Nucleic Acids Research Advance Access originally published online on November 12, 2008
Nucleic Acids Research 2009 37(1):e3; doi:10.1093/nar/gkn884
Nucleic Acids Research, 2009, Vol. 37, No. 1 e3
© 2008 The Author(s)
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Improvement of bacterial transformation efficiency using plasmid artificial modification
Kazumasa Yasui1,
Yasunobu Kano2,
Kaori Tanaka3,
Kunitomo Watanabe3,
Mariko Shimizu-Kadota4,5,
Hirofumi Yoshikawa5 and
Tohru Suzuki1,*
1The United Graduate School of Agricultural Science, Gifu University, 1-1 Yanagido, Gifu 501-1193, 2Department of Molecular Genetics, Kyoto Pharmaceutical University, 1 Shichono-cho, Misasagi, Yamashina-ku, Kyoto 607-8412, 3Division of Anaerobe Research, Life Science Research Center, Gifu University, 1-1 Yanagido, Gifu 501-1193, 4Department of Environmental Science, Musashino University, Shinmachi Nishitokyo-shi, Tokyo 202-8585 and 5Department of Bioscience, Tokyo University of Agriculture, Sakuragaoka, Setagaya-ku, Tokyo 156-8502, Japan
*To whom correspondence should be addressed. Tel: +81 58 293 2996; Fax: +81 58 293 2992; Email: suzuki{at}gifu-u.ac.jp
Received July 22, 2008. Revised October 11, 2008. Accepted October 20, 2008.
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ABSTRACT
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We have developed a method to improve the transformation efficiency
in genome-sequenced bacteria, using ‘Plasmid Artificial
Modification’ (PAM), using the host's own restriction
system. In this method, a shuttle vector was pre-methylated
in
Escherichia coli cells, which carry all the putative genes
encoding the DNA modification enzymes of the target microorganism,
before electroporation was performed. In the case of
Bifidobacterium adolescentis ATCC15703 and pKKT427 (3.9 kb
E. coli-Bifidobacterium shuttle vector), introducing two Type II DNA methyltransferase
genes lead to an enhancement in the transformation efficiency
by five orders of magnitude. This concept was also applicable
to a Type I restriction system. In the case of
Lactococcus lactis IO-1, by using PAM with a putative Type I methyltransferase
system,
hsdMS1, the transformation efficiency was improved by
a factor of seven over that without PAM.
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INTRODUCTION
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Recently, vast amounts of sequence information concerning bacterial
genomes have become available. Currently, 670 whole-genome bacterial
sequences have been published and over 1900 projects are in
progress. However, much of the data has been used inefficiently
in molecular biological studies since reverse genetic tools,
such as convenient shuttle vectors, an efficient transformation
method, gene knockout and random mutagenesis techniques, etc.,
have not been available. Accordingly, we have been working towards
developing simple methods that would establish transformation
techniques for bacteria for which the genome sequence is available.
It is well known that most bacteria carry a specific restriction modification (R–M) system which acts as a barrier against the invasion of foreign DNA by infected phages or conjugative plasmids, etc. (1). The restriction enzymes recognize a specific 4 bp–8 bp DNA sequence and cleave the DNA, but do not recognize the same sequence when modified by the sequence-specific DNA methylase (2). This prevents the degradation of the host's own DNA by the restriction enzyme. According to REBASE (3), 88% of bacterial genomes carry R–M systems and 43% carry four or more R–M systems. These multiple R–M systems, acting to prevent the incorporation of foreign DNA, make it difficult to apply reverse genetics techniques. To predict the gene-encoding modification enzyme from the genome sequence information is not difficult, since it is usually located in the region flanking that encoding the restriction enzyme, also the specific motifs of the DNA methylase have been well studied (3). It was conjectured that if all, or at least some, of modification enzymes were to be expressed in Escherichia coli, then a plasmid prepared in the E. coli would be modified as if it was replicated in the target bacterium. Thus, it would elude cleavage by restriction enzymes during the transformation of the target bacterium and greatly improve the efficiency. We term this approach ‘Plasmid Artificial Modification’ (PAM, Figure 1).
