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Antimicrobial Agents and Chemotherapy, January 1998, p. 190-193, Vol. 42, No. 1
0066-4804/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Cloning and Nucleotide Sequence of the DNA Gyrase
gyrA Gene from Serratia marcescens and
Characterization of Mutations in gyrA of Quinolone-Resistant
Clinical Isolates
Jeong Hoon
Kim,1
Eun Hee
Cho,2
Kwang Seo
Kim,3
Hak Yeop
Kim,1 and
Young Min
Kim3,*
Department of Pharmacology and Molecular
Biology, C&C Research Laboratories, Kyunggi-do
445-970,1
Department of Science
Education, Chosun University, Kwangju 501-759,2
and
Molecular Microbiology Laboratory, Department of
Biology, Yonsei University, Seoul 120-749,3
Korea
Received 27 May 1997/Returned for modification 7 August
1997/Accepted 24 October 1997
 |
ABSTRACT |
The sequence of the DNA gyrase gyrA gene of
Serratia marcescens ATCC 14756 was determined. An open
reading frame of 2,640 nucleotides coding for a polypeptide with a
calculated molecular mass of 97,460 was found, and its sequence
complemented the sequence of an Escherichia coli gyrA
temperature-sensitive mutation. Analysis of the PCR products of the
quinolone resistance-determining regions of gyrA genes from
six quinolone-resistant clinical isolates revealed a single amino acid
substitution, Ser-83 to Arg or Asp-87 to Tyr, in all six mutants,
suggesting that a mutational alteration in gyrA is a common
mechanism of quinolone resistance in S. marcescens.
 |
TEXT |
DNA gyrase, a type II DNA
topoisomerase, is an enzyme capable of transforming relaxed closed
circular DNA into a negatively supercoiled form (5). The
enzyme contains two protein subunits, subunits A and B. The A subunit,
coded for by the gyrA gene, is responsible for introducing
and rejoining double-stranded breaks in DNA, while the B subunit, coded
for by gyrB, mediates energy transduction and ATP hydrolysis
during the topological transformation of DNA (28, 40).
Fluoroquinolones are potent broad-spectrum antibacterial agents which
inhibit DNA gyrase activity (44). Mutations conferring high-level quinolone resistance have been mapped to a small region of
the 5' end of the gyrA gene, which has been designated the quinolone resistance-determining region (QRDR). Mutations in the Ser-83
and Asp-87 codons in particular have been found in the majority of
quinolone-resistant clinical isolates of Escherichia coli
(26, 38). Similar mutations were also identified in
quinolone-resistant strains from a diverse group of bacteria (1,
6, 7, 12, 14, 19, 21, 22, 24, 35, 39, 41, 47).
Serratia marcescens is recognized as a frequent cause of
extraintestinal human infections ranging from simple cystitis to life-threatening bloodstream and central nervous system infections (2, 15, 43). Several strains of S. marcescens
causing nosocomial infections were found to acquire resistance to
fluoroquinolones at higher frequencies than those for the E. coli strains (4, 30, 31, 42), but nothing has been
reported about the gyrA gene structure, the mutation in
gyrA responsible for quinolone resistance, or the genetic
basis of decreased susceptibility to quinolones in this bacterium. We
therefore carried out this study to characterize the wild-type
gyrA gene of S. marcescens and also to elucidate
the mutations in the gyrA genes of several
quinolone-resistant clinical isolates.
Cloning and sequencing of gyrA gene of S. marcescens ATCC 14756.
The type strain of S. marcescens, ATCC 14756, was purchased from the American Type
Culture Collection, Rockville, Md. The genomic restriction map for the
gyrA gene in ATCC 14756 was determined by Southern
hybridization with a randomly digoxigenin-labeled 582-bp
SacI-SmaI fragment of pDH24 (9) and a
2.8-kb HindIII fragment of pMK90 (18)
corresponding to the 5' and 3' ends of E. coli gyrA,
respectively (Fig. 1A). In addition, a
648-bp DNA probe which is specific for the 5' end of the S. marcescens gyrA gene comprising the putative QRDR was generated by
PCR and was also used in the hybridization studies. To generate the
648-bp probe, two primers, primers TACACCGGTCAACATTGAGG and
TTAATGATTGCCGCCGTCGG, the sequences of which are identical
to the nucleotide sequence from positions +24 to +43 of the E. coli gyrase gyrA gene and complementary to positions
+652 to +671 of the E. coli gyrA gene, respectively,
(26), were selected on the basis of the close genetic
relatedness of Serratia spp. to E. coli.
Preliminary data showed that the nucleotide sequence of the 648-bp
fragment was highly homologous to that of the 5' end region of the
E. coli gyrA gene.

