A restriction-free method for gene reconstitution using two single-primer PCRs in parallel to generate compatible cohesive ends
© The Author(s). 2017
Received: 8 December 2016
Accepted: 1 March 2017
Published: 17 March 2017
Restriction-free (RF) cloning, a PCR-based method for the creation of custom DNA plasmids, allows for the insertion of any sequence into any plasmid vector at any desired position, independent of restriction sites and/or ligation. Here, we describe a simple and fast method for performing gene reconstitution by modified RF cloning.
Double-stranded inserts and acceptors were first amplified by regular PCR. The amplified fragments were then used as the templates in two separate linear amplification reactions containing either forward or reverse primer to generate two single-strand reverse-complement counterparts, which could anneal to each other. The annealed inserts and acceptors with 5’ and 3’ cohesive ends were sealed by ligation reaction. Using this method, we made 46 constructs containing insertions of up to 20 kb. The average cloning efficiency was higher than 85%, as confirmed by colony PCR and sequencing of the inserts.
Our method provides an alternative cloning method capable of inserting any DNA fragment of up to at least 20 kb into a plasmid, with high efficiency. This new method does not require restriction sites or alterations of the plasmid or the gene of interest, or additional treatments. The simplicity of both primer design and the procedure itself makes the method suitable for high-throughput cloning and structural genomics.
KeywordsRestriction-free cloning Gene reconstitution Single-primer PCR High-throughput cloning
The manipulation of recombinant DNA molecules is an indispensable step in modern high-throughput protein crystallization studies . Restriction enzyme/ligase cloning, which relies on restriction enzyme digestion and ligation, is a simple and easy way to move a fragment of double-stranded DNA from one plasmid to another . However, this technique has two limitations : it is ineffective when lack of unique restriction sites and sometimes results in introduction of unwanted extra sequences. To circumvent these limitations, various restriction endonuclease cleavage site–independent cloning methods have recently been developed [4–21]. These methods have made cloning more accessible in cases in which conventional restriction site cloning was difficult or impossible. While as alternative cloning strategies are still required for more choices.
The restriction-free (RF) cloning strategy, as described extensively in the literature, was developed as a powerful tool for reconstituting genes in circular vectors [3, 22]. Because RF cloning requires no alterations in the plasmid or the gene of interest, it is exceptionally well-suited for high-throughput cloning. The gene of interest is amplified in a regular polymerase chain reaction (PCR), which produces a primer pair that, once annealed to the vector of interest, is extended in a linear amplification reaction. Thus, this method relies on amplified genes functioning as primers. However, this approach also has limitations. First, the motion of large DNA fragments and the formation of secondary structures will affect the efficiency of PCR. Second, this method relies on digestion with DpnI, which cleaves methylated DNA, to remove parental plasmids. The efficiency of DpnI treatment is influenced by many factors, and requires vector propagation in Dam+ strains .
In this paper, we describe a simple and fast method for performing gene reconstitution by modified restriction-free (MRF) cloning. In this method, two rounds of PCR generate two DNA fragments with compatible 5' or 3’ cohesive ends, which are therefore able to ligate to each other. This new method is independent of the existence of restrictions sites and DpnI treatment. Using this method, we made 46 constructs with inserts of variable size, with average cloning efficiency higher than 85%. The efficiency was not significantly affected by the insert length up to 20 kb.
Method overview and primer design
We used two pairs of primers, primer 1/primer 2 and primer 5/primer 6, to generate two DNA fragments, using the target gene and vector as templates, respectively. The resultant PCR products were gel purified. We then amplified these two DNA fragments in two separate PCR reactions containing either forward or reverse primer, which will add 5’ overhang or 3’ overhang to the PCR products (Fig. 1). Finally, DNA fragments with complementary overhang at the 5’ or 3’ end were able to anneal to each other, and were joined by DNA ligase (Fig. 1). The ligated products were then transformed into DH5α competent cells. The inserted genes were verified by colony PCR and further confirmed by DNA sequencing.
