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J Bacteriol, January 1998, p. 178-181, Vol. 180, No. 1
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Molecular Characterization of IS1541
Insertions in the Genome of Yersinia pestis
Monique
Odaert,1
Annie
Devalckenaere,1
Patrick
Trieu-Cuot,2 and
Michel
Simonet1,*
Laboratoire de
Bactériologie-Hygiène, Faculté de Médecine
Henri Warembourg, Lille,1 and
INSERM
U411, Faculté de Médecine Necker,
Paris,2 France
Received 16 July 1997/Accepted 25 October 1997
 |
ABSTRACT |
The genome of Yersinia pestis, the causative agent of
plague, contains at least 30 copies of an element, designated
IS1541, which is structurally related to IS200
(85% identity). One such element is inserted within the chromosomal
inv gene (M. Simonet, B. Riot, N. Fortineau, and P. Berche,
Infect. Immun. 64:375-379, 1996). We characterized other
IS1541 insertions by cloning 14 different Y. pestis 6/69M loci carrying a single copy of this insertion
sequence (IS) into Escherichia coli and, for each element, sequencing 250 bp of both flanking regions. In no case was this IS
element inserted into large open reading frames; however, in eight
cases, it was detected downstream (17 to 139 bp) of genes thought to be
transcribed monocistronically or which constituted the last gene of an
operon, and in only one case was it detected upstream (37 bp) of the
first gene of an operon. Sequence analysis revealed stem-loop
structures (
G, <
10 kcal) resembling rho-independent transcription terminators in 8 of the 14 insertion sites. These motifs
might constitute hot spots for insertion of this IS1541 element within the Y. pestis genome.
 |
TEXT |
Transposition of insertion sequences
(IS) is known to cause a number of effects, including insertions,
deletions, and cointegrate formation, which can result in silent
mutations, gene inactivation, or the modulation of expression of
downstream genes (8). Several classes of IS elements are
found in Yersinia pestis, the causative agent of bubonic and
pneumonic plague. They include IS100, IS285, and
IS1541 (previously designated IS200-like)
elements (7, 24, 30), which are integrated into the
chromosome and/or within the three virulence plasmids found in this
bacterium, pYV, pFra, and pPst (for a review, see references
3 and 23). Guiyoule et al.
(12) demonstrated by pulsed-field gel electrophoresis analysis that Y. pestis is subject to numerous and
spontaneous DNA rearrangements. These events might be due to the
presence of multiple IS in the bacterial genome allowing the fusion of replicons, such as the integration of virulence plasmids pYV and pFra
into the chromosome (25, 32), or the deletion of fragments, such as the 102-kb chromosomal pigmentation (pgm) locus.
This locus is a pathogenicity island involved in iron acquisition and can be lost spontaneously, at high frequency, by Y. pestis
(6).
IS1541 (approximately 708 bp) was recently discovered within
the chromosomal inv gene of Y. pestis, and gene
disruption results in the inability of this bacterium to invade
cultured mammalian cells (30). It displays 85% nucleotide
identity with IS200 (approximately 709 bp) found in
Salmonella spp., Shigella spp., and
Escherichia coli, and both elements encode homologous (93%
identity) 152-amino-acid putative transposases (1, 11, 14).
Unlike most IS elements, IS200 and IS1541 do not
contain inverted terminal repeats (15, 21, 30). As a
consequence of this unusual feature, the ends of these elements have
not been precisely characterized up to now. The IS1541
element has been found in multiple copies in the genome of Y. pestis (30), and in this report, we present additional sequence data for 14 insertion sites of the element other than in the
inv gene in Y. pestis 6/69M.
Distribution of IS1541 in the Y. pestis
6/69M genome.
We have previously shown that the genome of Y. pestis 6/69M, which belongs to biotype Orientalis and ribotype B
(12), contains multiple copies of the IS1541
element (30). By hybridization with an IS1541
probe of bacterial DNA digested by NotI or SpeI, restriction endonucleases that infrequently cut Y. pestis
DNA (12, 17) and do not cut the IS element, we found that 18 of 27 NotI fragments and 17 of 29 SpeI fragments
hybridized with this probe (Fig. 1A).
This result demonstrates that the IS1541 elements are not
clustered in the genome.

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FIG. 1.