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FIG. 1.
(A) Genomic restriction map for the gyrA
locus in S. marcescens ATCC 14756. The gyrA gene
is indicated by a thick line. (B) Detailed restriction map and
sequencing strategy for the gyrA gene. The open reading
frame (ORF) of the S. marcescens gyrA gene is shown by a
thick line. The 1.0-kb SalI fragment in pSC6-1 and the
3.2-kb SalI fragment in pSC6-2 are indicated by pSC6-1 and
pSC6-2, respectively. A strategy for nucleotide sequencing by the
dideoxy method (32) is shown under pSC6-1 and pSC6-2. Three
to five independent sequencing reactions were done for each arrow.
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|
To construct a subgenomic library, genomic DNA digested with
BamHI was ligated into bacteriophage lambda EMBL3 and was
packaged by using Gigapack III gold packaging extracts (Stratagene
Cloning System, La Jolla, Calif.). Replicate plaques were screened with the PCR-amplified QRDR-containing 648-bp probe. Positive clones all
contained identical 14-kb BamHI inserts (Fig. 1A). When
E. coli KNK453 gyrA43(Ts) (13) was
transformed with pBluescript II KS(+) containing a 3.8-kb
BamHI-EcoRI fragment of the gyrA gene
from a positive clone, bacteriophage lambda clone 6, the bacterium
exhibited full growth at the nonpermissive temperature of 42°C.
E. coli KNK453 transformed with pBluescript II KS(+) containing no insert DNA grew well at 30°C but did not grow at 42°C, indicating that the complete gyrA gene is present in
the 3.8-kb insert DNA and that an S. marcescens
GyrA-E. coli GyrB holoenzyme complex is functionally active.
On the basis of the detailed restriction map of the bacteriophage
lambda clone 6 constructed by Southern hybridization of
the cloned DNA
fragment with the three
gyrA probes (Fig.
1B),
two
subclones, pSC6-1 containing a 1.0-kb
SalI fragment and
pSC6-2
containing a 3.2-kb
SalI fragment (Fig.
1B), were
constructed
in pBluescript II KS(+) and were used for sequence
analysis. An
open reading frame of 2,640 nucleotides coding for a
polypeptide
of 880 amino acids with a calculated molecular weight of
97,460
was found. The G+C content of the
S. marcescens gyrA
gene was
60%, which is in agreement with the overall G+C content (57.5
to 60%) of this bacterium (
8). The derived amino acid
sequence
of the GyrA protein showed 85.8, 85.7, 89, and 75.5% overall
identities
with the
E. coli (
34),
Klebsiella pneumoniae (
3),
Erwinia carotovora (
29), and
Aeromonas salmonicida
(
25) GyrA proteins,
respectively. As expected, a high degree
of sequence homology
was found in the N-terminal region of the protein.
The catalytic
Tyr-122 involved in DNA breakage and reunion was also
present,
as in
E. coli and other bacteria (
9).
Amino acid residues from
positions 67 to 106 (ARVVGDVIGKYHPHGDSAVYDTIVRMAQPFSLRYMLVDGQ),
which are known to
cause quinolone resistance in mutated
E. coli GyrA
(
46), were perfectly conserved in
S. marcescens
GyrA.
A potential Shine-Dalgarno sequence (GAGGG) was present 12 bp upstream
of the ATG translation initiation codon. A putative