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Figure 1. The PAM concept. (A) The conventional method for the transformation of bacteria. The introduced shuttle vector is degraded by a restriction enzyme of the target bacterium. A small amount of vector survives and replicates in the target bacterium. (B) A PAM plasmid carries all the modification methylase genes of the target host. A shuttle vector plasmid is introduced to E. coli host, which had harboured the PAM plasmid (PAM host). The shuttle vector is methylated by the modification enzyme encoded by the genes on the PAM plasmid in the E. coli host. The shuttle vector is then extracted and introduced into the target host by electroporation. The shuttle vector is resistant against restriction enzymes and yields higher transformation efficiency. (C) The R–M system has a complicated structure, such as a gene cluster that includes subunits or unknown accessory genes. Alternatively, the PAM plasmid, containing a modification gene and also unknown parts, could be introduced into the transformant harbouring a shuttle vector. The restriction enzyme acts, but some copies of the plasmid could survive in the PAM host. The plasmid is then extracted and introduced the target bacterium.
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Bifidobacterium adolescentis is one of the dominant commensal
bacteria of the adult human large intestine. We have recently
analysed the whole-genome sequence of the strain
B. adolescentis ATCC15703 (DDBJ/EMBL/Genbank Accession# AP009256
[GenBank]
). However,
it was impossible to perform reverse genetic experiments using
standard methodology because the transformation efficiency was
at an extremely low level (1–3
x 10
0 CFU/µg DNA,
Table 3). Therefore, we used this strain to demonstrate the
experimental approach using PAM.
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MATERIALS AND METHODS
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Vectors and bacterial strains
pBAD33 (
4) was used as low copy number cloning vector. A
Bifidobactrium-E. coli shuttle vector, pKKT427 (
Figure 3), was modified from a
pBRASTA101 replicon (
5). The pKKT427, 3.9 kb vector, carried
a spectinomycin resistance gene, a multi-cloning site and two
replication origins including
repB from
B. longum and ColEI
ori (
Figure 3). The bacterial strains,
B. adolescentis ATCC15703
was obtained from the American Type Culture Collection. An
E. coli stain TOP10 (Invitrogen, Carlsbad, CA, USA) (
Table 1) was
used as a host for cloning and methyltransferase expression.
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Figure 3. Molecular structure of shuttle vector pKKT427. A Bifidobacterium–E. coli shuttle vector, pKKT427, was a modified pBRASTA101 replicon. This shuttle vector had been constructed by modification from a previously reported shuttle vector pBRASTA101, a composite plasmid of pUC18 and MCS and it excluded the β-galactosidase and ampicillin-resistant genes.
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Culture and transformation conditions for Bifidobacterium
Bifidobacterium adolescentis ATCC15703 was grown anaerobically
at 37°C in MRS medium (Becton, Dickinson and Company, Franklin
Lakes, NJ, USA) supplemented with 0.02%
L-cysteine (Nacalai
Tesque, Kyoto, Japan) and 0.34%
L-ascorbic acid sodium salt
(Nacalai Tesque). Spectinomycin hydrochloride (Wako, Osaka,
Japan) was added to an MRS agar plate at 150 µg/ml for
the transformation experiment.
Escherichia coli transformants
were grown in LB medium supplemented with 150 µg/ml of
spectinomycin and/or 20 µg/ml of chloramphenicol. The
electroporation of
B. adolescentis ATCC15703 was performed as
described by Matsumura
et al. (
6).
Cloning of the methyltranceferase genes
The putative methyltransferase genes of B. adolescentis ATCC15703 were chosen on the basis of a BLAST search at NCBI (http://www.ncbi.nlm.nih.gov/BLAST/) and REBASE (3) (http://rebase.neb.com/rebase/rebase.html). Genomic DNA of Bifidobacterium was extracted and purified (7), and was used as a template in PCR using KOD-Plus-DNA polymerase (TOYOBO, Osaka, Japan). PCR primers (Table 2) were designed using IMC (In Silico Biology, Inc., Yokohama, Kanagawa, Japan) and Primer 3 (8). The PCR products were ligated to pBAD33 using an In-Fusion Dry-Down PCR cloning kit (Clontech, Mountain View, CA, USA) (9).
