Appl Environ Microbiol, July 1998, p. 2710-2715, Vol. 64, No. 7
0099-2240/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Biotechnology Center for Agriculture and the Environment, Cook College, Rutgers University, New Brunswick, New Jersey 08901-8520
Received 12 February 1998/Accepted 5 May 1998
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ABSTRACT |
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A series of modular mini-transposon derivatives which permit the rapid cloning and mapping of the DNA flanking the minitransposon's site of insertion has been developed. The basic plasposon, named TnMod, consists of the Tn5 inverted repeats, a conditional origin of replication, rare restriction endonuclease multiple cloning sites, and exchangeable antibiotic resistance cassettes. The broad host range and low target DNA sequence specificity of the Tn5 transposase, in combination with the flexibility afforded by the modular arrangement of TnMod, result in a versatile tool for the mapping of insertional mutations and the rapid recovery of clones from gram-negative bacteria.
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TEXT |
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Transposon mutagenesis remains one of the most extensively utilized genetic techniques available for the characterization of bacteria. Transposon mutagenesis is especially useful for bacterial species with poorly described genetic systems or when existing molecular tools are inadequate. There are many well-characterized transposons available for the mutagenesis of both gram-positive and gram-negative bacteria (for reviews, see references 3, 20, and 25). Unfortunately, there are often certain difficulties associated with their use. Typically, transposons are large (making them difficult to manipulate) and may contain antibiotic resistance determinants which are not useful for selection in some bacterial species. Many transposons are inserted nonrandomly at particular target DNA sequences (31). In addition, many transposons have a limited host range or may transpose preferentially into plasmids rather than chromosomes (15, 18). Once a transposon integrates into a target DNA, it is potentially unstable since it is still capable of undergoing additional transpositional events or promoting DNA rearrangements within the cell. Finally, once a transposon is established in the host bacterium's genome, additional mutagenesis with a second transposon can be inhibited due to the resident transposon's production of an inhibitory protein (3, 4).
The problems associated with the use of transposons have largely been overcome with the development of minitransposons. Minitransposons are specialized transposons which arrange the cognate transposase outside of the transposon's inverted repeats (7, 10, 15, 18, 33). This arrangement permits the minitransposon to stably integrate into a target DNA without its transposase. Not only does this prevent further transposition and DNA rearrangements, it also allows for repeated rounds of minitransposon mutagenesis since no immunity protein is present in the cell. These synthetic minitransposons are small and stable and have been constructed to contain different antibiotic resistance determinants (7, 15). Minitransposons based on transposon Tn5 function in a wide range of gram-negative bacteria and exhibit virtually no preference for a specific target DNA sequence (4, 7).
In order to increase the cloning functionality of transposons, several investigators have included a conditional origin of replication within the basic transposon to produce "self-cloning" or "in vitro-cloning" transposons (11, 13, 17, 23, 36). The inclusion of a conditional origin of replication within the transposon allows the rapid cloning of the DNA adjacent to the transposon's site of insertion. The in vitro cloning is performed by digesting the total genomic DNA, self-ligating it, and transforming it into a permissive Escherichia coli host. The presence of a conditional origin of replication in the transposon expedites the cloning process and decreases the chances of obtaining a noncolinear DNA fragment during the cloning process.
The unique features of minitransposons and self-cloning transposons have been combined to construct new Tn5-based minitransposons for the rapid genetic analysis of gram-negative bacterial genomes. The basic minitransposon has been modified to include a conditional origin of replication and exchangeable antibiotic resistance determinants. The modular arrangement of the new TnMod minitransposons allows for different combinations of antibiotic resistance determinants and high- or low-copy-number origins of replication. Rare restriction endonuclease sites have been incorporated near the inverted repeats in order to facilitate the localization of the minitransposon's insertion site on a physical genome map. These rare restriction sites can also be used to construct a library of clones containing large DNA fragments surrounding the transposon's site of insertion.
