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Figure 4: Overview of exemplar DNA assembly techniques (from Ellis et al. (2011))
Genes to pathways (linking genes to construct pathways and devices)
The challenge of assembly at larger scales has led to methods utilising Type II restriction enzymes like the
Golden Gate assembly method: a parallel one-pot, one-step five min technique to assemble seamless
constructs. DNA is inserted into an entry clone shuttle vector which provides the Type II recognition sequences
immediately at both ends of the DNA pieces. Digestion then produces all the fragments for assembly which
ligate in parallel where overhangs are complementary. This method is also suitable to shuffle multiple
fragments.
Forbidden site requirement may occur – this problem can be solved with the oligo-based technology
RARE
(RecA-assisted restriction endonuclease), whereby sequence-specific blocking oligos prevent CpG methylation
of
tag digest sites,
via RecA-mediated binding to homologous sequences (Ferrin and Camerini-Otero 1991).
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Overlap assembly methods: These methods require DNA fragments for assembly to share at least 20 bp of
common sequence at ends that will be joined. This sequence is processed
in vitro by enzymes
that perform the
assembly. Several kits are available on the market, such as Gateway® (Life Technologies) and InFusion®
(Clontech). Suited particularly for parallel reactions with several DNA fragments, these techniques are also
often called “chew back and anneal”. They work by digesting back one strand of DNA from each exposed end
of a fragment, leaving a single-stranded overhang that anneals with the complementary overhang of a
fragment sharing the same overlap sequence. The order of the fragments is pre-determined by the sequence
overlap between them.
USER cloning and USER fusion (uracil-specific excision reagent) is a cloning technique which requires a PCR
amplification of fragments using primers that incorporate at least one uracil. No assembly scars are left
behind, seamless assembly is possible. The drawback of this method is that at least one thymidine is required
near the end of the sequence (replaced with uracil), so it is not truly sequence-independent.
Several other ligation-independent cloning methods also exist and a sequence-independent variation
SLIC
(sequence- and ligation-independent cloning) has recently been described (Ellis et al. 2011). The Craig Venter
Institute published several overlap methods where whole genomes were assembled
in vitro from directly
synthesised five kb fragments designed to have a more than 100 bp overlapping sequence. The
Gibson
isothermal assembly method requires a high fidelity DNA polymerase, T5 exonuclease and Taq DNA ligase and
foresees a single 30 min-incubation at one temperature.
Zhang et al. (2012) describe how to assemble multiple DNA fragments into recombinant DNA molecules in a
single
in vitro recombination reaction (SLiCE – Seamless Ligation Cloning Extract). This method is based on
bacterial extracts from common RecA
-
Escherichia coli laboratory strains, which can also be further optimised
by simple genetic modifications, and does not leave any unwanted sequences at the junction sites. SLiCE is also
capable of facilitating recombination between DNA fragments that contain flanking heterologous sequences
and of deleting the extra flanking sequences to generate precise junctions at the recombination sites.
Pathways to genomes
At this level parallel assembly reactions are essential; the order of assembly has to be defined.
TAR (Transformation Assisted Recombination cloning) is a common protocol for manipulation of DNA in yeast.
It undergoes homologous recombination during yeast spheroplast transformation and is predominantly used
to incorporate gene- and pathway-sized DNA assemblies into specific sites in the yeast genome. By including a
yeast artificial chromosome (YAC) replication sequence and a selective marker in one assembly fragment,
assembly of a circular self-propagations construct is achieved. The native recombination enzymes of yeast are
working, so the assembly in yeast is very accurate and it tolerates very large constructs.
For
genome sized assembly, yeast is not the only cellular chassis; also Bacillus was used by Itaya et al. (2008).
Gibson et al. (2010) reported the design, synthesis and assembly of the 1.08-mega-base pair
Mycoplasma
mycoides JCVI-syn1.0 genome starting from digitised genome sequence information and its transplantation
into a
Mycoplasma capricolum recipient cell to create new
Mycoplasma mycoides cells that are controlled only
by the synthetic chromosome.
With a combination of
in vitro enzymatic methods and
in vivo recombination in
Saccharomyces cerevisiae the
synthetic genome from
Mycoplasma genitalium was assembled in four stages out of an initial DNA cassette of
about 6 kb in size.
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Bacterial genomes grown in yeast (eukaryotes) are unmethylated and thus are not protected from the single
restriction system of the recipient cell (prokaryotes dispose of a restriction system). This restriction barrier can
be overcome by methylating the donor DNA with purified methylases or crude
Mycoplasma mycoides or
Mycoplasma capricolum extracts, or by simply disrupting the recipient cell’s restriction system.
Design of the
Mycoplasma mycoides JCVI-syn1.0 genomes was based on the highly accurate finished genome
sequences of the laboratory strains of
Mycoplasma mycoides subspecies
capri GM12. 4. Watermark sequences
were designed to further differentiate between the synthetic genome and the natural one. These watermark
sequences encode unique identifiers while limiting their translation into peptides.
The oligonucleotides used in
the cassettes were synthesised by Blue Horn (Bothell, Washington). The genome assembly was performed in
three stages: In the first step, 1080 bp cassettes (orange arrows) were recombined in sets of 10 to produce 109
~10 kb assemblies (blue arrows). These were then recombined in sets of 10 to produce 11 ~100 kb assemblies
(green arrows). In the final stage of assembly, these 11 fragments were recombined
into the complete genome
(red circle). With the exception of two constructs that were enzymatically pieced together
in vitro (white
arrows), assemblies were carried out by
in vivo homologous recombination in yeast.
Figure 5: The assembly of a Mycoplasma mycoides genome in yeast (Gibson et al. 2010)