Oligonucleotides O1 and O2 form adapter O1O2, which has an MseI-compatible overhang at its end. Oligonucleotides O3 and O4 form adapter O3O4, which has an ApeKI-compatible overhang at its end. These two adapters are used for tester preparation (Fig. 1). Here, the tester represents a DNA population in which both desired and undesired genes are included. The role of the poly(dA)36 adapter O1O2 is to remove fragments without adapters (Fig. 1, type 4 fragments) and Scientific Reports
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Removing PCR for the elimination of undesired DNA fragments cycle by cycle
Jiaojiao Huan, Kangkang Wan, [...], and Guoying Wang
Additional article information
Abstract
A novel removing polymerase chain reaction (R-PCR) technique was developed, which can eliminate undesired genes, cycle by cycle, with efficiencies of 60.9% (cDNAs), 73.6% (genomic DNAs), and ~ 100% (four DNA fragments were tested). Major components of the R-PCR include drivers, a thermostable restriction enzyme - ApeKI, and a poly(dA) adapter with mismatched restriction enzyme recognition sites. Drivers were generated from the undesired genes. In each cycle of R-PCR, drivers anneal to complementary sequences and allow extension by Taq DNA polymerase. Thus, ApeKI restriction sites in the undesired genes are recovered, and adapters of these undesired DNA fragments are removed. Using R-PCR, we isolated maize upregulated defense-responsive genes and Blumeria graminis specialized genes, including key pathogenesis-related effectors. Our results show that after the R-PCR reaction, most undesired genes, including very abundant genes, became undetectable. The R-PCR is an easy and cost-efficient method to eliminate undesired genes and clone desired genes.
Polymerase chain reaction (PCR), developed by Kary B. Mullis, is a technique used to amplify many copies of a region of DNA1,2,3. PCR is a fast, inexpensive and widely used technique to amplify desired sequence fragments. However, how undesired genes may be removed from a gene pool that includes both desired and undesired genes remains a challenge. Genes are turned on or off in different biological processes, such as cellular growth, organogenesis and disease development. To identify and clone specifically expressed genes is the first step and a key strategy to explore these biological processes4. To measure and isolate specifically expressed genes, a variety of methods have been developed including differential display PCR, RNA fingerprinting, serial analysis of gene expression (SAGE), real-time quantitative PCR, subtractive suppression hybridization (SSH), microarrays, and high-throughput next-generation sequencing technologies5,6,7,8,9.
Differential display PCR is a highly sensitive method to investigate regulated genes; however, it generates a large number of false positives and requires a large number of primer pairs. SSH is a very popular subtraction method and is available as a kit. However, there are two potential problems with SSH. The first is gene redundancy. If there are a few genes that are highly differentially expressed, then they will appear as a large number in the SSH results. The second problem is generating many false positives10. DNA microarrays represent a high-throughput technique to measure a large number of genes within a single experiment. The use of DNA microarrays holds considerable promise in our understanding of genes and their impact on disease, drug discovery and development. The disadvantages of the microarrays include insufficient sensitivity because of hybridization; sequences must be known in advance; lack of reproducibility; lack of standardization; and expense. Advantages of high-throughput sequencing technologies include that they are highly efficient, and sequences do not need to be known in advance. Their disadvantages include sequencing only very short sequences, complicated post-sequencing data analysis and expense11,12. Each method has its own advantages and drawbacks, and no method can easily and efficiently remove undesired DNA fragments. The PCR method is a highly effective technique with few drawbacks. Restriction enzymes cleave DNA at specific nucleotide sequences. The R-PCR method proposed in this study makes use of a restriction enzyme that has only restriction activity and cuts in a predictable and consistent manner. As a time-saver, the enzyme ApeKI can digest one unit of assay DNA substrate in 5 min (New England Biolab Inc., USA). SSH is still a popular technique that allows isolation and cloning of differentially expressed genes. Here, we describe a novel method R-PCR, which inherits the merits of PCR, restriction enzymes and SSH.
