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Applied and Environmental Microbiology, December 2003, p. 7002-7008, Vol. 69, No. 12
0099-2240/03/$08.00+0 DOI: 10.1128/AEM.69.12.7002-7008.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Department of Bacterial Genetics, Institute of Microbiology, Warsaw University, 02-096 Warsaw, Poland
Received 15 July 2003/ Accepted 18 September 2003
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Paracocci are among the most metabolically versatile bacteria. They are chemoorganoheterotrophs (utilizing a wide variety of organic compounds, including potentially polluting compounds, like acetone) or facultative chemolitoautotrophs (utilizing reduced sulfur compounds, such as sulfides, thiosulfate, thiocyanate, or molecular hydrogen, as energy sources). Methylotrophy is often observed (with such methyl compounds as formate, methanol, trimethylamine, tetramethylammonium), as is growth in anaerobic conditions (nitrate respiration) (1). Because of their versatile metabolism paracocci can play an important role in the cycling of elements in the environment.
It seems that all these physiological properties raise the possibility that Paracoccus species could be used in bioremediation systems, particularly since most species can use nitrate as an alternative electron acceptor. One of these species is P. solventivorans, which was first isolated at the site of a natural gas company as an acetone-degrading, nitrate-reducing bacterium (33). The second isolate of this species, DSM 11592 (used in this study), was found in biofilters used for the treatment of waste gases (22). So far, nothing is known about the genetics of P. solventivorans. It is only known that strain DSM 11592 carries a 5-kb plasmid, pSOV1 (2).
We initiated studies on transposable elements of Paracoccus spp. Such elements significantly enhance the variability and consequently the adaptative capacities of their hosts (7). By using the specific paracoccal entrapment vector pMEC1, we previously identified and characterized several insertion sequences (ISs) and transposons of different strains of P. pantotrophus (3). This analysis allowed us to distinguish transposable elements common in this species (e.g., ISPpa2 of the IS5 family and ISPpa5 of the IS66 family), as well as strain-specific elements (e.g., ISPpa1 of the IS256 family and Tn3434 of the Tn3 family). In order to study the occurrence and frequency of lateral transfer in this group of bacteria, we performed studies to identify and determine the distribution of transposable elements of other paracoccal strains. Here we describe molecular characteristics of ISs caught by pMEC1 in P. solventivorans DSM 11592. These ISs may be used for construction of novel tools for transposon mutagenesis, which may be very useful for genetic analysis of this interesting group of bacteria.
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View this table: [in a new window] |
TABLE 1. Distribution
of ISPso2-related elements in paracoccal strains
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Transformation.
Competent cells of E. coli
TG1 were prepared and transformed as described by Kushner
(21).
Triparental
mating.
Overnight cultures
of donor strain E. coli TG1 carrying a mobilizable vector, a
P. solventivorans recipient strain, and E. coli
DH5
carrying the helper plasmid pRK2013
(10) were spun down at
the maximum speed for 2 min at 4°C in a microcentrifuge and
washed twice with LB medium to remove the antibiotics. The donor,
recipient, and helper were mixed (1:2:1), and a 100-µl sample
of the mixture was spread on a plate containing solidified LB medium.
After overnight incubation at 30°C, the bacteria were washed
off the plate, and suitable dilutions were plated on selective media
containing rifampin (selective marker of the recipient strain) and
kanamycin to select transconjugants. Spontaneous resistance of the
recipient strains to kanamycin was undetectable under these
experimental conditions.
Isolation of
insertion mutants.
The
entrapment vector pMEC1 was introduced into a recipient P.
solventivorans strain by triparental mating. An overnight culture
of a Kmr transconjugant, carrying pMEC1, was spread on
plates containing solidified LB medium supplemented with tetracycline.
Appropriate dilutions of the culture were also spread on LB medium
lacking tetracycline in order to determine the frequency of
transposition. Spontaneous resistance of the strain to tetracycline was
undetectable under these experimental
conditions.
PCR amplification.
