Protocol: Streamline cloning of genes into binary vectors in Agrobacterium via the Gateway® TOPO vector system
© Xu and Li; licensee BioMed Central Ltd. 2008
Received: 24 October 2007
Accepted: 22 January 2008
Published: 22 January 2008
In plant functional genomic studies, gene cloning into binary vectors for plant transformation is a routine procedure. Traditionally, gene cloning has relied on restriction enzyme digestion and ligation. In recent years, however, Gateway® cloning technology (Invitrogen Co.) has developed a fast and reliable alternative cloning methodology which uses a phage recombination strategy. While many Gateway- compatible vectors are available, we frequently encounter problems in which antibiotic resistance genes for bacterial selection are the same between recombinant vectors. Under these conditions, it is difficult, if not sometimes impossible, to use antibiotic resistance in selecting the desired transformants. We have, therefore, developed a practical procedure to solve this problem.
An integrated protocol for cloning genes of interest from PCR to Agrobacterium transformants via the Gateway® System was developed. The protocol takes advantage of unique characteristics of the replication origins of plasmids used and eliminates the necessity for restriction enzyme digestion in plasmid selections.
The protocol presented here is a streamlined procedure for fast and reliable cloning of genes of interest from PCR to Agrobacterium via the Gateway® System. This protocol overcomes a key problem in which two recombinant vectors carry the same antibiotic selection marker. In addition, the protocol could be adapted for high-throughput applications.
With the rapid progress of many genome sequencing projects in plants, plant scientists are stepping up the pace of gene function studies. To this end, both gene cloning and subcloning have become routine procedures that traditionally rely on restriction enzyme digestion and ligation. In recent years, however, Gateway® cloning technology (Invitrogen Co.) has developed a fast and reliable alternative cloning methodology based on bacteriophage λ site-specific recombination. In plant functional genomics research, the gene of interest usually needs to be cloned into binary vectors of a larger size (5 to 12 kb) in order to obtain transgenic plants via Agrobacterium-mediated T-DNA transformation. Thus, in terms of its efficiency when compared to traditional DNA cloning, Gateway® cloning technology has proved to be extremely useful for gene cloning into a larger size of vectors [1, 2]. Many Gateway® compatible binary vectors have been made available [1, 2], including the pMDC series of binary vectors, which is freely available for noncommercial use. These vectors can be used for functional analysis of genes by constitutive or inducible ectopic expressions, antisense or RNAi expressions, promoter analyses, subcellular localizations, or complementation analyses .
In general, the Gateway® technology involves a two-step process . The gene of interest is first cloned into an Entry vector through the so-called BP reaction, which produces an Entry clone. When making the Entry clone, it is necessary to change the sequences of the gene's ends in order to be Gateway-compatible (recombinase recognition sites), but no restriction enzyme is involved during the entire cloning process. The system also takes advantage of the negative selection marker ccd B gene to eliminate the original vector after transformation . Next, the resulting cloned gene is subcloned into one of the Destination Vectors by the LR reaction between the Entry clone and the Destination vector. Thus, once an Entry clone with the gene of interest has been made, a further advantage of the Gateway technology lies in the ease of subcloning it into a wide variety of Destination vectors through the LR reaction.
When using these vectors in conjunction with the Gateway® cloning technology, however, we frequently encounter a situation in which antibiotic resistance genes for bacterial selection are the same between two recombinant vectors. Under these conditions, it is impossible to use the antibiotic resistance for the purpose of selecting desired transformants during cloning processes. One recently reported solution uses a restriction enzyme to digest the recombinant by-product , but it may often be impractical to select a unique restriction enzyme, especially when dealing with a large plant gene insert. Here, we present a method that uses differential selections of plasmids based on their replication origins and that, consequently, bypasses the problem indicated above.
We are particularly interested in expressing various plant genes encoding mRNA polyadenylation factor subunits through Agrobacterium-mediated transformation for gene functional studies. In our previous work , we created Entry clones by first generating a PCR product containing att B sites, which required two overlapping PCR reactions due to the addition of a 25 bp att B site on both ends of the PCR product. This att B-PCR product was then used in BP reactions with a donor vector pDONR™201 to generate the Entry clone. Since the pENTR™/D-TOPO® cloning kit (Invitrogen) has now become available, only a 4-bp (CACC) leader needs to be added to the 5'-PCR primer for gene amplification. This allows the PCR product to be directionally cloned into the TOPO vector to generate an Entry clone. Thus, the TOPO technology allows us to easily produce Entry clones.
