Designing Primers for Targeted Mutagenesis

Now that my lab is fully equipped, I’m taking on rotation students. Unfortunately, with the pandemic, it’s harder to have one-on-one meetings where I can sit down and walk the new students through every method. Furthermore, why repeat teaching the same thing to multiple students when I can just make an initial written record that everyone can reference and just ask me questions about? Thus, here’s my instructional tutorial on how I design primers in the lab.

First, it’s good to start out by making a new benchling file for whatever you’re trying to engineer. If you’re just making a missense mutation, then you can start out by copying the map for the plasmid you’re going to use as a template. Today, we’ll be mutating a plasmid called “G619C_AttB_hTrim-hCPSF6(301-358)-IRES-mCherry-P2A-PuroR” to encode the F321N mutation the CPSF6 region. This should abrogate the binding of this peptide to the HIV capsid protein. Eventually every plasmid in the lab gets a unique identifier based on the order it gets created (this is the GXXXX name). Since we haven’t actually started making this plasmid yet, I usually just stick an “X” in front of the name of the new file, to signify that it’s *planned* to be a new plasmid, with G619C being used as the template. Furthermore, I write in the mutation that I’m planning to make in it. Thus, this new plasmid map is now temporarily being called “XG619C_AttB_hTrim-hCPSF6(301-358)-F321N-IRES-mCherry-P2A-PuroR”

That’s what the overall plasmid looks like. We’ll be mutating a few nucleotides in the 4,000 nt area of the plasmid.

I’ve now zoomed into the part of the plasmid we actually want to mutate. The residue is Phe321 in the full length CPSF6 protein, but in the case of this Trim-fusion, it’s actually residue 344.

I next like to “write in” the mutation I want to make, as this 1) makes everything easier, and 2) is part of the goal of making a new map that now incorporates that mutation. Thus. I’ve now replaced the first two T’s of the Phe codon “TTT” with two A’s, making the “AAT” codon which encodes Asn (see the image above)

Next is planning the primers. So there are a few ways one could design primers to make the mutation. I like to create a pair of overlapping (~ 17 nt), inverse primers, where one of the primers encodes the new mutation in it. PCR amplification with these primers should result in a single “around-the-circle” amplicon, where there is ~ 17 nt of homology on the terminal ends. These ends can then be brought together and closed using Gibson assembly.

So first to design the forward primer. This is the primer that will go [5’end] –[17 nt homology] — [mutated codon] — [primer binding region] — [3’end]. So the first step is to figure out the primer binding region.

In a cloning scheme like this, I like to start selecting the nucleotides directly 3′ of the codon to be mutated, and select enough nucleotides such that the melting temperature is ~ 55*C. In actuality, the melting temperature will be slightly higher, since 1) we will end up having 17 nt of matching sequence 5′ of the mutated codon, and 2) the 3rd nt in the codon, T, will actually be matching as well.

Now that I’ve determined how long I need that 3′ binding region to be, I select the entire set of nucleotides I want in my full primer. In this case, this ended up being a primer 36 nt in length (see below).

Since this is the forward primer, I can just copy the “sense” version of this sequence of nucleotides.

OK, so next to design the reverse primer. This is simpler, since it’s literally just a series of nucleotides going in the antisense orientation directly 5′ of the codon (as it’s shown in the sense-stranded, plasmid map). I shoot for ~ 55*C to 60*C, usually just doing a little bit under 60*C.

Since this is the reverse primer, we want the REVERSE COMPLEMENT of what we see on the plasmid map.

Voila, we now have the two primers we need. We just now need to order these oligos (we order from ThermoFisher, since it’s the cheapest option at CWRU, and can then perform the standard MatreyekLab cloning workflow).