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Ligation Independent Cloning Primer Design

Posted in: DNA / RNA Manipulation and Analysis

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Ligation Independent Cloning Primer Design

Ligation independent cloning (LIC) is an easy and effective method to ensure successful cloning, all without the need for ligation. As easy as the technique is, designing primers can be a bit tricky. In this article, we will present a quick overview on primer design for ligation independent cloning.

The easiest way to start is to look at the treated vector that the insert will be annealed into. In the diagram below, panel 1 shows the treated vector waiting for the insert and panel 2 shows the corresponding insert, which would fit into it. It’s easy to see how those would fit together, but how do you make the insert like that?

lic-primer-design.gifPanel 3 shows how the insert is made from the PCR product by T4 DNA polymerase and dTTP. The crucial residues are the ones in red. They are added to the oligos to so that they are the first thymidine residues, going back from the 3′ ends. This allows the T4 DNA polymerase and dTTP mix to generate the long sticky ends.

Just like in the vector treatment, the 3′-5′ exonuclease activity of the polymerase digests the 3′ ends of the insert because no are dNTP’s available. This activity stops once it reaches dTTP, where the polymerase and exonuclease activities cancel each other out, resulting in stable, long sticky ends.

It is crucial to note that the forward primer is designed so that the addition of the extra T does not disrupt the reading frame of the N-terminal fusion leading up to the coding sequence.

Panel 4 shows the primers required to amplify the insert with the ligation independent cloning vectors. Of course, one of the primer sequences – the reverse primer – has to reversed to that it reads 5-3′ before it is sent off for sequencing. A common error is to reverse complement this sequence, which of course is wrong – it only needs to be reversed as it is already the complement.

The good news is that you only have to design these adaptor parts of the primers once. As long as you always use the same ligation independent cloning site, the adaptor parts will always should be the same. The parts of the primer that are specific to the gene to be amplified should be designed as normal, and the adaptors simply added to the ends. Note also that since the start codon is vector-borne, the start codon of the gene itself should be omitted.

If you are still confused (you may well be), the best thing to do is try it yourself – get a pencil and a piece of paper, and sketch out the process to become more comfortable with understanding how it works.

You may also find this tool useful for helping to design LIC primers.

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14 Comments

  1. Max on February 19, 2008 at 4:59 pm

    Hey, what happens if someone doesn’t have the right primers? Maybe you have 50 primer pairs WITHOUT the overlaps. Is it possible to ligate them later to the PCR products, to these cute protruding As? I remember that there are protocols to add extensions to PCR products using the As. I just cannot remember the keyword and can’t find anything on the internet without them…
    This would add one step with the advantage of more flexibility (one primer set per plasmid)

  2. Nick on February 19, 2008 at 4:54 pm

    Max,

    I meant that the oligos need the extra base to insert a T that stops the T4 DNA polymerase from going any further during the reaction to generate the sticky ends.

  3. Max on February 19, 2008 at 4:45 pm

    OK. I’d prefer using one of those new polymerases (like Finnzyme’s Phusion or the very similar PfuUltra Fusion II from Strategen) to get rid of the Ts and remove mutations from your PCR product (if it’s long).

  4. Nick on February 19, 2008 at 11:26 am

    Thanks Max…

    Your description simplifies things a bit, but remember that an extra base (the ones I have colored in red) has to be added on both primers to stop the polymerase from eating into the coding sequence.

    I think that the best way to learn how to design these primers is to read the background, then sit down with a piece of paper and work it through for yourself.

  5. Max on February 19, 2008 at 10:58 am

    Hm. How about this description:

    Design your primers pF and pR with primer3 as usual, write them down in the usual 5?-3? direction. Before ordering, you have to add some basepairs:

    Take 15 bp from the left side (pleft) and 15 bp from the right side (pright) of your restriction site in the plasmid. Then:

    a) LIC primer left: pleft pF
    b) LIC primer right: reverse complement(pright) pR

    So: The reverse overlap has to be reverse complemented. Then add your overlapping regions 5? to each primer

    Hm. Don’t know if this is easier to understand…

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