Plasmid DNA preparations from transformed B. adolescentis ATCC15703
A plasmid preparation from
Bifidobacterium was obtained based
on the alkaline-SDS method (
7), using the lytic enzyme, mutanolysin.
A 15 ml Bifidobacterial culture transformant was centrifuged
and the cell pellet was suspended in 15 ml of 0.9% NaCl, recentrifuged
and then resuspended in 100 µl of TE-glucose [50 mM glucose,
10 mM EDTA, 25 mM Tris–HCl (pH 8.0)]. The suspension (100
µl) was treated with 25 µl of 25 mg/ml lysozyme,
5 µl of 10 U/µl mutanolysin (Sigma-Aldrich, St.
Louis, MO, USA) and 1 µl of 10 mg/ml RNase A (Roche Diagnostics,
Basel, Switzerland) at 37°C for 30 min. To this, 200 µl
of an alkaline-SDS solution (0.2 N NaOH, 1% SDS) was added and
the mixture was incubated for 10 min, following which it was
neutralized by adding 100 µl of 3 M potassium acetate
(pH 4.8) and was then centrifuged at 10 000
g for 15 min at 4°C.
The supernatant was treated with the same volume of phenol–chloroform–isoamylalcohol
(25 : 24 : 1). The upper layer was collected and to this, 2.5
times its volume of ethanol was added following which the mixture
was subjected to further centrifugation. The pellet was rinsed
with 70% ethanol, dried and then dissolved in 50 µl of
TE buffer.
Transformation of Lactococcus lactis IO-1
The culture and transformation conditions for L. lactis IO-1 were as described previously (10). The plasmid pGKV11 was used as a shuttle vector. For cloning of hsdM1 and hsdS1 of L. lactis IO-1, E. coli BL21(DE3) and pETNH were used as a host-vector system (11), where the cloned gene was tightly repressed in the absence of the inducer.
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RESULTS AND DISCUSSION
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Two putative R–M clusters were found in the annotated
genome of
B. adolescentis ATCC15703 (
Figure 2A). Each cluster
contained one putative gene-encoding DNA methyltransferase,
BAD_1233 (M.
Sau3AI homologue) and BAD_1283 (M.
Kpn2kI, homologue).
These two genes have been amplified and introduced into pBAD33
(
4), a low copy number vector. Three plasmids, carrying BAD_1233
and/or BAD_1283 were constructed and designated pPAM1233, pPAM1283
and pPAM1233–1283 (
Figure 2B).
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Figure 2. Construction of pPAM plasmids. (A) The B. adolescentis ATCC15703 genome includes two R–M clusters, BAD_1227–1234 and BAD_1279–1284. Red boxes show putative restriction genes. The blue boxes (BAD_1233 and BAD_1283) show putative methyltransferase genes. (B) The putative methyltransferase genes were amplified by PCR using primers as listed in Table 2. The PCR products were joined by in vitro homologous recombination to plasmid vector pBAD33, which had been cleaved by HincII, using the In-Fusion Dry-Down PCR cloning kit (Clonetech) to obtain pPAM plasmids. Overlap extension PCR was used for BAD_1233–1283. The pPAM1233–1283 plasmid was a constructed operon of BAD_1233 and BAD_1283. In the first PCR, the coding region of BAD_1233, which was added to the downstream 19 bases from the 5'-end of BAD_1283 was amplified. BAD_1283 was obtained in the same manner 20 bases from the 3'-end of BAD_1233. In the second PCR, the first PCR products were used as a DNA template and PMT1-F and PMT2-R primers were used. The amplified DNA fragment was ligated to the same vector and the plasmid pPAM1233–1283 then obtained.
