Method for assembling PCR fragments of DNA
A process for assembling a series of DNA fragments generated by PCR into an ordered circular arrangement for replication and genetic work in cells. The PCR fragments are made with a modified nucleotide in the primers that can be removed with a DNA excision repair enzyme to generate a 3′ overhang. The 3′ overhangs are designed to allow directional annealing and thus sequential PCR fragments can be assembled by annealing the overhangs and subsequent ligation. Sequential addition of PCR fragments is facilitated by growing the chain on a solid support, and the assembled chain can be removed with a site specific recombinase if the first and last primers contain the recombinase site. The circularized assembled fragment can be directly used for cell transformation if the appropriate sequences are included, such as an origin of replication and a selectable marker.
1. A method of assembling PCR fragments, comprising
a) making a first PCR fragment with first and second primers, wherein the second primer comprises a modified nucleotide that can be removed by a DNA repair enzyme, resulting in a 3′ overhang, and wherein the first PCR fragment comprises a first site specific recombinase site;
b) treating the first PCR fragment with a DNA repair enzyme to generate a 3′ overhang and immobilizing the first PCR fragment on a solid support or vice versa;
c) making a second PCR fragment with third and fourth primers, wherein the third and fourth primers each comprises a modified nucleotide that can be removed by a DNA repair enzyme resulting in a 3′ overhang;
d) treating the second PCR fragment with a DNA repair enzyme to generate a 3′ overhang;
e) annealing and ligating the first and second PCR fragments;
f) optionally repeating steps c, d and e until a last PCR fragment is added to the growing chain to produce an assembled fragment, wherein the last PCR fragment comprises a second site specific recombinase site; and
g) simultaneously removing and circularizing the assembled fragment from the solid support with a site specific recombinase in a single step.
2. The method of claim 1 , where one of the PCR fragments comprises an origin of replication and a selectable marker.
3. The method of claim 1 , wherein the first PCR fragment or the last PCR fragment comprises an origin of replication and a selectable marker.
4. The method of claim 1 , wherein the site specific recombinase is CRE and the site specific recombinase site is lox.
5. The method of claim 1 , wherein the nucleotide is deoxyuridine and the DNA repair enzyme is uracil-DNA-glycosylase followed by T 4 endonuclease V.
6. The method of claim 5 , wherein the assembled DNA is greater than 30 kb.
7. The method of claim 5 , wherein the assembled DNA is greater than 30, 40, 50, 75, 100, 125, 150, 200, 250, 300, 350, 400, 450, 500, 750, 1000 or 1500 kb.