Results
Outline of the R-PCR method
The R-PCR method is essentially divided into three main sections (Fig. 1 and Supplementary Fig. 1) and includes specifically designed testers, drivers, a single primer, a thermostable restriction enzyme (ApeKI), a thermostable Taq DNA polymerase and dNTPs. A brief description of the R-PCR method is as follows: 1) Section 1, tester and driver preparation. The preparation of tester and driver starts from samples digestion with ApeKI and MseI. The tester is made by ligation with an adaptor containing a polyA tail and then oligo-dT column purification. The driver is made by ligation of different adaptors, PCR amplification, and digestion with MseI. 2) Section 2, R-PCR reactions. The tester and driver are mixed and subjected to R-PCR with a single primer in the presence of ApeKI, which results in linear amplification of the desired fragments without ApeKI digestion due to design of a mismatch in the adaptor. In contrast, common undesired fragments are extended from the 3′ end of the driver to create the ApeKI site, which is cut, removing those fragments from further amplification. 3) Section 3, recovery of the desired fragments and the products are cloned. Recovery of the desired fragments from linear amplification in the previous step is carried out using selective PCR primers, and the products are cloned by Invitrogen's TOPO TA cloning system. Detailed procedure refers to Figure 1 and Supplementary Figure 1.
Figure 1
Figure 1
Three main sections of the R-PCR method.
The R-PCR design
A critical design aspect for R-PCR is the following nine oligonucleotides and related adapters.
O1: 5′-TTACCACGACCACCCTATTGCTGCTGC-3′
O1-short: 5′-TTACCACGACCACCCTATTGCTG-3′
O2: 5′-TAGCAGAAGCAATAGGGTGGTCGTGGTAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA-3′
O3: 5′-ACGAGGTGCGGTCTTGGACTACTT-3′
O4: 5′-CWGAAGTAGTCCAAGACCGCACCTCGT-3′
O5: 5′-CGACATTCTGTAGGAAACACTAGGACTT-3′
O6: 5′-TAAAGTCCTAGTGTTTCCTACAGAATGTCG-3′
O7: 5′-GGGTTGCGATACGATTGTTATAGGTCAC-3′
O8: 5′-CWGGTGACCTATAACAATCGTATCGCAACCC-3′
Oligonucleotides O1 and O2 form adapter O1O2, which has an MseI-compatible overhang at its end. Oligonucleotides O3 and O4 form adapter O3O4, which has an ApeKI-compatible overhang at its end. These two adapters are used for tester preparation (Fig. 1). Here, the tester represents a DNA population in which both desired and undesired genes are included. The role of the poly(dA)36 adapter O1O2 is to remove fragments without adapters (Fig. 1, type 4 fragments) and fragments that have O3O4 adapters at both ends (Fig. 1, type 3 fragments) via oligo-dT spin column purification. It is essential that adapter O1O2 harbors two overlapping ApeKI recognition sites with one mismatched base pair (Fig. 1). The mismatched base pair can save this adapter from digestion, but once a correct match is recovered in R-PCR cycling, our results show that ApeKI can efficiently cut off the overlapping recognition sites. To construct the tester, DNA was digested by ApeKI and MseI, and then two adapters, O1O2 and O3O4, were added (Fig. 1 and Supplementary Fig. 1).Oligonucleotides O5 and O6 form adapter O5O6, which has an MseI-compatible overhang at its end. Oligonucleotides O7 and O8 form adapter O7O8, which has an ApeKI-compatible overhang at its end. These two adapters are for the driver preparation (Fig. 1 and Supplementary Fig. 1). Here, the driver represents a DNA population in which only undesired genes (a reference) are included. To obtain the excess driver, we used primers O5 and O7 to amplify DNA fragments of undesired genes (genes from the control sample). The driver was obtained after the PCR products were digested by MseI and then purified by a commercial spin column.
Four types of DNA fragments were generated in the tester preparation (Fig. 1). Type 1 fragments: one end has the adapter O1O2 and the other end has the adapter O3O4. These are the only expected fragments for R-PCR. Type 2 fragments: both ends have adapter O1O2. Annealing and extension of primer O1 introduced the ApeKI recognition sites, which were digested. Thus, fragments with these adapters were removed. Type 3 fragments: both ends have adapter O3O4. As with random amplified polymorphic DNA (RAPD) and single primer amplification reaction (SPAR)13,14, this type of fragment could also be exponentially amplified during PCR15,16,17. Fortunately, they are removed by the oligo-dT spin column purification. There are many fragments that do not have any adapters in Type 4 fragments. These non-adapter-ligated fragments can serve as the driver and remove desired genes. Fortunately, they are also removed by oligo-dT spin column purification.