Distribution of IS1541 elements in Y. pestis 6/69M. Bacterial DNA was digested by appropriate
restriction endonucleases according to the manufacturer's
recommendations (New England Biolabs), and DNA fragments were separated
by electrophoresis. Pulsed-field gel electrophoresis of macrorestricted
genomic DNA was carried out as described by Guiyoule et al.
(12). After separation, DNA fragments were depurinated,
denaturated, and then transferred to nylon membranes (Boehringer Gmbh,
Mannheim, Germany) by the Southern technique (28). The
IS probe preparation, [32P]dCTP (Amersham France,
Courtaboeuf, France) labeling, and DNA-DNA hybridization were
performed as previously described (20). (A) Lanes 1 and 2, pulsed-field gel electrophoresis of DNA digested by
NotI (lane 1) or SpeI (lane 2); lanes 3 and 4, Southern blot hybridization of DNA digested by NotI (lane 3)
or SpeI (lane 4) with the radiolabeled IS probe. (B) Lane 1, agarose gel electrophoresis of a total plasmid extract (pYV, pPst, and
pFra) prepared by an alkaline lysis procedure and digested by the
restriction endonuclease EcoRV; lane 2, Southern blot
hybridization of EcoRV-digested DNA with the radiolabeled IS
probe. (C) Southern blot hybridization of HincII-digested
DNA from Pgm+ (lane 1) and Pgm
(lane 2) cells with the radiolabeled IS probe. Molecular
size markers (in kilobases) are indicated on the left. A strong
hybridization signal given by some bands may correspond to the presence
of multiple IS.
|
|
As in almost all strains of Y. pestis (5), strain
6/69M contains one plasmid each of 110 (pFra), 70 (pYV), and 9.5 (pPst) kb. Plasmid DNAs were extracted from this strain, digested with EcoRV (which does cleave within IS1541),
separated by electrophoresis, transferred onto a nylon membrane, and
hybridized with a 32P-labeled IS1541-specific
DNA probe (28). A single hybridizing band of about 3.2 kb
was detected, and on the basis of the known restriction maps of
individual plasmids, this fragment was assigned to pFra (Fig. 1B).
Thus, almost all IS1541 copies were located on the bacterial
chromosome.
Finally, we looked for the presence of the IS within the
pgm
locus. This was indirectly achieved by comparing the IS
1541
hybridization
profiles of DNA from the parental strain Pgm
+
and its derivative strain Pgm

, digested by
HincII, an enzyme which does not cleave within
IS
1541.
As depicted in the autoradiogram presented in Fig.
1C, there was
no detectable difference in the IS patterns exhibited by
the two
strains. This result suggested that there is no
IS
1541 element
within the
pgm locus. Moreover,
E. coli recombinant clones carrying
DNA fragments spanning
the
pgm locus did not hybridize with an
IS
1541-specific DNA probe (
4).
Cloning of IS1541 flanking regions of the Y. pestis 6/69M genome.
The autoradiogram presented in Fig. 1C
revealed the presence of numerous hybridizing bands, mostly ranging
from 2 to 6 kb in size. To identify regions of the Y. pestis
genome in which the IS element was inserted,
HincII-restricted DNA from strain 6/69M was separated by
electrophoresis through a 1% agarose gel in Tris-acetate buffer, and
2- to 6-kb DNA fragments were extracted from the gel by using
-agarase (New England Biolabs, Beverly, Mass.) and ligated with T4
DNA ligase into a SmaI-digested pUC19 vector (Appligene,
Illkirch, France) (28). Recombinant plasmids were introduced
into E. coli JM105 by transformation (28), and transformants were selected on Luria-Bertani agar containing ampicillin (100 µg ml
1) and X-Gal
(5-bromo-4-chloro-3-indolyl-
-D-galactopyranoside). White
transformants resistant to ampicillin were screened for the presence of
the IS1541-related sequences by colony blot hybridization (28) with the specific probe. Endonuclease restriction
(EcoRI, HindIII, and SphI)
followed by Southern blot hybridization with the IS1541
probe of plasmids from recombinant clones allowed discrimination of 14 different inserts having a size less than 4.5 kb and containing a
single copy of the IS element. Following digestion with the appropriate
enzymes (EcoRI and/or SphI), these inserts were
purified and put into M13mp18 and -mp19.
IS1541 insertion specificity in Y. pestis
6/69M.
