As synthetic biology constructs grow in complexity, traditional restriction cloning becomes impractical. Gibson Assembly and Golden Gate cloning allow multi-fragment assembly in a single, one-pot reaction.
Gibson Assembly
Gibson Assembly joins multiple DNA fragments with overlapping ends in a single isothermal reaction at 50 °C. The reaction mix contains three enzymatic activities:
- 5′ exonuclease (T5 exonuclease or RecE): chews back the 5′ ends of double-stranded DNA, leaving single-stranded 3′ overhangs that allow complementary fragments to anneal.
- DNA polymerase (Phusion or Taq): fills in the gaps left after annealing.
- DNA ligase (Taq ligase): seals the nicks in the assembled DNA.
Each fragment must have 15–40 bp of homology with its neighbor at the junction. These overlaps are added by PCR primers. Gibson Assembly can join 2–15 fragments simultaneously. The assembly is scarless — no restriction sites are left in the final product.
The reaction requires 50–100 ng of each fragment in equimolar amounts and is complete in 15–60 minutes at 50 °C. The product is transformed directly into chemically competent E. coli. Gibson Assembly is ideal for:
- Assembling synthetic genes from oligonucleotides.
- Building entire plasmids from overlapping fragments.
- Constructing genome engineering cassettes with homology arms.
- Creating DNA libraries for directed evolution.
Golden Gate Cloning
Golden Gate cloning uses Type IIS restriction enzymes (BsaI, BpiI, Esp3I) that cut outside their recognition sequence, producing defined 4 bp single-stranded overhangs. These overhangs can be specified by the user.
The reaction is a simultaneous digest-ligation: the Type IIS enzyme and T4 DNA ligase are added together in a single tube, and the reaction mixture is cycled between the enzyme’s optimal temperature (37 °C for BsaI) and 16 °C or 37 °C (for ligation). Because the recognition sites are removed from the final assembled product, the reaction is driven to completion — the assembled product cannot be re-digested.
Golden Gate is the basis of modular cloning standards such as MoClo, Golden Braid, and the Plant Toolbox. It is ideal for:
- Combinatorial assembly of genetic parts (promoters, 5′ UTRs, CDS, terminators, tags).
- Building TALEN arrays from individual repeat modules.
- Constructing pooled sgRNA libraries for CRISPR screens.
The overhangs are designed to be mutually compatible in a defined order, allowing the same backbone to accept different combinations of parts. Golden Gate is more scalable than Gibson Assembly for modular combinatorial projects, though it requires the fragments to lack internal recognition sites for the enzyme used.