Development of TnMod plasposons. In order to construct TnMod, an XbaI-SfiI DNA fragment containing the 19-bp "inside-end" inverted repeat and a NotI-EcoRI DNA fragment containing the 19-bp "outside-end" inverted repeat were isolated from pUT/mini-Tn5Km2 (7). As shown in Fig. 1, these DNA fragments were individually ligated to two sets of complementary oligonucleotides which, when annealed, contain sites for restriction enzymes which rarely cut bacterial DNA. The Tn5 inside-end rare multiple cloning site (RMCS1) oligonucleotide contains sites for SfiI, FseI, SgfI, SgrAI, AscI, NsiI, SalI, and KpnI. The Tn5 outside-end RMCS2 oligonucleotide contains restriction sites for ClaI, SstI, PacI, SwaI, XbaI, SpeI, and NotI. A 700-bp PCR product containing the pUC (pMB1/ColE1) origin of replication was amplified from pUC19 (35) with PCR primers 5'-GGGTACCAGGAAAGAACATGTG-3' and 5'-CCATCGATTTCGTTCCACTGAG-3'. This fragment was digested with KpnI and ClaI and in a four-way ligation was cloned into pBBR1MCS (22), creating pBBO. A gentamicin resistance cassette isolated from p34S-Gm was cloned into the SstI site of pBBO, creating pBBOGm. In order to construct the delivery DNA fragment, the origin of transfer from broad-host-range plasmid RP4 (29) was isolated from pMOB3 (27) and ligated into pUC18 (35), creating pT18. The Tn5 transposase was isolated from pUT/mini-Tn5Km2 (7) on a 1.5-kb XbaI-SalI fragment. The ends were polished with mung bean nuclease, and this fragment was blunt end ligated into pT18 digested with BamHI and mung bean nuclease, forming pTT18. The XbaI-EcoRI fragment from pTT18 containing the RP4 oriT and Tn5 transposase was isolated and ligated to the XbaI-EcoRI fragment isolated from pBBOGm. The resulting plasmid/transposon, shown in detail at the bottom of Fig. 1, was sequenced to verify its correctness and designated pTnMod-OGm. All derivatives containing different antibiotic resistance cassettes and origins of replication are modifications of this basic construct.
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Construction of TnMod plasposon origin-of-replication
derivatives.
In order to obtain a plasmid origin which will not
replicate upon introduction into Enterobacteriaceae, a
modified origin of replication from plasmid R6K was utilized
(21). Since the R6K origin of replication requires the
gene product to function properly, plasmids containing an R6K origin of
replication and not the
gene will not replicate unless the
bacterial host can supply the
replication protein in trans
(21). E. coli lambda phage lysogens which
contain the
gene serve as hosts which allow the maintenance of
TnMod derivatives containing the modified R6K origin of
replication. TnMod variants containing the modified R6K
origin of replication will not replicate in
Enterobacteriaceae not expressing
, which allows
the selection of cells whose plasposons have integrated into the
chromosome. The R6K origin of replication was isolated from
pJM703.1 (24) on a 420-bp BamHI fragment, treated with mung bean nuclease, and ligated into pGEM7Z
(Promega, Madison, Wis.) digested with SmaI. The
resulting plasmid, designated pJDR, contains the R6K origin of
replication flanked by a KpnI site and a ClaI
site. This KpnI-ClaI DNA fragment was
cloned into TnMod, replacing the origin of replication from
pMB1. These plasposon constructs are designated
TnMod-R.
Construction of exchangeable antibiotic resistance cassettes. Because many gram-negative bacteria are naturally resistant to high levels of antibiotics, TnMod was constructed in such a way as to allow the addition of multiple antibiotic resistance cassettes or the exchange of different antibiotic resistance cassettes based on the antibiotic resistance characteristics of the target bacteria. Antibiotic resistance cassettes for chloramphenicol, gentamicin, kanamycin, streptomycin, tetracycline, and trimethoprim were constructed in the cassette vector p34E (30) from resistance genes originating from Tn9, Tn1619, Tn903, E. coli aac(3)-IV, pBR322, and R388, respectively. As shown in Fig. 2, the resulting antibiotic resistance cassettes are flanked by duplicate multiple cloning sites. These antibiotic resistance cassettes can be inserted into either the left or right side of the origin of replication in TnMod. Alternatively, multiple antibiotic resistance cassettes can be inserted into both ends of TnMod in order to select for TnMod insertions by using two or more antibiotics for bacteria especially resistant to antibiotics.