IS200 does not possess terminal inverted repeats
and preferentially inserts into T/A-rich short segments (15,
21). Consequently, the ends of this element have not been
definitively assigned and a duplication event at the target site, if
any, has not been precisely characterized. We previously reported the
insertion of IS1541 within the inv gene of
Y. pestis, and in the present work we sequenced about 350 nucleotides of both IS-chromosome junction fragments in 14 different
Y. pestis loci (MO951, MO952, MO953, MO954, MO955, MO956,
MO957, MO958, MO959, MO960, MO962, MO963, MO964, and MO965). In all
cases, the nucleotide sequence (100 bp) of both termini of
IS1541 were identical to those of the prototype sequence
(30). Sequence analysis revealed that the right extremity of
IS1541, as defined in Fig. 2A,
is almost identical to that of IS200, whereas their left
extremities display sequence divergence within their outermost 7-bp
segments. However, the ends of IS1541, like those of
IS200, do not contain inverted repeats. Comparison of the
sequences of the 14 target sites of IS1541 revealed that in
all cases insertion occurred within a T/A-rich short (
10-bp) segment
(Fig. 2B). Two T's were invariably present in both sides of the IS
element, but since the sequence of targets prior to insertion is not
known, it is not possible to determine if these motifs belong to the element or whether they result from an IS-mediated duplication event.
More interestingly, in 13 of the 14 targets studied, sequence analysis
revealed that the region upstream of the IS could form stem-loop
structures having a minimum energy formation of <
10 kcal/mol. These
motifs resemble rho-independent transcription terminators and might
constitute hot spots for insertion of IS1541 within the
Y. pestis genome. Precise characterization of the
transposition behavior of IS1541 will require the
development of a qualitative and quantitative transposition assay.

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FIG. 2.
Nucleotide sequences adjacent to IS1541 in
Y. pestis 6/69M. Sequencing of the IS flanking regions was
achieved by the dideoxynucleotide chain termination method
(29) with modified T7 DNA polymerase (Sequenase version 2.0;
Amersham France) and primers located at the left
(5'-CATTTGCAGTTGCCAG-3') and right (5'-GTTTACGGGCCGTAA-3') ends of the
IS (30). In each case, the sequence of the complementary
strand was carried out by using synthetic oligonucleotides designated
from the sequence previously determined. (A) The 5' and 3' ends of
IS200 and IS1541 are shown. Asterisks indicate
identical nucleotides. (B) Nucleotide sequences (36 bp) upstream and
downstream of 15 IS elements are shown. Inverted and complementary
nucleotides are depicted by arrows below the sequences. The loop free
energies ( G, in kilocalories) were calculated as reported
by Zuker (33).
|
|
In none of the 14 target sequences was IS
1541 inserted
within large open reading frames (ORFs) (data not shown). However,
sequence analysis using the BlastX program revealed that, in 9
of 14 cases, this IS inserted in the vicinity of an ORF (designated
orf1 to
orf9, of which
only
orf4 was entire) coding for a peptide
highly homologous (47 to 98% identity) to a known bacterial protein
from
E. coli or
Salmonella typhimurium (Fig.
3). In eight cases,
IS
1541
inserted downstream of genes thought to be either transcribed
monocistronically (
cspG,
envZ,
gltX,
and
suhB) or which constituted
the last gene
(
crr,
ilvH,
rpoC, and
secF)
of an operon. These
findings are consistent with our proposal that the
hairpin structures
present at the corresponding IS
1541
insertion sites might correspond
to transcriptional terminators.
Insertion of IS
1541 in the 5'
region of
leuA, the
first gene of the leucine operon, constitutes
the only case of
insertion of this IS upstream from a gene. By
analogy with the
S. typhimurium system, it is likely that this
insertion has occurred
in a hairpin structure belonging to the
transcriptional regulatory
region of the
leu operon (
10).

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FIG. 3.
Insertions of IS1541 in eight coding regions
of the Y. pestis 6/69M genome. Arrows represent ORFs.
