Gibson Assembly of a Multi-Fragment Expression Construct with PCR-Amplified Insert and Linearized Vector

Run the assembly reaction alongside a vector-only (no-insert) negative control to quantify background recircularization, and a positive control assembly provided with the kit. Pick 6–8 colonies per assembly for colony PCR; sequence-verify 2–3 PCR-positive clones. Use a 2–3:1 insert:vector molar ratio with 50–100 ng total DNA. Perform single assembly reactions but evaluate at least two independent transformations if the correct rate is low.

BSL-1 for standard non-pathogenic E. coli cloning strains. Wear gloves and coat; use a Bunsen burner or sterile technique near an open flame with care. DNA stains (e.g., ethidium bromide) are mutagenic — prefer safer stains (SYBR Safe, GelRed) and dispose of gels as chemical waste. UV transilluminator: wear face shield/UV-blocking goggles. Autoclave or disinfect all bacterial waste, plates, and liquid cultures before disposal.

Negative control: vector-only assembly (no insert) and its transformation — colony count indicates recircularization/background; the assembly plate should have substantially more colonies. Positive control: kit-provided control fragments and transformation to confirm reagent and competent-cell performance. No-DNA transformation control to confirm plate selectivity (no growth). DpnI control: verify template removal by including a no-DpnI mock to compare background. Sanger sequencing is the final verification control for junction fidelity.

A successful assembly yields tens to hundreds of colonies, clearly above the vector-only control. Colony PCR should give the expected junction band size in ≥ 80% of picked colonies for 2–3 fragment assemblies (lower for 4–6). Sanger reads should show seamless, scarless junctions matching the design with no indels at overlaps. Vector-only background should be a small fraction of the assembly plate count.

• Linearized expression vector (restriction-digested or PCR-amplified, gel-purified, ~50 ng/µL)
• High-fidelity polymerase (Phusion or Q5) for insert amplification
• Gibson/isothermal assembly master mix (T5 exonuclease + Phusion + Taq DNA ligase, 2×)
• Primers with 20–40 bp 5' overlaps complementary to adjacent fragment ends
• dNTPs (10 mM each), gel-extraction and PCR-cleanup kits
• DpnI (to remove methylated template plasmid after PCR)
• Chemically competent E. coli (DH5α or NEB 5-alpha), SOC medium
• LB agar plates with appropriate antibiotic (e.g., 100 µg/mL ampicillin or 50 µg/mL kanamycin)
• 1% agarose, DNA ladder, gel stain; thermocycler and 50 °C heat block/PCR block

To construct a scarless recombinant plasmid by designing 20–40 bp overlaps between a PCR-amplified insert and a linearized vector, joining them in a single isothermal exonuclease/polymerase/ligase reaction, and recovering sequence-verified clones. The objective is a directional, ligation-independent assembly with a high correct-clone rate (≥ 80%).

1. Design primers adding 20–40 bp overlaps; amplify insert with Phusion (98 °C 30 s; 30× 98 °C 10 s / 60–65 °C 20 s / 72 °C 30 s per kb; 72 °C 5 min).
2. Add 1 µL DpnI to the PCR, incubate 37 °C 30 min to digest template; clean up or gel-purify the insert.
3. Quantify insert and vector by NanoDrop; compute a 2–3:1 insert:vector molar ratio (50–100 ng total).
4. Combine fragments to 10 µL total; add 10 µL 2× assembly master mix (20 µL final).
5. Incubate 50 °C for 60 min (15 min for 2–3 fragments; 60 min for 4–6); hold on ice.
6. Transform 2 µL of the assembly into 50 µL competent cells: ice 30 min, 42 °C 30 s heat shock, ice 2 min.
7. Add 950 µL SOC; recover 60 min at 37 °C, 250 rpm; plate 100–200 µL on selective agar; incubate 16 h at 37 °C.
8. Pick 6–8 colonies; screen by colony PCR with junction-spanning primers; run on 1% agarose.
9. Miniprep PCR-positive clones and confirm junctions by Sanger sequencing before use.

Independent variables: number of fragments, overlap length, and insert:vector molar ratio. Dependent variables: total colony count, fraction of colony-PCR-positive clones, and fraction sequence-correct. Controlled variables: assembly temperature/time (50 °C, 15–60 min), DNA total mass (50–100 ng), competent-cell lot and efficiency, recovery time, and antibiotic concentration. Overlap GC content and Tm are held in a design window (40–60% GC, Tm ≥ 48 °C).

Fragments sharing 20–40 bp of homologous, GC-balanced overlap will be assembled by T5 exonuclease chew-back, annealing, Phusion gap-fill, and Taq ligase into a covalently closed plasmid, yielding more correct colonies than the vector-only background and producing the designed junctions confirmed by Sanger sequencing.

Count colonies on assembly vs vector-only plates and compute a fold-over-background ratio. Score colony-PCR gels for correct-size bands and calculate percent positive. Align Sanger traces to the in-silico assembled sequence in SnapGene/Benchling; confirm overlap regions, reading frame, and absence of mutations. Record which clone is archived (glycerol stock) and its verified sequence.

No colonies on assembly plate: low DNA, poor competent cells, or no overlap — verify fragment concentrations, retest cells with the positive control, recheck overlap design. High vector-only background: incomplete linearization — gel-purify the cut vector and add a phosphatase or longer digest. Many colonies but PCR-negative: template carryover — ensure DpnI digestion. Mutations at junction: polymerase error — use high-fidelity enzyme and sequence multiple clones. Small/no insert band: mispriming — optimize annealing temp and re-amplify.

Assembly is typically descriptive rather than inferential, but to compare conditions (e.g., overlap length or ratio) score correct-clone counts as proportions and compare with Fisher's exact test (α = 0.05) across ≥ 2 independent assemblies. Report correct-clone percentage with a 95% binomial CI from the number of colonies screened. For colony-count comparisons across conditions use a Poisson or negative-binomial test if quantifying transformation efficiency.