8-PCR of the f237 filamentous phage (
21), and
multilocus sequence typing (
8). Nevertheless, the occurrence
of group-specific PCR-positive but
orf8-negative pandemic isolates
has recently been reported (
4,
8,
14), and the group-specific
toxRS marker has been detected in TDH gene-negative O3:K6 strains
of
V. parahaemolyticus (
23).
To date, only a limited number of virulence-associated factors have been described for pandemic strains of V. parahaemolyticus (17, 25, 26). Comparison of genomic differences between O3:K6 and nonpathogenic strains by subtractive hybridization could reveal novel mechanisms relevant to the study of the enteropathogenicity of V. parahaemolyticus. Here we describe identification of a putative virulence-associated DNA methyltransferase (MTase) gene on a 22.79-kb pathogenicity island-like element on chromosome 1 of V. parahaemolyticus.

Identification of a unique genomic locus.
The differential subtraction chain method of Luo et al. (
16)
was used to identify unique genomic sequences of a clinical
V. parahaemolyticus O3:K6 isolate (strain QM98284) by subtractive
hybridization with DNA from an environmental isolate (strain
CECT611). Genomic DNAs of clinical strain QM98284 (tester) and
environmental strain CECT611 (driver) were digested with SphI,
and tester DNA was ligated to adaptors A and B (Table
1); this
was followed by 10 PCR cycles consisting of 94°C for 30
s, 60°C for 1.5 min, and 72°C for 1.5 min. Driver fragments
(100-fold excess) were added to adaptor-ligated tester fragments,
and two rounds of subtractive hybridization were performed.
Tester fragments were then amplified by PCR with adaptor-specific
primers RDSCA and RDSCB (Table
1) for 40 cycles consisting of
94°C for 30 s, 65°C for 30 s, and 72°C for 1.5 min,
and the products were cloned into the pCR2.1-TOPO vector (Invitrogen).
Genomic Southern hybridization showed that clone 32 (containing
a 254-bp insert) was present only in the tester strain (QM98284)
and not in the driver strain (CECT611) (data not shown), and
the clone was extended using a universal GenomeWalker kit (Clontech,
United States) to produce a 5,195-bp fragment (designated CLONE-32)
(Fig.
1). Comparison of CLONE-32 to the genome sequence of
V. parahaemolyticus RIMD2210633 (
17) showed that it exhibited >99%
identity with a homologous region on chromosome 1 of this strain
at positions 390994 to 396188 (accession no. NC_004603).

Characterization of the V. parahaemolyticus MTase gene.
The
V. parahaemolyticus MTase gene encodes a 233-amino-acid
protein (Fig.
2), and a homology search showed that this protein
exhibits a high level of amino acid identity (72%) with the
mannose-fucose-resistant hemagglutinin (MFRHA) of
Vibrio cholerae O1 (
9). Analysis of the
V. parahaemolyticus MTase and MFRHA
deduced proteins using an HMM (hidden Markov models) search
(at
http://pfam.wustl.edu/hmmsearch.shtml) against the Pfam
protein family database (
3) showed that both proteins contain
nine highly conserved sequence motifs (Fig.
2) typical of N4-N6
MTases (
18). Specifically, motif I (DXFXGXG, where X is any
amino acid residue) and motif IV [(D/N/S)PP(Y/F)] are the two
most highly conserved motifs, and they are essential for
S-adenosyl-
L-methionine
(AdoMet) binding and catalytic activity, respectively. Based
on the arrangement of the nine conserved motifs, the linear
order of the AdoMet-binding, catalytic, and target recognition
domains, and alignment with the well-known cell cycle-regulated
methyltransferase (CcrM; a group ß MTase found so
far only in

-proteobacteria [
15]), it seems likely that
V. parahaemolyticus MTase and MFRHA are members of the ß class of m
6-
N-adenine
MTases (
11,
18). The fact that a mutant of
V. cholerae defective
in MFRHA exhibited significant attenuation in virulence potential
(
9) suggests that the MTase gene has an important role in virulence,
and by extrapolation, it is likely that the
V. parahaemolyticus MTase gene also has a similar pathogenic role in
V. parahaemolyticus.