10 promoter
sequence (TATAGT) similar to the proposed

10
E. coli consensus sequence (TATAAT) was found 46 bp
upstream of the
ribosome-binding site. The

35 region (TTGACA)
was not apparent,
but a sequence (ATTTTCC) similar to
the conserved sequence (GTTTACC)
found in the
E. coli (
34),
K. pneumoniae (
3), and
Bacillus subtilis (
20)
gyrA promoter
regions was present 26 bp upstream
of the

10 region. The sequence is
also similar to the sequences
GTTTGCC, GTTTCCC, and
GTTTAAG found in the
gyrA promoter regions
of
Pseudomonas aeruginosa (
14),
E. carotovora (
29), and
A. salmonicida
(
25), respectively. The formation of a cruciform
structure
is theoretically possible in the 5-bp inverted repeated
sequences AAAGC
and GCTTT, located 12 and 5 bp upstream of the

10 region,
respectively, as has been predicted to occur in the
corresponding
region of the
K. pneumoniae (
3),
E. carotovora (
29), and
A. salmonicida
(
25)
gyrA promoters. The presence
of an inverted
repeated sequence indicates that the supercoiling-dependent
regulation
of
gyrA transcription may work in
S. marcescens,
as
suggested for
E. coli (
10,
17). Located 46 bp
downstream of
the ochre stop codon is another 13-bp inverted repeat
with 4 bases
in between; this inverted repeat may act as a
transcription terminator
sequence. The modified relaxation-stimulated
transcription sequence
(TTGTGA
TATAGTTTTACACC)
which includes the nucleotides surrounding
the

