Design and validate Type IIS scarless DNA assembly (Golden Gate, MoClo) using Biopython Bio.Restriction. Screens parts for internal BsaI/BsmBI/BbsI/SapI sites (domestication), previews the fusion overhangs a digest exposes, and validates a fusion-overhang set for distinctness and fidelity. Use when designing a Golden Gate or MoClo assembly, domesticating a part by removing internal Type IIS sites, or choosing and checking fusion overhangs for one-pot assembly.
Design and validate Type IIS scarless DNA assembly (Golden Gate, MoClo) using Biopython Bio.Restriction. Screens parts for internal BsaI/BsmBI/BbsI/SapI sites (domestication), previews the fusion overhangs a digest exposes, and validates a fusion-overhang set for distinctness and fidelity. Use when designing a Golden Gate or MoClo assembly, domesticating a part by removing internal Type IIS sites, or choosing and checking fusion overhangs for one-pot assembly.
tool_type
python
primary_tool
Bio.Restriction
Version Compatibility
Reference examples tested with: BioPython 1.83+ (API verified on 1.86)
Before using code patterns, verify installed versions match. If versions differ:
Python: pip show biopython then help(Bio.Restriction.BsaI.search) to confirm the search API
If code throws ImportError, AttributeError, or TypeError, introspect the installed
package and adapt the example to match the actual API rather than retrying.
Golden Gate / Type IIS Assembly
"Design (or check) my Golden Gate assembly" -> Make each part free of the assembly enzyme's internal sites, and give every junction a distinct, well-behaved fusion overhang, so one tube of enzyme plus ligase builds the construct directionally and scarlessly.
Python: Bio.Restriction to find internal Type IIS sites and read the overhang a cut exposes; the overhang-set rules are sequence logic, not a database call.
The whole method rests on one property: a Type IIS enzyme cuts outside its recognition sequence, so the 4-nt overhang it leaves is set by the user's flanking DNA, and the recognition site is placed to be removed from the final product. Because the ligated junction no longer contains the site, the enzyme cannot re-cut it, so digestion and ligation run together in one pot. Two design obligations follow, and both are what this skill checks: (1) domestication -- no part may contain an internal copy of the assembly enzyme's site, or it will be fragmented during assembly; (2) a set of distinct, non-palindromic fusion overhangs -- one per junction -- so parts assemble in exactly one order.
Choosing The Assembly Enzyme
Enzyme
Recognition
Overhang
Typical role
BsaI (Eco31I)
GGTCTC(1/5)
4 nt 5'
The default Golden Gate / MoClo Level 1 enzyme; BsaI-HFv2 for fidelity
BsmBI (Esp3I)
CGTCTC(1/5)
4 nt 5'
MoClo Level 0 / Level 2 (alternates with BsaI between levels)
BbsI (BpiI)
GAAGAC(2/6)
4 nt 5'
Alternative when BsaI/BsmBI sites cannot be domesticated out
SapI (LguI)
GCTCTTC(1/4)
3 nt 5'
3-nt (codon-length) overhangs for reading-frame-preserving fusions
Hierarchical systems (MoClo, Golden Braid) alternate enzymes between levels so each assembly round removes the previous level's sites: assemble Level 0 -> 1 with one enzyme, 1 -> 2 with the other. Pick the level's enzyme first, then domesticate every part against it.
Method Context
Golden Gate (Type IIS)
Classic restriction-ligation (Type IIP)
Gibson assembly
Junction defined by
user-designed 4-nt overhang (3 for SapI)
the enzyme's fixed overhang
~20-40 bp designed homology
Scar
none (site removed from product)
a restriction-site scar at each junction
none
Reaction
one-pot, one-step (37/16 C cycling)
sequential digest -> purify -> ligate
isothermal 50 C
Fragments per reaction
many (20-30+, more with optimized sets)
few
several
Main constraint
domestication; distinct overhang set
needs available compatible sites
terminal homology only
Choose Golden Gate when assembling several parts repeatedly from a standardized library; classic digestion for a one-off two-piece clone with convenient sites; Gibson when parts cannot be domesticated or no site layout works.
Domesticate: Find Internal Sites
Goal: Confirm a part carries no internal copy of the assembly enzyme's site (on either strand), and locate any that must be removed.
Approach:enzyme.search(seq) finds Type IIS sites on both strands (the recognition sequence is asymmetric, so its reverse-complement is detected too). Any hit inside a part is a defect to silently mutate away.
from Bio import SeqIO
from Bio.Restriction import BsaI, BsmBI, BbsI, SapI
record = SeqIO.read('part.fasta', 'fasta')
for enzyme in (BsaI, BsmBI, BbsI, SapI):
hits = enzyme.search(record.seq) # both strands; recognition site is asymmetric
status = 'clean'ifnot hits elsef'{len(hits)} internal site(s) at {hits} -> domesticate'print(f'{enzyme} ({enzyme.site}): {status}')
Domesticate: Break A Site By A Silent Mutation
Goal: Remove an internal site from a coding part without changing the protein.