Association of the V. parahaemolyticus MTase gene with a pathogenicity island.
Inspection of the sequences flanking the CLONE-32 locus in the
V. parahaemolyticus RIMD2210633 genome resulted in identification
of a putative phage-like integrase (
int) gene (VP0380) 9,936
bp upstream of
hsdS (VP0387) and a methionine-specific tRNA
(tRNA
Met) gene sequence 7,656 bp downstream of
hsdR (VP0395).
Further analysis of this 22.79-kb genomic region revealed that
it is flanked by a 47-bp direct repeat sequence (Fig.
1). These
features suggest that CLONE-32 is carried on a pathogenicity
island-like element (designated VPAI) that contains 25 open
reading frames (ORFs) (Table
2). Ten of these ORFs were homologous
to genes having known functions, including the genes encoding
the type I restriction-modification complex (VP0387, VP0388,
and VP0395),
V. parahaemolyticus MTase (VP0394), phage P4-like
integrase (VP0380), and proteins involved in DNA replication
(VP0400), transcription regulation (VP0399), signal transduction
(VP0382), and general metabolism (VP0386 and VP0392). Further
analysis showed that the mean G+C content of the 25 coding genes
of VPAI (43.79% ± 3.93%) is significantly lower than
(but within 1 standard deviation of) the mean G+C content of
the ca. 5,000 coding genes of the complete genome (accession
no. NC_004603 and NC_004605; 4,992 genes; G+C content, 45.8%
± 4.19%) of
V. parahaemolyticus (
P < 0.05, as determined
by a one-tailed
t test), which suggests that VPAI was recently
acquired by lateral gene transfer.
To define the chromosomal insertion site of VPAI, primers VP0379-F
and VP0404-R flanking VPAI (Fig.
1 and Table
1) were used in
PCRs with a number of
V. parahaemolyticus MTase gene-negative
and -positive isolates of
V. parahaemolyticus. As expected,
no PCR product was detected for the
V. parahaemolyticus MTase
gene-positive isolates examined (since VPAI is too large to
be amplified), while a 0.9-kb PCR product was obtained for 10
of the 15
V. parahaemolyticus MTase gene-negative isolates tested
(Table
3). DNA sequencing showed that the 22.79-kb VPAI is precisely
inserted into the 47-bp direct repeat sequence at the 3' end
of the methionine tRNA gene (Fig.
1), and this was corroborated
by Southern blot analysis of
V. parahaemolyticus MTase gene-positive
clinical (pandemic and nonpandemic) isolates and
V. parahaemolyticus MTase gene-negative clinical and environmental isolates (Table
3).
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TABLE 3. Distribution of VPAI in V. parahaemolyticus MTase gene-positive and V. parahaemolyticus MTase gene-negative strains of V. parahaemolyticus
|

Distribution of the genes encoding V. parahaemolyticus MTase, TDH, TRH, and type III secretion system 2 (TTSS2).
The clinical isolates of
V. parahaemolyticus used in this study
were kind gifts from W. C. Yam (University of Hong Kong, Hong
Kong), H. C. Wong (Soochow University, Taiwan), and G. B. Nair
(International Centre for Diarrheal Disease Research, Bangladesh).
V. parahaemolyticus strains were confirmed by PCR using the
species-specific primers Vpara-F and Vpara-R (Table
1) (
13).
Serotyping of
V. parahaemolyticus was performed using commercial
antisera purchased from Denka Seiken Co., Ltd., Tokyo, Japan.
PCRs were carried out in 100-µl mixtures containing 50
ng DNA, each primer at a concentration of 0.2 to 0.25 µM,
each deoxynucleoside triphosphate (dATP, dCTP, dGTP, and dTTP)
at a concentration of 0.2 mM, 1.5 mM MgCl
2, and 5 U of
Taq DNA
polymerase. Each PCR consisted of an initial denaturation step
at 94°C for 2 min, followed by 30 cycles of denaturation
at 94°C for 1 min, annealing at 55°C for 1 min, and
elongation at 72°C for 1 min. The PCR products were analyzed
by agarose gel electrophoresis.
A total of 108 clinical isolates of V. parahaemolyticus (55 pandemic strains consisting of 34 O3:K6, 13 O4:K68, 3 O1:K25, and 5 O1:KUT isolates and 53 nonpandemic strains having diverse serotypes) and 39 environmental isolates (Table 4) were tested by PCR for the presence of the V. parahaemolyticus MTase gene and the three known virulence genes, the genes encoding TDH, TRH, and TTSS2 of V. parahaemolyticus (17, 25). As reported previously (17), the TTSS2 gene was detected by PCR in all TDH gene-positive pandemic and nonpandemic strains. The V. parahaemolyticus MTase gene was detected in 98.2% (54/55) of TDH gene-positive pandemic strains and in 55.2% (16/29) of TDH gene-positive nonpandemic strains but was not detected in any of the TDH gene-negative clinical strains examined. Intriguingly, all TDH gene-positive "prepandemic" clinical isolates (collected before 1996) from Taiwan (H. C. Chung, personal communication) were PCR negative for the V. parahaemolyticus MTase gene (Table 4), and a few strains were confirmed to lack VPAI in the genome by PCR and Southern blot analysis (Table 3). The V. parahaemolyticus MTase gene was detected in only 2.56% (1/39) of the TDH gene-negative environmental isolates examined. Taken together, these results indicate that the V. parahaemolyticus MTase gene (and/or VPAI) may confer unique virulence or fitness traits to the TDH gene-positive pandemic strains of V. parahaemolyticus, enabling them to cause disease more frequently than strains belonging to other serovars. However, it is possible that some other ORFs (which encode hypothetical proteins whose functions are not known) in VPAI may code for virulence-associated factors. Clearly, further experiments are needed to understand the relevance of VPAI (on chromosome 1) and its relationship to the TDH gene (on chromosome 2) for the pathogenesis and virulence potential of V. parahaemolyticus.
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TABLE 4. Source and characteristics of the clinical and environmental V. parahaemolyticus isolates used in this study
|

ACKNOWLEDGMENTS
This work was supported by an Area of Excellence grant from
the University Grants Committee of the Hong Kong Special Administrative
Region, China (project AoE/P-04/04).

FOOTNOTES
* Corresponding author. Mailing address: Department of Biology and Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Kowloon, Hong Kong SAR, China. Phone: 852-2788-7794. Fax: 852-2788-7406. E-mail:
bhrkong{at}cityu.edu.hk.


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Applied and Environmental Microbiology, June 2006, p. 4455-4460, Vol. 72, No. 6
0099-2240/06/$08.00+0 doi:10.1128/AEM.02095-05
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
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