However, a major problem still remains and involves the bacterial selection marker for the TOPO vector and the binary destination vector pMDC series . Since these vectors all harbor a kanamycin antibiotic-resistant gene for transformant selections, the antibiotic selection will not work when screening the recombinant clones after the LR reaction. One convenient solution would be to change a TOPO vector, but, based on our consultations with Invitrogen, there is no other directional TOPO Entry clone vector available. Alternatively, as noted above, the use of a restriction enzyme to digest the undesired plasmid after the LR reaction has been reported . However, this solution is flawed because it involves the selection of an enzyme that only digests the recombinant by-product plasmid, not the target gene and the binary vector. This may be impractical, in some cases, especially with large plant genes. Therefore, we report here our development of a streamlined procedure which is based on the characteristics of the Gateway vectors, but which avoids all of the potential problems described above.
Reagents and Consumables
Template DNA or cDNA for PCR amplification of the gene of interest.
PCR Primers – the 5' end of the forward primer should add the 4 nucleotides, CACC, which are required for directional cloning using the pENTR/D-TOPO® cloning kit.
High fidelity DNA polymerase (e.g., Pfu DNA polymerase) and appropriate 10× PCR buffer.
The PCR product and gel purification kit (e.g., QIAquick® from Qiagen Inc.).
The pENTR/D-TOPO® Cloning kit (Invitrogen Cat#KNM4500-01).
Chemically competent (or electro-competent, optional) E. coli strain, any rec A, end A E. coli strain, e.g., One Shot® TOP10, DH5α™, XL-1 Blue or equivalent.
Gateway-compatible binary vectors (e.g., the pMDC series), which are freely available for noncommercial use at the Arabidopsis Biological Resource Center .
LB liquid medium (10 g/L bacto-trypton, 5 g/L yeast extract, 10 g/L NaCl) and plates (LB + 15 g/L agar) supplemented with antibiotics (kanamycin, 50 μg/mL), as appropriate.
YEP liquid medium (10 g/L bacto-trypton, 10 g/L yeast extract, 5 g/L NaCl) and plates (YEP + 15 g/L agar) supplemented with antibiotics (kanamycin, 50 μg/m L), as appropriate.
Plasmid miniprep kit (e.g., Bio-Rad Quantum Prep®)
The Gateway® LR Clonase™ enzyme mix kit (Invitrogen)
TE buffer, pH8.0
1.5 ml centrifuge tubes
Sterile culture tubes
PCR thermal cycler
Long wavelength UV transilluminator – long wave ultraviolet light is less damaging to DNA during excision of bands from gels.
42°C water bath (or Electroporator, e.g., Bio-Rad E. coli Pulser, if using electro-competent E. coli strains).
Horizontal gel electrophoresis equipment
Shaking and non-shaking temperature-controlled incubators
A. Creating the Entry clone using pENTR™ Directional TOPO® cloning kit
Design PCR primers for gene amplification, with an addition of 4-nucleotide CACC at the 5' end of the forward primer.
Perform PCR to amplify the gene of interest with high fidelity DNA polymerase.
Run agarose gel electrophoresis to check PCR product, after which the desired band is cut and purified using the QIAquick® Gel Purification Kit (Qiagen) or other substitutes.
Use the pENTR™ Directional TOPO® cloning kit to set up TOPO® cloning reaction: mix the PCR product with the pENTR™ TOPO® vector (0.5:1 to 2:1 molar ratio) in the reaction buffer and incubate 5 minutes at room temperature, according to manufacturer's recommendation.
Take 2 μL of the TOPO® cloning reaction mixture to transform competent E. coli cells (e.g., TOP10, DH5α™, XL-1 Blue), using heat shock at 42°C, spread the cells on LB plate containing 50 μg/mL kanamycin, and incubate overnight at 37°C.
Select 5–10 colonies and culture them by shaking overnight at 37°C in 2 mL LB liquid media containing 50 μg/mL kanamycin.
Isolate plasmid DNA by the plasmid miniprep kit.
Confirm the insert in the plasmids by restriction analysis or PCR or DNA sequencing. This is the Entry clone.
B. Performing the LR recombination reaction
Add the following components to a 1.5 mL microcentrifuge tube and mix:
Entry clone (100–300 ng) 1–5 μL
Destination vector, e.g., pMDC (150 ng/μL) 1 μL
5× LR Clonase Reaction Buffer 2 μL
Add 2 μL LR Clonase™ enzyme, mix well by votexing briefly twice and spin down. Then, incubate the reaction mixture at 25°C for 1 hour to overnight.
To stop the LR reaction, add 1 μL of the Proteinase K (2 μg/μL) solution and incubate for 10 minutes at 37°C.