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There are no reported cryptic plasmids in
B. adolescentis, accordingly
a
B. longum–E. coli shuttle vector pKKT427 (
Figure 3)
was used in the transformation experiments, compacted to 3.9
kb, and this gave a high transformation efficiency of the shuttle
vector for
B. longum 105-A (1–3
x 10
6 CFU/µg DNA,
Figure 4) (
5). The plasmid, pKKT427, was introduced into PAM
hosts
E. coli TOP10 and its recombinants carrying pPAM1233,
pPAM1283 or pPAM1233
–1283 (
Table 1). A colony resistant
to the antibiotics spectinomycin and chroramphenicol was selected,
the vector, pKKT427 was then extracted and introduced into
B. adolescentis ATCC15703 by electroporation (
12). It was then
spread onto a MRS agar plate and cultured at 37°C under
anaerobic conditions (
5,
6).
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Figure 4. Comparison of PAM effects on transformation efficiencies. (A–D) Bifidobacterium adolescentis ATCC15703 was transformed by electroporation using the PAM method. The plasmid pKKT427 was prepared from E. coli TOP10 carrying pPAM1233-1283 (A), pPAM1233 (B), pPAM1283 (C) or without pPAM plasmid (D). An alkaline-SDS method using purification by agarose gel electrophoresis was used to isolate the PAM plasmids which were then introduced into B. adolescentis ATCC15703 by electroporation, as described previously (6). The electroporated samples were 100 times diluted in (A–C), but not in D. (E) Schematic presentation of transformation efficiencies. Plasmid pKKT427 was prepared from the PAM host (blue), B. longum 105-A (green) or B. adolescentis ATCC15703. The numbers beside arrows indicate transformation efficiencies (CFU/µg DNA).
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The shuttle vector pKKT427 acted as a replicon in the target
cells, as confirmed by plasmid extraction
(Figure 5). The transformation
efficiency without the PAM plasmid was 1–3
x 10
0 CFU/µg
DNA. The CFU with a single gene PAM (pBAD1233 or pBAD1283) was
around 10
4 CFU/µg DNA (
Table 3). Dual gene PAM (pBAD1233–1283),
carrying both methylase BAD_1233 and BAD_1283, yielded 10
5 CFU/µg
DNA (
Table 3,
Figure 4). The transformation efficiency with
pPAM1233–1283 was higher than that with pPAM1233 or pPAM1283.
It is postulated that the recognition sites of the two methyltransferases,
encoded by BAD_1233 and BAD_1283, were different and act synergistically.
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Figure 5. The transformation of Bifidobacterium was confirmed by plasmid isolation followed by agarose gel electrophoresis. Plasmids extracted from PAM host E. coli TOP10 harbouring pPAM1233–1283 (Lane 1) and from recombinant B. adolescentis ATCC15703 (Lane 2). Vector pKKT427 (Lane 3) and pPAM1233-1283 (Lane 4).
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A combination of two the putative modification enzymes yielded
a synergistic effect. When pKKT427 was extracted from transformed
B. adolescentis ATCC15703 and introduced into the same cells,
the efficiency was almost the same, 6–9
x 10
4 CFU/µg
DNA, as for PAM with pBAD1233–1283 (
Table 3). When the
plasmid pKKT427 was prepared from
B. longum 105-A, the transformation
efficiency into
B. adolescentis ATCC15703 was also improved
by a factor of 10
3 compared with that from
E. coli TOP10 without
PAM plasmid, which was almost the same as that with pPAM1233
or 1283 (
Table 3). It should be noted that the R–M system
of
B. longum 105-A was not determined, while that of
B. longum NCC2705 (
13) carried a BAD_1233 orthologue (78% identical to
BAD_1233). It appears that 105-A carried a BAD_1233-like modification
enzyme, which also improved the transformation efficiency of
B. adolescentis by a factor of 10
3.
These experiments clearly demonstrate that the PAM concept (Figure 1B) is an effective approach for constructing a transformation system for a bacterium for which the whole-genome sequence is known. A reverse sequence procedure was also tried (Figure 1C) and this also improved the efficiency.