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35 region of the tetracycline gene
lost during construction, two complementary oligonucleotides (5'-CATGTTTGACAGCTTATCAT-3' and
5'-CGATGATAAGCTGTCAAACATGAGCT-3') were annealed,
reproducing the
35 region, and the double-stranded oligonucleotide was ligated into p34S-Tc digested with ClaI
and partially digested with SstI. The ClaI site
between the
10 and
35 regions was eliminated by digesting the
plasmid with ClaI and filling in the ends of the DNA
fragment with Klenow DNA polymerase and deoxynucleoside triphosphates.
The nucleotide sequences of these plasmid constructs were verified by
DNA sequence analysis.
The basic TnMod can be outfitted with multiple antibiotic
resistance cassettes in a variety of ways, making a numerical
designation for each derivative uninformative. Instead,
TnMod derivatives are named according to the physical
locations and orientations of their components. An antibiotic
resistance cassette located next to the inside inverted repeat is
designated with a two-letter abbreviation (Cm, Gm, Km, Sm, Tc, or Tp),
which appears immediately after "TnMod-" in the name.
The origin of replication, located in the center of TnMod,
is abbreviated with a one-letter abbreviation (O for pMB1
oriR, R for R6K oriR, or S for pSC101
oriR). An antibiotic resistance cassette following the
origin of replication, near the outside inverted repeat, is similarly
designated with a two-letter abbreviation. The orientation of an
antibiotic resistance cassette, which is especially important in
designing primers to sequence DNA adjacent to TnMod, is
denoted by a prime when antibiotic resistance cassette genes are
oriented away from the origin of replication; the lack of a prime
indicates the reverse orientation. By this format, a TnMod
derivative containing a kanamycin resistance cassette in the
KpnI site oriented toward the R6K origin and a
chloramphenicol resistance cassette in the SstI site
oriented away from the origin would be named
TnMod-KmRCm'. If this TnMod derivative exists in plasmid form (i.e., attached to the delivery DNA fragment containing the RP4 oriT and Tn5 transposase), it is denoted
pTnMod-KmRCm'.
Due to the large number of structural variations possible with the
TnMod mutagenesis system, including the type of origin of replication and the number, types, and orientations of the antibiotic resistant cassettes, only a small fraction of these variants
were constructed. The TnMod variants constructed, including their salient characteristics, are listed in Fig.
3. (A form for requesting the
pTnMod derivatives can be downloaded from
http://www.rci.rutgers.edu/~zylstra/plasposon.) In order to
facilitate DNA sequencing primer design and the subcloning of the
plasmids formed by the self-ligation of the excised plasposons and
flanking chromosomal DNA, the complete DNA sequences of the constructed
TnMod variants and the p34S antibiotic resistance cassette
plasmids have been submitted to GenBank.
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TnMod mutagenesis procedure. TnMod insertional mutagenesis and clone recovery is simple and rapid. Once the antibiotic resistance MICs have been determined for the target bacteria, pTnMod is introduced by a triparental mating. Briefly, the recipient bacterial cells, the E. coli host containing the pTnMod derivative, and E. coli HB101 containing the helper plasmid pRK2013 (12) are grown in liquid culture to mid-log phase. Equal volumes of these cells (approximately 1 ml) are combined and centrifuged at low speed at room temperature. The supernatant is discarded, and the cells are resuspended in 50 µl of Luria-Bertani (LB) broth or 10% glycerol and plated in a small pool on a nonselective LB plate. This mating plate is left for 6 to 18 h at 25 to 37°C, depending upon the optimum growth temperature of the target bacteria. The cells are then scraped off the plate and resuspended in 1 to 2 ml of buffer (e.g., 10 mM HEPES or 10% glycerol), and appropriate dilutions are plated on selective medium. This medium should contain nutrient limitations or antibiotic levels which select for exconjugant target cells and eliminate the donor E. coli strains. The plates are incubated for 1 to 2 days, and individual exconjugant colonies are picked to a second selective-medium plate. Alternatively, the recipient cells can be transformed directly with purified pTnMod by electroporation (8).
