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Rapid Amplification of cDNA Ends

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Typically, with novel mRNAs only a portion of its complete sequence is known. The missing nucleotide sequences at the ends of the mRNA can be determined using a PCR-based method called Rapid Amplification of cDNA Ends, or RACE.

In eukaryotes, mature mRNAs have distinctive structural features at both ends. At the five-prime end, most have a methylated guanosine residue connected to the mRNA via a five-prime to five-prime triphosphate linkage. This is also known as the five-prime cap. At the three-prime end, most eukaryotes have a tail of 20 to 250 adenylate residues, called the poly(A) tail.

Now, if the nucleotide sequence of even a small segment is known anywhere within the mRNA, the sequence up to its three-prime end can be amplified using a gene-specific primer and a non-specific oligo(dT) primer that anneals to the poly(A) tail at the three-prime end. This subset of the RACE technique is called three-prime RACE, and it allows for the detection of transcript variants and different three-prime Untranslated Regions, or UTRs.

The sequence at the five-prime end can be amplified similarly. To do this, a poly(A) tail is first attached at the five-prime end. Then, using a non-specific oligo(dT) primer that anneals to the appended tail and a gene-specific primer, the sequence up to the five-prime end can be amplified. This subset of RACE is known as five-prime RACE and is used to find differential five-prime splice variants and alternative five-prime UTRs.

To perform three-prime RACE to identify different transcripts encoded by a given gene, RNA is first isolated from the organism or tissue of interest. Next, cDNA is synthesized from the isolated mRNAs with a reverse transcription reaction that utilizes an oligo(dT) primer and a reverse transcriptase enzyme, which generates complementary DNA from an RNA template. Next, from the generated pool of cDNAs, the target cDNA's unknown three-prime end is extended via PCR, utilizing the non-specific oligo(dT) primer and a gene-specific primer, and amplified during the PCR reaction.

However, due to the generic nature of the non-specific primer and random mispriming by the gene-specific primer, they can anneal to off-target cDNAs, causing them to amplify as well. To overcome this problem, a second round of PCR is conducted using nested primers, which bind downstream of the first set of primers. This second set of primers further amplifies the cDNA of interest but not the non-specific product, increasing the specificity and yield of the reaction.

Finally, the PCR products are separated using agarose gel electrophoresis, which separates different transcripts of the target gene based on their sizes, producing distinct bands. The band of interest can then be excised, purified, and finally sequenced to obtain the transcript's complete sequence.

In this video, we will demonstrate the three-prime RACE technique to identify and isolate different transcripts encoded by the Drosophila dSmad2 gene.

Before beginning the synthesis and amplification of the cDNA, use a primer design software to create a five-prime specific primer for the gene of interest, dSmad2 in this example. To ensure the primer is highly specific, it should be around 24 nucleotides and have a Tm ranging from 55 to 65 degrees Celsius. Next, design a second nested primer located three-prime of the first primer, which is also specific for the target sequence.

To begin, extract RNA from 10 whole adult flies with a commercially available RNA extraction kit. Once the RNA is extracted, resuspend the pellet in 20 microliters of nuclease-free water. For a 50-microliter reaction, add 5 microliters of the reaction buffer to each sample tube, and bring up the volume of the reaction by adding 24 microliters of nuclease-free water. Then, add 1 microliter of DNase I to prevent amplification of genomic DNA. Finally, incubate the samples at 37 degrees Celsius for 15 minutes.

After DNase treatment, inactivate the enzyme with 1 microliter of 25 millimolar EDTA in each of the tubes. Add the sample to a spin column to purify the RNA. Next, use a microvolume spectrophotometer to measure the RNA concentration. Adjust the concentration to 100 nanograms per microliter by adding nuclease-free water.

Now, prepare a master mix for the reverse transcription reaction. In addition to the test DNA samples, include one negative control by omitting reverse transcriptase from the appropriate tube. Incubate the reactions for 90 minutes at 42 degrees Celsius. Then, inactivate the reverse transcriptase by incubating the tubes at 85 degrees Celsius for five minutes. Next, dilute the DNA levels by adding 80 microliters of TE buffer to each tube to bring the reaction total to 100 microliters.

Now that the cDNA is synthesized, amplify the gene of interest via PCR. To do this, first prepare the first round amplification PCR mix. In addition to the cDNA template, include a negative control reaction that uses the template from the non-reverse-transcribed product, as well as a reaction that does not have the gene-specific primer as a no-primer control.

Once the reactions are prepared, carry out PCR amplifications in a thermocycler equipped with a heated lid. Next, dilute the products 1 to 20 by adding 1 microliter of the PCR products to 19 microliters of TE buffer. Using these diluted products, prepare the second round amplification PCR mix. Then, use a thermocycler to run the second PCR.

To isolate the PCR fragments, prepare a 1% agarose gel by adding 1 gram of agarose per 100 milliliters of TAE buffer. Melt the mixture for two minutes in the microwave, and then add SYBR Safe stain or ethidium bromide. Pour the molten mixture in a gel tray and allow it to set.

While the gel is setting, pipette 1 microliter droplets of 6X Loading Dye onto a piece of parafilm corresponding to the number of samples. Add 5 microliters of sample to each droplet. Now, load the samples, along with a 1 kilobase DNA ladder, onto the gel. Run the gel at 120 volts for approximately 45 minutes or until the dye front is 75% of the way down the gel.

When the gel has finished running, check the gel bands under a UV transilluminator. Locate the bands and cut them out using a scalpel. Purify the cDNA fragments using a commercially available spin column kit. Once purified, the cDNA fragments can be stored at minus 20 degrees Celsius or used immediately for further analysis.

Prior to this experiment, two transcripts for Drosophila dSmad2 to were annotated in the Drosophila genomic database, FlyBase. Based on the expected splicing of this gene, two products should be identified by the three-prime RACE protocol.

The results from this experiment, however, reveal three different transcripts for dSmad2. Among the predicted products, one transcript is predominant, and one is expressed at a lower level. In addition, a previously undescribed smaller product, visible at 750 base pairs, was detected.

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