For amplification of transposable
elements captured in pMEC1, five nested pairs of cartridge-specific
forward and reverse primers were used as described previously
(3). For differentiation
of ISPso2 and the related element ISPpa2, as well as
IS1248, in various strains of Paracoccus the
following pairs of forward and reverse primers were used: LPSO2
(5'-AGGATGCATTGATTTCTGTT-3') and
RPSO2 (5'-ATAACCAATAGATGACGAGA-3');
LPPA2 (5'-AGGATGCATTGATTTTCGAC-3')
and RPPA2
(5'-ATAACCAGTAGATGACGACC-3'); and
L1248 (5'-CAGGATGCATTGATTTTCAG-3')
and R1248
(5'-ATAACCAATAAATGACGGTT-3').
Amplification was performed with a Mastercycler (Eppendorf)
by using the synthetic oligonucleotides described above, Taq
polymerase from Qiagen (supplied with buffer), and appropriate template
DNAs. Each 50-µl reaction mixture for amplification contained 2
mM MgCl2, each deoxynucleoside triphosphate at a
concentration of 200 µM, 50 pmol of each primer, 0.5 U of
Taq polymerase, and 1x Q solution (Qiagen). The
amplification program was 96°C for 5 min, followed by 35 cycles
of 48°C for 30 s, 72°C for 1 min, and
94°C for 1 min; the last cycle was followed by an additional
annealing step and a final 10-min extension
step.
DNA sequencing and
analysis.
The nucleotide
sequence was determined by using a terminator sequencing kit and an
automatic sequencer (ABI 377; Perkin-Elmer). The transposable elements
(present in pMEC1 derivatives) were sequenced first with the
appropriate sets of cartridge-specific starters
(3) and then with primers
complementary to the previously determined sequence. Sequence analysis
was done with programs included in the Wisconsin Genetic
Computer Group Sequence Analysis Software Package, version 8.1
(9). Comparison searches
were performed with IS Finder
(http://www-is.biotoul.fr/is.html)
and with the BLAST program provided by the National Center for
Biotechnology Information
(http://www.ncbi.nlm.nih.gov/BLAST).
Phylogenetic analysis was performed by using the parsimony method
(DNAPARS in the software package PHYLIP, version 3.57c)
(14), as well as the
programs SEQBOOT, CONSENSE, and DRAWTREE to perform bootstrap
analysis.
Nucleotide sequence accession
numbers.
Nucleotide
sequences determined in this study have been deposited in the GenBank
database under accession numbers
AY237733
(ISPso1),
AY237732
(ISPso2), and
AY311510
(ISPso3).
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and the gene coding for the
CI repressor. Inactivation of the repressor gene (e.g.,
through insertion of an IS) results in constitutive expression of
tetracycline resistance. The pMEC1 vector was introduced into P.
solventivorans DSM 11592, and Tcr clones were selected
as described in Materials and Methods. Tcr mutants appeared
with a frequency of 2.9 x 10-5. We tested
100 Tcr colonies for plasmid content. Most of the plasmids
tested (92%) carried inserts that were smaller than 2 kb, while
8% were the size of pMEC1. Restriction analysis followed by
hybridization analysis (with digoxigenin-labeled internal fragments of
randomly chosen inserts as probes) revealed the presence of three
different classes of elements caught in pMEC1 (0.8, 1.2, and 1.4 kb)
(data not shown). To localize the insertion sites of the elements, we
used five sets of previously described cartridge-specific primers
(3) together with
pMEC1-derived plasmids (as template DNAs) in PCRs. We amplified all the
inserts, which confirmed that the insertions were within the
cI gene (data not shown). The representative elements of each
of the classes distinguished were sequenced. A comparison with the
nucleotide sequences in databases revealed that these sequences were
novel ISs, and they were designated ISPso1, ISPso2,
and ISPso3. The G+C contents of the sequences
identified were in the range from 60 to 67 mol% (Table
2), while that of P. solventivorans total DNA was 68.5 to 70
mol%
(1). |
View this table: [in a new window] |
TABLE 2. Characteristic
features of ISs of P. solventivorans
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ISPso1 (1,400 bp) carries one large ORF (1,197 bp) (ORF1) encoding a putative basic protein (pI 9.25) consisting of 398 amino acids and having a predicted molecular mass of 44.65 kDa. The ORF1 product contains the DDE motif, in which the acidic residues are separated by 65 and 105 amino acids, respectively (Fig. 1); this motif is highly conserved among elements belonging to the IS256 family (23). Additionally, the nucleotide sequences of the IRs of ISPso1 (Fig. 2A), as well as the size (8 bp) of the DRs resulting from its transposition (Table 2), are also typical of the members of this family (7). Database comparison searches revealed that the predicted translation product of ORF1 exhibited the highest levels of similarity with a hypothetical protein (product of the rrub3476 gene) of Rhodospirillum rubrum (accession no. ZP 00016443) (65% identity and 72% similarity) and with TnpA of Sphingopyxis macrogoltabida (accession no. BAB07803) (57% identity and 67% similarity), both of which exhibit similarity to Tnps encoded by members of the IS256 family. All these data show that ISPso1 can be classified as a new member of the IS256 family.