Next, we applied this method to another bacterium, L. lactis IO-1 (14), which is capable to utilizing xylose and produce lactic acid efficiently. This strain has the potential to utilize biomass for lactic acid production; however, the transformation efficiency was too low to produce using the gene knockout technique. For this strain, the shuttle plasmid, pGKV11 (10), prepared from L. lactis IO-1(pGKV11), showed a 26-fold higher efficiency than that of E. coli BL21(DE3) harbouring pGKV11 (Table 4), suggesting that the strain IO-1 has R–M system(s). Primers were constructed based on the sequences of L. lactis IL1403 hsdMS (15) and used to amplify putative hsdMS genes using the IO-1 genomic DNA as a template. Then, we constructed a PAM plasmid, which is carrying a methyltransferase subunit gene, designated hsdM1 and its specificity subunit gene, hsdS1 of the IO-1 strain, and designated pETMS1. The transformation efficiency was successfully improved by a factor of seven times under the induced conditions (in the presence of IPTG), the transformation efficiency using pGKV11 increased by a factor of 7 compared with pETNH, which is an empty vector of the PAM plasmid pETMS1 (Table 4). These results suggest that the R and S subunits of this strain were duly reconstituted and showed activity in the E. coli host.
From the various trails with PAM, we found three requirements
for it to work effectively. First, a strong promoter, such as
the
lac promoter, should be avoided. In some cases, over-production
of DNA methylase leads to cell death or diminishes the growth
rate of the PAM host. In the case of
B. adolescentis,
araBAD was used as a promoter (
4), but under a repressing condition
in the absence of an arabinose inducer. Second, an
E. coli host
should be used, which lacks Type IV restriction systems. This
type of enzyme degrades the DNA, which has been methylated by
the R–M system of other bacterium.
Escherichia coli K-12
carries four genes-encoding Type IV restriction enzymes (
mcrA,
mrr and
mcrBC). In many cases it is not possible to clone foreign
DNA methylase in the wild-type
E. coli host (
mcrA+,
mrr+ and
mcrBC+), this is because the cloned enzyme will methylate the
host
E. coli genomic DNA, the Type IV enzyme then cleaves its
own DNA, causing cell death. From our experience TOP10 was the
most effective strain (
Table 1). Third, an
E. coli strain which
is
dam, dcm and
hsdMS deficient should be used as the PAM host.
These genes (
dam, dcm) encode DNA methylases of the
E. coli K-12 Type II R–M system. If the target strain carries
a Type IV system, the plasmid from (
dam+,
dcm+) yields around
a 10
3 decrease in the transformation efficiency compared with
that from a (
dam–,
dcm–) host. Otherwise, a strictly
regulated expression system for the methylase gene was applicable
for PAM plasmids.
In summary, a new method for constructing a transformation system for bacteria has been developed. With this system it is feasible to increase efficiency to >105 CFU/µg DNA for B. adolescentis ATCC15703, at which point it becomes relatively easy to set up other molecular tools, such as site-directed mutagenesis, etc. Using the L. lactis IO-1 strain, we also demonstrated that this system is applicable not only to a Type II R–E system but also to a Type I multi-subunit R–E system. This simple but powerful method may be generally applicable for other bacterial strains, which carry R–M systems. It could potentially promote post-genomic research into bacterial molecular biology.
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FUNDING
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Grant-in-Aid for Scientific Research on Priority Areas Applied
Genomics from the Ministry of Education, Culture, Sports, Science
and Technology of Japan and KAKENHI (C)(20510189) from the Japan
Society for the Promotion of Science Funding for open access
charge: KAKENHI (C).
Conflict of interest statement. None declared.
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ACKNOWLEDGEMENTS
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Analysis of
L. lactis IO-1 has been done as a part of a collaboration
of H.Y. and M.S-K. with Prof. K.Sonomoto at Kyushu University.
H.Y. and M.S-K. thank J.Oshima, N.Ikawa and T.Tozaki for their
experimental contributions.
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ABSTRACT
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