MRF cloning can assemble insert DNA fragment into target vector
PCR reaction components
Forward primer (100 μM)
Reverse primer (100 μM)
Phusion GC Buffer (5×)
dNTPs (10 mM)
Phusion High Fidelity DNA
Add water to
Long DNA fragment cloning
Genes cloned and efficiency of reconstitution
Gene cluster 1
Gene cluster 2
Gene cluster 3
Application of MRF cloning in genes reconstitution
In routine application of our cloning method, we created 46 constructs from E. coli genomic DNA and human cDNA (Clontech) with inserts of variable size (Table 2). E. coli genes and gene clusters (Additional file 5: Table S2, Additional file 6: Table S3, and Additional file 4: Table S4) were cloned from the E. coli genome. The E. coli genes and gene clusters were subcloned into pET22b, with the gene under the control of a T7 promoter, and with a C-terminal His6-tag to facilitate protein purification (Additional file 1: Figure S1). Human genes were subcloned into the expression vector pcDNA™ 3.1 (+) (Invitrogen) (Additional file 2: Figure S2). Under our test conditions, we achieved an average cloning efficiency of 86.9%. DNA sequencing revealed that all genes were correctly placed in the plasmid.
In this study, we describe a new cloning method. The technique uses two rounds of PCR to obtain inserts and acceptors with compatible cohesive ends, which are then ligated. Using this method, we made 46 constructs with inserts of variable size. The average cloning efficiency was 86.9%, as determined by colony PCR and sequencing of the cloned genes. Because the method relies on PCR to generate cohesive 5’ or 3’ ends for DNA ligation, restriction sites are not needed, which facilitates cloning of the gene of interest. For convenience, we only used the vector pET22b for E. coli genes and pcDNA™ 3.1 (+) for human genes, but used inserts of variable size. Our results showed that cloning efficiency was not significantly affected by the different inserts, thus providing a glimpse of the wide choice in inserts that can be used as a template, which then can be used as an alternative method for multiple fragment assembly and library construction.
We noticed that cloning efficiency was not altered dramatically by fragment length. As shown in Table 2 and Additional file 7: Figure S3, this method was suitable for the cloning of large DNA sequences up to 20 kb in size. In contrast to traditional restriction enzyme cloning, the method described here provides a much more flexible approach to gene cloning. Therefore, it represents a cost-effective and simple solution for high-throughput cloning applications. Because this method relies on PCR amplification of the DNA sequences, the most crucial requirement is high-fidelity DNA polymerase. Fortunately, the high-fidelity polymerases recently developed for cloning, e.g., Phusion® High-Fidelity DNA Polymerase and KOD Hot Start DNA Polymerase, have extremely low error rates. Therefore, it is no longer challenging to amplify large DNA fragments for use in our method.
We developed a novel cloning method that provides an alternative approach to DNA assembly. This method is independent of restriction sites and DpnI treatment, and does not introduce undesired operational sequences at the junctions of functional modules. This new method simplifies complex cloning procedures in which long stretches of DNA can be inserted into circular plasmids in an unrestricted way, and the efficiency does not decrease for long inserts up to 20 kb. The simplicity of both primer design and the procedure itself makes the method suitable for high-throughput studies. The protein of interest is expressed without the addition of extra residues originating from the cloning procedure, making it an attractive alternative method for structural genomics.
Phusion® High-Fidelity DNA Polymerase, DNA marker, Taq DNA polymerase, and T4 DNA ligase were purchased from New England Biolabs, and cloning kits from Qiagen. pET22b, pcDNA™ 3.1 (+), and host strain Escherichia coli DH5α were obtained from Invitrogen. Human cDNAs were purchased from Clontech. Oligonucleotide primers were purchased from Invitrogen. PCR purification and gel extraction kits were purchased from Qiagen. Plasmids were isolated using a QIAprep Spin Miniprep Kit (Qiagen). All other chemicals used in the study were of molecular biology grade.
PCR reactions were performed to generate DNA fragments in a final volume of 50 μL using Phusion® High-Fidelity DNA Polymerase (New England Biolabs) and the primer pair as shown in Additional file 3: Table S1. After the initial denaturation step at 98 °C for 5 min, the PCR was conducted for 20 cycles with denaturation at 98 °C for 20 s; primer annealing from 60 °C to 50 °C with a step of -0.5 °C each cycle for 20 s; extension at 72 °C for 30 s/kb; and 10 cycles with an annealing temperature at 52 °C. When all cycles were completed, the samples were maintained at 72 °C for 10 min to finish all DNA synthesis.