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FIG. 1. Comparison
of the amino acid sequences of the predicted DDE motifs of the putative
Tnps encoded by ISPso1, ISPso2, and ISPso3
with the appropriate family- and group-specific consensus sequences
(23). In the consensus
sequences uppercase letters indicate conservation within the family,
lowercase letters indicate predominant amino acids, and dashes indicate
nonconserved residues. Residues forming the DDE motif are indicated by
a black background. The N2, N3, and C1 domains are enclosed in boxes
and labeled. The residues conserved in the domains of the Tnps analyzed
and the consensus sequences are indicated by boldface type. The numbers
in parentheses are the distances (in amino acids [aa])
between the residues forming the DDE
motif.
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FIG. 2. Alignment
of the terminal nucleotide sequences of ISPso1 (A),
ISPso2 (B), and ISPso3 (C) and their
relatives. The identical residues of the termini of an IS of P.
solventivorans are indicated by a black background. The putative
IRs are indicated by bars. The nucleotides of other ISs identical to
those of ISPso1 (IS1194 [accession no.
Y13626]
and ISBli2 [accession no. AF195203]),
ISPso2 (ISPpa2 [accession no.
AY179508]
and IS1248 [accession no. PDU08856]), or
ISPso3 (IS1246 [accession no.
NC
003350] and ISSsp126 [accession no.
SSP277295]) are indicated by boldface type and a gray background.
The consensus sequence (Cons) of the IRs compared is included in each
panel. L, sequences at the 5' (left) end; R, complementary
sequences at the 3' (right) end of the
elements.
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Analysis of the nucleotide sequence of ISPso3 (1,286 bp) revealed the presence of one major ORF (ORF1), spanning 84% of the element. ORF1 encodes a putative peptide consisting of 374 amino acids with predicted molecular mass of 39.4 kDa and a pI of 9.91. The amino acid sequence of the ISPso3-encoded putative Tnp contains the N3 and C1 domains of the invariant DDE motif (Fig. 1) conserved in the IS4 and IS5 families (29) with a distance of 49 bp, which is characteristic of the IS5 family (23). In fact, comparison searches with the ORF1 product in databases revealed homology with Tnps encoded by ISs classified as members of a distinct subgroup of the IS5 group of the IS5 family (http://www-is.biotoul.fr/is.html). The highest levels of similarity were observed with Tnps encoded by ISSsp126 of Sphingomonas sp. strain LB126 (51% identity and 60% similarity) (40), IS1169 residing in plasmid pIP421 of Bacteroides fragilis BF-F238 (approximately 43% identity and 52% similarity) (35), and IS1168 present in pIP417 of Bacteroides vulgatus BV17 (approximately 43% identity and 52% similarity) (17). ISPso3 is flanked by identical 11-bp terminal IRs, which exhibit similarity to the IRs of the ISs mentioned above (Fig. 2C). Transposition of this IS resulted in generation of 4-bp DRs (Table 2), a size typical of other elements of the IS5 group (23).