Take 3–5 μl of the LR reaction mixture to transform competent E. coli cells (e.g., TOP10, DH5α™, XL-1 Blue), spread the transformation on LB plate containing 50 μg/ml kanamycin and incubate overnight at 37°C. Note: do not transform the Gateway ® LR reaction mixture into E. coli strains that contain the F2 episome (e.g., TOP10F2). These strains contain the ccdA gene and will prevent negative selection with the ccdB gene. Alternatively, transformants can also be selected by liquid culture with the antibiotic without spreading on a plate, and the selected culture can be directly subject to plasmid isolation.
Harvest all colonies from the plate and isolate plasmid DNA mixture containing the non-reacted plasmids of the Entry clone, as well as the recombination plasmid between the Entry clone and the Destination vector.Note: E. coli cells containing non-reacted plasmids of the Destination vector and by-product plasmids of the LR recombination reaction fail to grow because these plasmids contain the ccdB gene which is toxic to the cell growth of E. coli stain, e.g., TOP10, DH5α™, XL-1Blue .
C. Performing Agrobacterium transformation to select the recombinant expression clone
Preparation of chemically competent Agrobacterium cells.
Grow a colony of Agrobacterium in 2 mL YEP liquid media by incubating at 28°C overnight with shaking (~250 rpm).
Transfer the cells to 50 mL YEP liquid media and incubate for 3 ~ 4 hr with shaking until OD600 = 0.5 ~1.0.
Chill the culture on ice for 5 min, then centrifuge at 3000 × g for 5 min at 4°C and discard the supernatant.
Rinse the cell pellet with 10 mL of 20 mM ice-cold CaCl2 and spin briefly again.
Add 1 mL of 20 mM ice-cold CaCl2 to resuspend the cells gently on ice.
Take an aliquot of 0.1 mL of the competent cells into a pre-chilled microcentrifuge tube.
Add 1 μg or more plasmid DNA mixture (Step 2.4) to the 0.1 mL of competent cells, mix gently, but thoroughly, and then freeze in liquid nitrogen or dry ice-EtOH slurry. Note: When using -70°C stored competent cells, thaw the cells until just becoming liquid before adding plasmid DNA.
Thaw the cells in a 37°C water bath for 5 min. Add ~150 μL YEP liquid media and incubate at 28°C for 2~4 hr with gentle shaking; then transfer and spread all cells on YEP plate containing 50 μg/mL kanamycin and/or other antibiotics, if appropriate, and incubate 2 to 3 days at 28°C. Note: Since the unreacted Entry clones are removed by the selection due to lack of Agrobacterium replication origin, only the recombinant Destination plasmid can be selected in Agrobacterium.
The positive colonies will grow. Note: According to our experience, all the positive colonies contain the desired recombination plasmid.
[Optional] verification of the positive colonies can be done by a colony PCR method. If desired, plasmid can be isolated and used for verification by restriction analysis or DNA sequencing. Note: For isolation of the binary plasmid from Agrobacterium, culture a single colony in YEP liquid media overnight at 28°C. Then, we use Quantum Prep ® Plasmid Miniprep kit (Bio-Rad) to isolate plasmid DNA with the following minor modifications: prior to cell lysis, add 10 μL freshly-prepared solution of lysozyme (100 mg/mL in 10 mM Tris-HCl, pH8.0) into the cell suspension followed by incubation for 5 minutes at room temperature. As the final step, elute plasmid DNA with pre-heated TE buffer at 70°C. In case a low yield or low quality plasmid DNA is obtained from Agrobacteria, the plasmid can be transformed into E. coli to produce sufficiently pure plasmid DNA for sequencing purposes.
The Agrobacterium colonies are ready to be used for plant transformation.
Examples of successful application
When two recombination vectors carry the same antibiotic resistance gene, it is difficult, if not sometimes impossible, to use antibiotic resistance in selecting the desired transformants. Our protocol solves this problem by employing the Gateway® cloning technique to produce expression clones for Agrobacterium-mediated plant transformation. Moreover, our protocol improves upon previous methods by utilizing a more straightforward strategy which eliminates the potentially time-consuming step of selecting an appropriate enzyme to digest and remove the unreacted Entry clones from the LR reaction, as previously suggested . Our experience indicates that this is an efficient and reliable method for cloning genes of interest by PCR and introduced into Agrobacterium for plant functional genomic studies. Finally, our protocol offers the potential for use in high-throughput applications because many of the reactions can be modified for parallel processing.
We are grateful to Mark D. Curtis and Nam-Hai Chua for providing the pMDC series Gateway-compatible vectors, to Denghui Xing, Hongwei Zhao and Xinfu Ye for helpful discussion and testing the protocol, and to David Martin for editorial assistance. This research was supported by grants from US NSF (MCB-0313472 to QQL) and from NIH (1R15GM077192-01A1 to QQL).
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