DNA ligation reactions were performed to fuse DNA fragments in a final volume of 20 μL using T4 DNA ligase (New England Biolabs) following the standard protocol from New England Biolabs. In brief, the longer and shorter DNA fragments were mixed at a molar ratio of 1:3–1:10. The reaction was incubated at room temperature for 2 h. After heat inactivation at 65 °C for 10 min, the reaction was chilled on ice. A 10 μL aliquot of the reaction was used to transform 50 μL of competent cells.
For each transformation, eight colonies were selected randomly for colony PCR to verify insertion. The colony PCR included 5 units of Taq DNA polymerase (New England Biolabs) and 1× ThermoPol® Buffer (New England Biolabs) in the presence of 200 μM dNTP, 1 mmol each of a primer from the vector and a primer from the insert gene, and a small amount of cells picked from the colony, all in a final volume of 20 μL. The colony PCR conditions were as follows: 95 °C for 2 min; 25 cycles of 95 °C for 30 s, 50 °C for 30 s, and 68 °C for 1 min/kb; and a final extension at 68 °C for 10 min. Insert-positive constructs were confirmed by DNA sequencing.
Isolation and purification of total genomic DNA from E. coli
Mid-log phase E. coli DH5α cells were collected by centrifugation at 4 °C for 10 min. The pellet was resuspended in 190 μL of TE supplemented with 10 μL of 10% SDS and 1 μL of 20 mg/mL protease K, and then incubated at 37 °C for 1 h. After 1 h, 30 μL of 5 M NaCl and 30 μL of CTAB/NaCl were added to the solution, and the sample was incubated at 65 °C for 20 min. After incubation, 300 μL of phenol/chloroform/isoamyl alcohol (25:24:1, v/v) was added, and the sample was immediately mixed and centrifuged at 5000 rpm in a table-top microcentrifuge for 10 min. To the upper (aqueous) phase 300 μL of chloroform/isoamyl alcohol (24:1) was added, which was mixed and centrifuged at 5000 rpm. To the resultant aqueous phase 300 μL of isoamyl alcohol was added; after mixing, the sample was incubated at room temperature for 10 min to precipitate DNA. To pellet DNA, the sample was centrifuged at 5000 rpm for 10 min. The pellet was resuspended in 500 μL of 70% ethanol and centrifuged at 5000 rpm for 10 min. The supernatant was discarded, and the pellet was dried and dissolved in 20 μL of TE buffer.
Plasmid transformation and isolation
The competent DH5α cells were prepared by calcium chloride method . The ligation product (10 μL) was added directly to 50 μL of competent DH5α cells, incubated for 15 min on ice, heat-shocked at 42 °C for 1 min, and then transferred on ice for 5 min. After adding 500 μL of LB, the cells were incubated in a shaker at 37 °C for 60 min. After incubation, cells were pelleted and resuspended in 100 μL of LB, which was then spread on LB plates containing ampicillin (100 μg/mL). After overnight incubation at 37 °C, eight colonies from each transformation were randomly picked and checked by colony PCR. Plasmids were isolated using the Spin Miniprep kit (Qiagen).
Mix of 2’-deoxynucleoside 5’-triphosphates
Modified restriction-free cloning
Polymerase chain reaction
The authors wish to thank Esteve Padros (Department of Biochemistry and Molecular Biology, Medical School, and Center of Biophysics, Autonomous University of Barcelona) for critical reading of the manuscript.
This work was supported by Starting Grant from Hebei Agricultural University to Fanli Zeng (grant number ZD201622) and grants from the National Natural Science Foundation of China to Jingao Dong (grant number 31271997) and Zhimin Hao (grant number 31301616).
Availability of data and materials
The data sets supporting the results of this article are included within the article and its additional files.
FZ, JD, and YL designed the experiments and drafted the manuscript. FZ, ZH and PL carried out the practical work. YM was involved in the research discussions and helped to finalize the manuscript. All authors read and approved the manuscript.
The authors declare that they have no competing interests.
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