Occurrence
of ISPso1, ISPso2, and ISPso3 in different
strains of Paracoccus spp.
Hybridization analysis was performed to
study the distribution of the ISs studied in different strains of
Paracoccus spp. To do this, ISs, PCR performed with
cartridge-specific primers
(3) were probed with total
DNAs of paracocci (listed in Table
1) digested with
EcoRI and PstI. These restriction enzymes did not cut
the ISs analyzed, and therefore the number of hybridized DNA fragments
was thought to be equivalent to the minimum number of copies of a given
element within the genome.
We detected two copies of ISPso1 in parental strain DSM 11592 (Fig. 3A, lane 1) and a single copy of homologous sequences in all of the P. pantotrophus strains tested (DSM 65, LMD 82.5, DSM 11072, DSM 11073) (Fig. 3A, lanes 8 to 11). The presence of an ISPso1-like sequence (on restriction fragments that were the same size) in all strains of P. pantotrophus strongly suggests that this element was acquired by the common ancestor before branching of these strains. A weak hybridization signal was also observed with a single band of total DNA of P. versutus UW1 (Fig. 3A, lane 14), which is phylogenetically closely related to P. pantotrophus (1). On the other hand, ISPso3 (a member of the IS5 family) was present only in one strain of P. pantotrophus (LMD 82.5) (Fig. 3C, lane 8) in addition to the parental strain (two copies in DSM 11592) (Fig. 3C, lane 1), which suggests that it was recently acquired by a lateral transfer event.
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FIG. 3. Analysis
of the distribution of ISPso1-like (A), ISPso2-like
(B), and ISPso3-like (C) elements by DNA
hybridization. The lanes contained
EcoRI-PstI-digested DNAs of P.
solventivorans DSM 11592 (lane 1), P. alkenifer DSM 11593
(lane 2), P. aminophilus JMC 7686 (lane 3), P.
aminovorans JCM 7685 (lane 4), P. denitrificans DSM 413
(lane 5), P. denitrificans LMD 22.21 (lane 6), P.
methylutens DM12 (lane 7), P. pantotrophus DSM 65 (lane
8), P. pantotrophus LMD 82.5 (lane 9), P.
pantotrophus DSM 11072 (lane 10), P. pantotrophus DSM
11073 (lane 11), P. alcaliphilus JCM 7364 (lane 12), P.
thiocyanatus IAM 12816 (lane 13), P. versutus UW1 (lane
14), and pSOV1 of P. solventivorans DSM 11592 (lane 15). The
positions of the markers are indicated on the
left.
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None of the ISs identified hybridized with the only natural plasmid (2) harbored by P. solventivorans DSM 11592, plasmid pSOV1 (Fig. 3, lane 15); therefore, all of them reside in the chromosome of this strain.
Phylogenetic analysis.
Comparisons of the Tnps encoded by
ISPso1, ISPso2, and ISPso3 in databases
allowed identification of their closest homologues. These sequences
were used for construction of a phylogenetic tree, which showed the
evolutionary relationships (Fig.
4) and allowed estimation of possible directions of lateral transfer of
transposable elements between various bacterial hosts. As expected, the
phylogenetic analysis separated the Tnps into three subgroups
(comprising sequences with similarity to IS256 products and
products of two groups belonging to the IS5 family), which
were well supported by bootstrap values (Fig.
4).
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FIG. 4. Unrooted
bootstrap tree (1,000 replicates) for putative Tnps encoded by
ISPso1, ISPso2, and ISPso3 and their closest
relatives (identified in available databases), constructed by the
parsimony method. The tree was constructed by using SEQBOOT, DNAPARS,
CONSENSUS, and DRAWTREE in the software package PHYLIP
(14). The numbers at the
nodes indicate bootstrap values for the nodes based on 1,000 bootstrap
resamplings (values less than 80 are not shown). The taxonomic groups
(as described by Garrity and Holt
[15]) are
indicated in brackets, as follows: A, Actinobacteria; B,
Bacteroidetes; P, alpha subgroup of the
Proteobacteria; ßP, beta subgroup of the
Proteobacteria; P, gamma subgroup of the
Proteobacteria; Bl, bacilli. The sequences used for this
analysis were sequences of Agrobacterium tumefaciens
(accession no.