10 region
(underlined) was also found 38 bp upstream of the
ribosome-binding
site, as in the
gyrA genes of
E. coli
(
33),
K. pneumoniae (
3), and
E. carotovora (
29).
In this experiment, approximately 600 bp upstream from the coding
region of
S. marcescens gyrA gene was sequenced. Analysis
of
this region, however, failed to identify an open reading frame
greater
than 74 amino acids long and revealed no homology with
the
E. coli gyrB sequence. The arrangement of DNA gyrase genes
on the
S. marcescens chromosome thus appears to be noncontiguous,
which is common in many gram-negative bacteria (
3,
14,
25,
27,
29,
34,
41,
45).
Characterization of gyrA mutations in
quinolone-resistant clinical isolates of S. marcescens.
The
susceptibilities of seven uropathogenic clinical isolates of S. marcescens isolated from patients in Dong Suwon Hospital, Suwon,
Korea (Table 1), to several quinolones were tested by the agar dilution
method established by the National Committee for Clinical Laboratory
Standards (23).
Two-step nested PCR (
11) and direct DNA sequencing were
carried out to analyze mutations in the
gyrA genes of the
clinical
isolates. For the first step, a 866-bp region of the
gyrA gene
was amplified by using two 25-mer oligonucleotide
primers, GGGCTTGTCTGGCTCCTTGTTCTCG
and
GGGAAGTCCGGCCCCGGGATGTGTT, the sequences of which are
identical
to the sequences at nucleotide positions 212 to 187 bp
upstream
of the ATG translation start codon and complementary to the
sequence
at positions 626 to 651 bp downstream of the ATG codon,
respectively.
A 245-bp
gyrA fragment was then amplified from
the 866-bp PCR
product by using two 17-mer primers,
TTGGGTAACGACTGGAA and GCCAACAGTTCGTGAGC,
the
sequences of which are identical to the sequences at nucleotide
positions 160 to 176 bp and 388 to 404 bp downstream of the translation
initiation codon, respectively. After removal of primers and free
nucleotides, PCR products were directly sequenced by the dideoxy
method
(
32).
Table
1 presents the nucleotide and amino
acid changes in the QRDR of the PCR-amplified fragments of the
gyrA genes from
clinical isolates. In six of the seven
isolates, a nucleotide
change leading to an amino acid substitution was
observed. No
amino acid substitution was found in strain DSWH 102. Among the
six mutants, four (DSWH 103, DSWH 105, DSWH 106, and DSWH
107)
were found to have the same amino acid substitution at position
83. These four mutants were characterized by a C-to-A mutation
at
nucleotide position 249, resulting in a Ser-to-Arg substitution.
Two
other mutants, DSWH 101 and DSWH 104, were characterized by
a G-to-T
mutation at codon 87, leading to a Asp-to-Tyr substitution.
All other
nucleotide changes detected in the seven isolates were
silent. A silent
mutation at the wobble position of codon 117
(GCG to GCC) was observed
in all isolates. The substitution of
Ser-83 with Arg has also been
found in quinolone-resistant clinical
isolates of
Enterococcus
faecalis (
12,
36) and
A. salmonicida (
24). The Ser-83-to-Arg substitution might lead to
high-level
quinolone resistance in the
S. marcescens GyrA
protein by introducing
a bulky amino acid residue into the protein and
also by decreasing
the hydrogen-bonding capacity between amino acid
residues. The
Asp-87-to-Tyr substitution, which results in the exchange
of a
negatively charged residue for a larger polar amino acid, has
also
been observed in quinolone-resistant
P. aeruginosa
(
14),
E. coli (
38), and
Salmonella saint-paul (
7).
In most previous reports on the resistance of clinical isolates or
spontaneous mutants of
S. marcescens to quinolones, the
decreased susceptibilities have been attributed to alterations
in outer
membrane proteins, resulting in the decreased permeation
of quinolones
(
30,
37,
42). Although mutations resulting
in quinolone
resistance have not yet been described at the nucleotide
level for the
S. marcescens gyrA gene, it has been demonstrated
in
reconstitution experiments with mutant GyrA and wild-type GyrB
proteins
that an alteration in the GyrA protein is sufficient
to render the DNA
gyrase resistant to quinolones in vitro (
4,
16). This,
together with the present observation that an amino
acid substitution
at position 83 or 87 was present in all clinical
isolates for which the
ciprofloxacin MIC was high (Table
1),
strongly suggests that DNA gyrase
is the primary target of quinolones
in
S. marcescens and
that these two residues of the GyrA protein
are especially important in
the formation of the quinolone-gyrase-DNA
complex. The possibility that
other mutations in the
gyrA gene
or that other mechanisms of
resistance (e.g., changes in outer
membrane proteins and mutations in
gyrB or
parC) may modulate
the ultimate MICs
cannot be excluded, however, since we have examined
only a small
portion of the
gyrA gene sequence from clinical isolates.
The unique MIC pattern for DSWH 105 among the four Ser-83-to-Arg
mutants supports this possibility (Table
1).
Nucleotide sequence accession number.
The nucleotide sequence
of the gyrA gene of S. marcescens ATCC 14756 has
been assigned accession number U56906 in the GenBank/EMBL database.
 |
ACKNOWLEDGMENTS |
We are especially grateful to James C. Wang, Kenneth N. Kreuzer,
Martin Gellert, Mary H. O'Dea, and Nicholas R. Cozzarelli for
providing the E. coli strains and plasmids. We also thank Sang S. Han, Department of Clinical Microbiology, Dong Suwon Hospital, Suwon, Korea, for generously providing clinical isolates of S. marcescens.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Molecular
Microbiology Laboratory, Department of Biology, Yonsei University,
Seoul 120-749, Korea. Phone: 82-2-361-2658. Fax: 82-2-312-5657. E-mail: young547{at}bubble.yonsei.ac.kr.
 |
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Antimicrobial Agents and Chemotherapy, January 1998, p. 190-193, Vol. 42, No. 1
0066-4804/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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