Asterisks and periods indicate identical and similar amino acids,
respectively. Symbols: , IS insertion site;
|, hairpin
structure. Left and right extremities of IS1541 are
indicated by L and R, respectively. LeuA is the E. coli
-isopropylmalate synthase (26); SuhB is the extragenic
suppressor protein of E. coli (31) which
possesses inositol monophosphatase activity (18); RpoC is
the ' subunit of the E. coli RNA polymerase
(22); CspG is an E. coli cold shock protein
(19); EnvZ is the osmotic sensor from the two-component
system OmpR-EnvZ of S. typhimurium (16); Crr is
the component III (IIIGlc) of the phosphoenolpyruvate-dependent
phosphotransferase system of E. coli (27); SecF
is a protein export membrane protein of E. coli
(9); IlvH is an acetolactate synthase of S. typhimurium (13) involved in isoleucine and valine
synthesis; GltX is the glutamyl-tRNA synthetase of E. coli
(2).
|
|
Characterization of IS1541 insertions in unrelated
Y. pestis strains.
The next step was to consider
whether, in other strains of Y. pestis, the
IS1541 element flanked the same genes as in strain 6/69M.
For this purpose, strains Hambourg 12 (biotype Orientalis, ribotype A),
PKR XXIV (biotype Medievalis, ribotype O), Sénégal Th
(biotype Orientalis, ribotype B), and Saïgon 55-1239 (biotype Orientalis, ribotype E) from the Centre National de
Référence des Yersinia (Institut Pasteur, Paris,
France) were investigated. As previously reported (30),
these strains, isolated from different periods of time and geographical
areas, displayed different IS1541 hybridization patterns.
Y. pestis DNA was amplified by PCR with different sets of
primers, each of them consisting of (i) a forward primer located within
one of the previously defined ORFs (F1, 5'-CTGAGCGATAGTACG-3'
[orf1]; F2, 5'-CTATTGGTTCGTGAG-3'
[orf2]; F3,
5'-CGTGATGAATTGCGTGGC-3' [orf3];
F4, 5'-GAAGTATCTCTATGTC-3' [orf4];
F5, 5'-CGTGTTAAATCAGCG-3' [orf5];
F6, 5'-CGTATCGCTGAAGAA-3' [orf6];
F7, 5'-CGTCGTGGAACGCCT-3' [orf7];
F8, 5'-CTCACATGTTGAGCG-3' [orf8]; or F9,
5'-CGCCATCACCGAATG-3'[orf9])
and (ii) a reverse primer chosen in the IS element sequence (R1,
5'-CATTTGCAGTTGCCAG-3' [used in PCR with F2, F5, F7, F8,
and F9] or R2, 5'-TGCGGTCTGGCAACT-3' [used in PCR with F1,
F3, F4, and F6]). PCRs were performed as previously described
(30), and amplimers were analyzed by 1% agarose gel
electrophoresis. For the five unrelated strains, the IS element was
always found, on the basis of product size, in the vicinity of
leuA, suhB, rpoC, crr,
secF, ilvH, envZ, or gltX but was not associated with suhB in strain PKR XXIV (data
not shown). We previously reported that an identical copy of
IS1541 was inserted at the same nucleotide position within
the inv gene of strains 6/69M, Hambourg 12, PKR XXIV,
Sénégal Th, and Saïgon 55-1239 (30).
This indicates that acquisition of IS1541 by these strains
was prior to their divergence. Data also suggest that insertions of
IS1541 at a given loci constitute a stable event, which, in
turn, raises the question of the efficiency of the intracellular mobility of these IS elements in Y. pestis. This last point
is currently being investigated in the laboratory.
Nucleotide sequence accession numbers.
The sequences of
orf1, orf2,
orf3, orf4,
orf5, orf6,
orf7, orf8, and
orf9 have been submitted to the EMBL nucleotide
sequence database under accession numbers Z97975, Z97976, Z97977, Z97978, Z97979, Z97980, Z97981, Z97982, and Z97983, respectively.
 |
ACKNOWLEDGMENTS |
We thank C. Buchrieser and A. Guiyoule for their collaboration and
E. Carniel for a critical reading of the manuscript. We are also
grateful to P. Berche for his constant interest in this work.
This research was supported partly by INSERM, the Université
René Descartes (Paris), and the Conseil Régional Nord
Pas-de Calais.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Laboratoire de
Bactériologie et Hygiène, Faculté de Médecine
Henri Warembourg, 1 Place de Verdun, 59045 Lille, France. Phone: (33)
03 20 44 55 73. Fax: (33) 03 20 52 93 61. E-mail:
simonet{at}univ-lille2.fr.
 |
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J Bacteriol, January 1998, p. 178-181, Vol. 180, No. 1
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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