X53945),
Azotobacter vinelandii
(ZP
00089333 [superscript 1] and
ZP
00088382[superscript 2]), Bacteroides
fragilis
(S44507
[superscript 1] and
I40184[superscript
2]), Bacteroides vulgatus
(I40597),
Bradyrhizobium japonicum
(NP
769117 [superscript 1],
NP 769117
[superscript 2],
NP 771828
[superscript 3], and
NP 774822
[superscript 4]), Brevibacterium linens (AAF09242),
Brucella melitensis
(NP
540341 [superscript 1] and
NP 540319
plus NP
540320 [superscript 2]), Brucella ovis
(Q08082
[superscript 1] and
M94960
[superscript 2]), Corynebacterium efficiens
(NP
737799 [superscript 1],
NP 737110
[superscript 2], and
NP 702969
[superscript 3]), Corynebacterium jeikeium
(NP
848205), Corynebacterium striatum (AAG03374),
Mesorhizobium loti
(NP
109452 plus
NP 109453
[superscript 1] and
NP 108276
[superscript 2]), Micrococcus sp. (AAK62483),
Mycobacterium avium (AAA69904 [superscript 1],
CAA11709 [superscript 2], and AAC71696 [superscript
3]), Mycobacterium celatum (CAA65977), Mycobacterium
gordonae (AAB54010), Mycobacterium tuberculosis (NP
218157 [superscript 1], NP 338287 [superscript 2],
and NP 337078 [superscript 3]), Neisseria
meningitidis (Z49092), Novosphingobium aromaticivorans
(ZP 00093225 plus ZP 00093229), P. denitrificans (AAC43507
plus AAC43509), P. pantotrophus (AAO21198 plus AAO21199),
P. solventivorans (AAO84921 [superscript 1],
AAP76386 [superscript 2], and AAO48787 plus AAO48788
[superscript 3]), Porphyromonas gingivalis
(CAA10225), Ralstonia solanacearum (BAA97979 plus BAA97980),
Rhodococcus opacus (AAB57888), Rhodococcus
rhodochrous (BAA11042), Rhodospirillum rubrum (ZP
00016443), Sphingomonas sp. (CAB87573), Sphingopyxis
macrogoltabida (BAB07803), and Streptococcus thermophilus
(AF454495). Some of the Tnps of the IS427 group are encoded by
two overlapping ORFs; therefore, two accession numbers are given above.
In these cases the sequences of the transframe fusion Tnps (generated
in silico within the putative frameshift motif) were used for the
analysis. The designations of the defined ISs are the GenBank and
ISDatabase
designations.
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Interestingly, Tnps of the ISPso1 type were found exclusively in bacteria belonging to two phylogenetically unrelated taxa, the alpha subgroup of the Proteobacteria and the Actinobacteria. The ISPso1 sequence has a high G+C content (Table 1), which is typical of both groups of bacteria. The Tnps of members of the alpha subgroup of the Proteobacteria (P. solventivorans, Rhodospirillum rubrum, and Sphingopyxix macrogoltabida, which are classified in different orders) do not form a separate cluster, which might suggest that they were acquired by different transfer events.
The ISPso3-like Tnps, located on the same branch of the phylogenetic tree (Fig. 4), are produced by members of different groups of bacteria (including the gram-negative alpha and gamma subgroups of the Proteobacteria and Bacteroidetes, as well as gram-positive bacilli), indicating their broad host range and frequent transfer between various bacterial genera.
The ISs described in this paper are the first transposable elements identified in P. solventivorans. Some of these elements might be useful for strain or species identification. These elements are members of two different families, the IS256 family (ISPso1) and the IS5 family (ISPso2 and ISPso3). Several elements classified in the IS5 family (which is relatively heterogeneous and comprises several groups) were also identified in P. pantotrophus (3), which suggests that ISs belonging to this family predominate in paracocci.
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