dna-assembly

name	dna-assembly
description	Guidance for Golden Gate assembly primer design and DNA assembly tasks. This skill should be used when designing primers for Golden Gate cloning, Type IIS restriction enzyme assembly, or multi-fragment DNA assembly workflows. It covers overhang selection, primer structure, assembly simulation, and verification strategies.

DNA Assembly

Overview

This skill provides procedural knowledge for Golden Gate assembly primer design, a molecular cloning technique that uses Type IIS restriction enzymes (like BsaI) to create scarless multi-fragment assemblies. The skill emphasizes rigorous verification and simulation to ensure designed primers produce the expected assembled product.

Workflow

Phase 1: Understand the Assembly Requirements

Before designing primers, thoroughly analyze:

1. Input sequences: Read and parse all input FASTA files to understand the fragments to be assembled
2. Output sequence: Understand the expected final assembled product
3. Assembly topology: Determine if the assembly is linear or circular (circular plasmids require the last overhang to match the first)
4. Fragment order: Identify the correct order of fragments in the final assembly
5. Reading frame considerations: Note which fragments require start codons, stop codons, or neither

Phase 2: Overhang Design

Overhang selection is critical for efficient Golden Gate assembly. Follow these principles:

Use established overhang sets: Rather than designing arbitrary overhangs, use validated overhang sets from NEB or published literature. See references/overhang_design.md for recommended sets.

Overhang requirements:

• 4-nucleotide overhangs for BsaI-based assembly
• Overhangs must be sufficiently different (Hamming distance >= 2 recommended)
• Avoid palindromic overhangs that could self-ligate
• Avoid overhangs with high GC content at ligation junction
• For N fragments in circular assembly, exactly N unique overhangs are needed

Common mistake: Selecting overhangs that differ by only one nucleotide (e.g., AACC, AACG, AACT). These similar overhangs can cause mis-ligation and reduce assembly efficiency.

Phase 3: Primer Structure Design

Golden Gate primers have a specific structure. Understanding orientation is crucial:

Forward primer structure (5' to 3'):

[5' extension (optional)] - [BsaI recognition site: GGTCTC] - [N spacer] - [4nt overhang] - [gene-specific binding region]

Reverse primer structure (5' to 3'):

[5' extension (optional)] - [BsaI recognition site: GAGACC] - [N spacer] - [4nt overhang (reverse complement)] - [gene-specific binding region (reverse complement)]

Critical orientation check: The BsaI site must always be at the 5' end of the primer as written. A common mistake is placing the recognition site at the 3' end, which will not produce the intended cut.

Gene-specific binding region requirements:

• Typically 18-25 nucleotides
• Melting temperature (Tm) between 55-65 degrees C
• GC content of 40-60% preferred
• Avoid runs of >4 identical nucleotides
• Check for secondary structure (hairpins) that could affect PCR

Phase 4: Pre-Assembly Verification

Before finalizing primers, perform these checks:

1. Internal restriction site check: Verify that insert sequences do not contain BsaI recognition sites (GGTCTC or GAGACC). If present, consider silent mutations or alternative enzymes.

2. Backbone check: Also verify the plasmid backbone being amplified does not contain internal BsaI sites.

3. Overhang uniqueness verification: Confirm all overhangs are unique and sufficiently different from each other.

4. Primer quality checks:

   • Self-complementarity analysis (avoid hairpins)
   • Primer-dimer formation potential
   • 3' end GC content (1-2 G/C in last 5 bases ideal for specificity)
   • Overall GC content (40-60%)

Phase 5: Assembly Simulation

This is the most critical verification step. Before declaring success:

1. Simulate PCR products: For each primer pair, determine the exact PCR product sequence including the BsaI sites and overhangs.

2. Simulate BsaI digestion: Apply the enzyme cut to each PCR product to determine the digested fragment with overhangs.

3. Simulate ligation: Assemble all digested fragments in silico based on overhang complementarity.

4. Compare to expected output: Perform a nucleotide-by-nucleotide comparison of the simulated assembled product against the expected output sequence.

Common mistake: Claiming assembly will work without actually simulating the complete product and comparing it to the expected output.

Phase 6: Output Generation

Generate primer output in a clear format:

• Primer name (indicating fragment and direction)
• Primer sequence (5' to 3')
• Calculated Tm for binding region
• Overhang produced after digestion

Verification Checklist

Before finalizing any primer design, confirm:

• All input sequences parsed correctly
• Expected output sequence understood
• Overhang set uses established/validated sequences
• All overhangs differ by Hamming distance >= 2
• No internal BsaI sites in inserts or backbone
• BsaI sites positioned at 5' end of all primers
• Primer Tm values within acceptable range (55-65 degrees C)
• No significant primer secondary structures
• Full assembly simulated in silico
• Simulated product matches expected output exactly
• Circular topology handled correctly (if applicable)
• Start/stop codons correctly included/excluded per fragment

Common Pitfalls

1. Inconsistent overhang reporting: Track overhangs carefully throughout the design process. If reported overhangs change between steps, this indicates a bug.

2. Primer orientation confusion: Remember that reverse primers are written 5' to 3' but bind to the opposite strand. The overhang sequence in a reverse primer should be the reverse complement of the desired overhang.

3. Circular assembly errors: For circular plasmids, the overhang connecting the last fragment back to the first must be correctly designed. Verify the plasmid closes in the correct orientation.

4. Incomplete verification: Checking that "the fusion matches at position X" is insufficient. Verify the entire assembled sequence matches the expected output.

5. Tm calculation inconsistencies: If Tm values differ between reports, investigate the calculation method. Use a consistent, reliable Tm calculation approach.

6. Script development approach: Test primer design logic incrementally on simple cases before applying to complex multi-fragment assemblies. Avoid writing large scripts that fail with cryptic errors.

Resources

references/

• overhang_design.md: Validated overhang sets and selection criteria for Golden Gate assembly