Methods for in vitro joining and combinatorial assembly of nucleic acid molecules
The present invention relates to methods of joining two or more double-stranded (ds) or single-stranded (ss) DNA molecules of interest in vitro, wherein the distal region of the first DNA molecule and the proximal region of the second DNA molecule of each pair share a region of sequence identity. The method allows the joining of a large number of DNA fragments, in a predetermined order and orientation, without the use of restriction enzymes. It can be used, e.g., to join synthetically produced sub-fragments of a gene or genome of interest. Kits for performing the method are also disclosed. The methods of joining DNA molecules may be used to generate combinatorial libraries useful to generate, for example, optimal protein expression through codon optimization, gene optimization, and pathway optimization.
1. An in vitro method of joining a set of two or more double-stranded (ds) or single-stranded (ss) DNA molecules, wherein adjacent DNA molecules to be joined contain overlapping sequences at their termini, said method comprising contacting in vitro the two or more DNA molecules in a single vessel with reaction components comprising
(a) an isolated non-thermostable 3′ to 5′ exonuclease active in the presence of dNTPs,
(b) a crowding agent selected from the group consisting of polyethylene glycol (PEG) and dextran,
(c) a heat-activated DNA polymerase comprising an inactivating moiety conjugated to the polymerase in a temperature-sensitive manner,
(d) a thermostable ligase,
(e) a mixture of dNTPs, and
(f) a suitable buffer,
under conditions that are effective for joining the two or more DNA molecules to form a first assembled dsDNA molecule in a one-step thermocycled reaction.
2. The method of claim 1 wherein the heat-activated DNA polymerase is active above 65° C.
3. The method of claim 2 wherein the non-thermostable exonuclease is inactive above 65° C.
4. The method of claim 1 , wherein the exonuclease of (a) is Exonuclease III.
5. The method of claim 1 , wherein DNA polymerase of (c) is heat-activated by the removal of the inactivating moiety bound to the polymerase in a temperature-sensitive manner.
6. The method of claim 5 wherein the inactivating moiety is an antibody bound to the DNA polymerase.
7. The method of claim 5 wherein the inactivating moiety is biotin bound to the DNA polymerase.
8. The method of claim 3 , wherein
the crowding agent of (b) is PEG, and/or
the ligase of (d) is Taq ligase.
9. The method of any of claim 1 , further comprising repeating the method to join a second set of two or more DNA molecules to one another to obtain a second assembled DNA molecule, and then joining the first and the second assembled DNA molecules to obtain a third assembled ds DNA molecule.
10. The method of claim 1 comprising a phase conducted at about 30-45° C. wherein the DNA polymerase is inactive and the exonuclease is active, and a phase conducted above 65° C. wherein the DNA polymerase is active and the exonuclease is inactive.
11. The method of claim 1 wherein the two or more DNA molecules are contacted with the components (a) (f) at about 37° C., and the temperature is subsequently raised and the components incubated at about 75° C.
12. The method of claim 3 wherein when the temperature is lowered to about 60° C. the ligase is active.
13. The method of claim 1 wherein the ligase is active at a temperature of 75° C.
14. The method of claim 1 wherein the two or more DNA molecules are contacted with the components (a) (f) at a temperature of 30-45° C., and the temperature is subsequently raised to above 65° C.
15. The method of claim 14 further comprising that subsequent to raising the temperature to above 65° C., the temperature is subsequently lowered to about 60° C.
16. The method of claim 3 wherein the exonuclease of (a) is exonuclease III.
17. The method of claim 1 , wherein the crowding agent is polyethylene glycol (PEG).
18. The method of claim 3 , wherein the crowding agent is polyethylene glycol (PEG).