Approach: Anchor on the recognition sequence itself (on both strands), not the cut position -- a Type IIS enzyme cuts outside its site, so mutating the codon at the cut would not touch the site. Walk the codons overlapping each recognition-site occurrence, swap one for a synonymous codon that breaks the site, and assert the protein is unchanged. This needs the reading frame.
from Bio.Data import CodonTable
from Bio.Seq import Seq
defdomesticate_cds(cds, enzyme, frame=0):
'''Remove an enzyme's internal sites from a CDS by synonymous codon swaps (reading frame `frame`).'''
table = CodonTable.unambiguous_dna_by_id[1]
syn = {}
for codon, aa in table.forward_table.items():
syn.setdefault(aa, []).append(codon)
s = list(str(cds).upper())
end = frame + 3 * ((len(s) - frame) // 3)
protein = str(Seq(''.join(s[frame:end])).translate())
site = str(enzyme.site)
motifs = (site, str(Seq(site).reverse_complement())) # both strands; Type IIS sites are unambiguousfor _ inrange(len(s)):
seqstr = ''.join(s)
ifnot enzyme.search(Seq(seqstr)):
break
hit = max(seqstr.find(m) for m in motifs) # a recognition-site start (either strand)for ci inrange(((hit - frame) // 3) * 3 + frame, hit + len(site), 3):
if ci < frame or ci + 3 > len(s):
continue
codon = ''.join(s[ci:ci + 3])
alt = next((a for a in syn.get(table.forward_table.get(codon), ())
if a != codon andnot enzyme.search(Seq(''.join(s[:ci] + list(a) + s[ci + 3:])))), None)
if alt:
s = s[:ci] + list(alt) + s[ci + 3:]
breakassertstr(Seq(''.join(s[frame:end])).translate()) == protein # protein unchangedreturn Seq(''.join(s))
# A site that overlaps only Met/Trp codons (no synonyms) is rare but cannot be silently broken;# always confirm the result is clean: assert not enzyme.search(domesticate_cds(cds, enzyme))
Preview The Fusion Overhangs A Digest Exposes
Goal: Read the actual 4-nt overhang each Type IIS cut would leave, to confirm junctions match as designed.
Approach: Anchor on the literal forward recognition sequence so only forward-oriented sites are read (a reverse-oriented site cuts on the other side and would otherwise return a misleading overhang). The 5' overhang starts fst5 bases after the recognition-site start.
from Bio.Restriction import BsaI
defforward_overhangs(seq, enzyme=BsaI, width=4):
'''4-nt overhangs at FORWARD-oriented Type IIS sites, anchored on the recognition sequence.'''
s, site, out = str(seq).upper(), str(enzyme.site), []
i = s.find(site)
while i >= 0:
cut = i + enzyme.fst5 # top-strand cut offset from the site start
out.append(s[cut:cut + width])
i = s.find(site, i + 1)
return out
# Reverse-oriented sites cut on the other side; for a full construct, design the overhangs# explicitly (below) rather than inferring every one from sequence.
Validate A Fusion-Overhang Set
Goal: Check that the overhangs chosen for all junctions assemble uniquely and ligate efficiently.
Approach: Apply the design rules: every overhang distinct; none palindromic (self-ligates); no overhang equal to the reverse complement of another (cross-ligates); avoid all-identical bases. High-throughput ligation-fidelity data (Potapov 2018) underlies curated high-fidelity sets used for large assemblies.
from Bio.Seq import Seq
defvalidate_overhang_set(overhangs):
issues = []
iflen(set(overhangs)) != len(overhangs):
issues.append('duplicate overhangs (parts assemble ambiguously)')
for o in overhangs:
if o == str(Seq(o).reverse_complement()):
issues.append(f'{o} is palindromic (self-ligates)')
iflen(set(o)) == 1:
issues.append(f'{o} is a homopolymer (low ligation fidelity)')
rc = {o: str(Seq(o).reverse_complement()) for o in overhangs}
for a in overhangs:
for b in overhangs:
if a != b and rc[a] == b:
issues.append(f'{a} is the reverse complement of {b} (cross-ligates)')
return issues or ['overhang set OK']
print(validate_overhang_set(['AATG', 'GCTT', 'TACT', 'GGGG']))
Common Errors
Symptom
Cause
Fix
Assembly drops or scrambles a part
An internal Type IIS site fragmented it
Domesticate every part against the level's enzyme before assembly
Junctions ligate in the wrong order or orientation
Two junctions share an overhang, or one is the reverse complement of another
Use a distinct, non-self-complementary overhang per junction; check with validate_overhang_set
Empty or low-efficiency assembly
Palindromic or homopolymer overhang, or wrong enzyme/buffer cycling
Avoid palindromic/homopolymer overhangs; cycle 37/16 C with a Type IIS enzyme + T4 ligase
search() misses a reverse-oriented site
Site too close to the sequence end so the cut falls off it
Domesticate on the full part in context, not a trimmed fragment
Recognition site still present in the product
Site placed so the cut does not remove it
Orient Type IIS sites so cleavage excises them from the assembled junction
Related Skills
enzyme-selection - Choose a classic restriction enzyme when scarless assembly is not needed
restriction-sites - Find any enzyme's sites in a part
fragment-analysis - Predict fragments to verify an assembly digest
genome-engineering/grna-design - Design constructs that this assembly will build
sequence-manipulation/transcription-translation - Confirm domestication kept the reading frame
References
Engler C, Kandzia R, Marillonnet S. A one pot, one step, precision cloning method with high throughput capability. PLoS One. 2008;3(11):e3647. doi:10.1371/journal.pone.0003647
Engler C, Gruetzner R, Kandzia R, Marillonnet S. Golden gate shuffling: a one-pot DNA shuffling method based on type IIs restriction enzymes. PLoS One. 2009;4(5):e5553. doi:10.1371/journal.pone.0005553
Weber E, Engler C, Gruetzner R, Werner S, Marillonnet S. A modular cloning system for standardized assembly of multigene constructs. PLoS One. 2011;6(2):e16765. doi:10.1371/journal.pone.0016765
Potapov V, Ong JL, Kucera RB, et al. Comprehensive profiling of four base overhang ligation fidelity by T4 DNA ligase and application to DNA assembly. ACS Synth Biol. 2018;7(11):2665-2674. doi:10.1021/acssynbio.8b00333