Once a desired mutation is identified, DNA flanking the insertion site can be rapidly cloned. Minichromosomal preparations are performed as described by Ausubel et al. (2). One microgram of total genomic DNA is digested with a restriction enzyme that cuts outside of or within one end of TnMod. The enzyme is removed, and the DNA is self-ligated under conditions favorable for intramolecular reactions. A portion of the ligation mixture is transformed into an appropriate E. coli host by electroporation (i.e., strain JM109 for pMB1- and pSC101-based TnMods and strain CC118
pir (16) for R6K-based TnMods). The
resulting plasmids containing TnMod are selected with the
antibiotic resistance determinant(s) encoded by TnMod. In
order to determine the identity of the gene into which the plasposon
has been inserted, a plasmid minipreparation of the E. coli
clone yields plasmid DNA ready for sequencing. This entire procedure
can be performed in as little as 2 days. Once a desired clone is
obtained, the entire chromosomal DNA insert can rapidly be sequenced by
primer walking or by using exonuclease III to construct nested
deletions in the cloned DNA fragment (6).
In order to generate a clone library by using TnMod,
TnMod mutagenesis is performed as described above. The use
of rare-cutting restriction enzymes will produce clones each of which
contains a large segment of the genome. After selection for recipient
cells by TnMod integration, genomic DNAs are isolated from
pools of TnMod exconjugants, cleaved with a rare-cutting
restriction enzyme, self-ligated, and transformed into a permissive
E. coli host. A plasposon library generated in this fashion
is a relatively rapid and simple alternative to the construction of
cosmid libraries.
Applications of TnMod.
Because Tn5 has a
broad host range (4), TnMod should work
efficiently in gram-negative bacterial hosts such as
Acinetobacter, Aeromonas,
Agrobacterium, Bordetella,
Caulobacter, Moraxella, Rhizobium, and
Vibrio spp., as well as in the
Enterobacteriaceae. We have successfully tested
TnMod mutagenesis with several different genera of bacteria
including Burkholderia, Escherichia,
Pseudomonas, and Sphingomonas. In
Pseudomonas putida F1 (14), for example, TnMod-OKm was found to be inserted in the chromosome
randomly and stably, with an operational transpositional frequency of
approximately 10
3 to 10
4. In all insertions
examined, the transposon delivery DNA fragment did not transpose with
TnMod (data not shown). We have successfully used the
TnMod mutagenesis system to identify and isolate genes involved in solvent tolerance from P. putida S12
(19).
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Nucleotide sequence accession numbers. The sequences of p34S antibiotic resistance cassette plasmids and those of TnMod variants have been submitted to GenBank and have been assigned the accession numbers shown in Fig. 2 and 3, respectively.
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ACKNOWLEDGMENTS |
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This work was supported by a National Science Foundation Young Investigator Award to G.J.Z. and cooperative agreement CR822634 from the U.S. Environmental Protection Agency Gulf Breeze Environmental Research Laboratory.
We thank D. DeShazer, H. P. Schweizer, P. A. Sokol, and K. N. Timmis for sharing bacterial strains and plasmids. We thank L. Newman in this laboratory for helpful discussions and M. Murillo for excellent technical assistance.
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FOOTNOTES |
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* Corresponding author. Mailing address: Biotechnology Center for Agriculture and the Environment, Foran Hall, 59 Dudley Rd., Cook College, Rutgers University, New Brunswick, NJ 08901-8520. Phone: (732) 932-8165, ext. 320. Fax: (732) 932-0312. E-mail: zylstra{at}aesop.rutgers.edu.
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