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Intact synthetic
Mycoplasma mycoides genomes from the sMmYCp235 yeast clone were transplanted into
restriction-minus
Mycoplasma capricolum recipient cells.
To aid in testing the functionality of each 100 kb synthetic segment, semisynthetic genomes were constructed
and transplanted. By mixing natural pieces with synthetic ones, the successful construction of each synthetic
100 kb assembly could be verified without having to sequence these intermediates (Gibson et al. 2010)
.
It was also demonstrated that the progeny of this cell will not contain any protein molecules that
were present
in the original recipient cell.
3.2.3
Modification on other level than DNA
Non-Natural Amino Acids (NNAA):
DNA is a chemically and mechanically robust biopolymer that serves as a code and a memory but has almost
no enzymatic or catalytic activity (Noireaux et al. 2011). However, all necessary transcription and translation
components are encoded in genes,
i.e. DNA, which is also the basis for proteins that upon translation perform
the desired tasks within an organism.
Artificial cells would be ideal containers for inserting new genetic modules or modifying existing ones. With the
ability to generate a synthetic specific genome, a modified genetic code could be defined thus affording
additional codons for the incorporation of non-natural amino acids (NNAA) in cellular proteins. Such an
incorporation of a variety of NNAA would allow the fabrication of novel proteins with novel functions (Moya et
al. 2009).
Non-natural amino acids have already been successfully incorporated into proteins using several strategies
involving orthogonally evolved tRNA and rRNA synthases (Hendrickson et al. 2004),
but this approach has been
hampered by low efficiencies of incorporation due to competition with already existing codon recognition
factors. Expanding the repertoire of possible amino acids that the cell can use to build proteins is a powerful
capability that will be readily available to any recoded chassis (Esvelt and Wang 2013).
In all living cells the genetic code is limited to the common 20 amino acids (AA), with the rare exceptions of
selenocysteine and pyrolysine. The genetic code is expressed by aminoacyl-tRNA synthetases, the enzymes
that load specific AAs onto tRNAs. It was demonstrated (Liu and Schultz 2010) that mutating the active site of
the
Methanococcus jannaschii tyrosyl-tRNA synthetase enabled it to load a variety of non-natural AA onto
target tRNA that were subsequently assembled into proteins in
Escherichia coli. Using these methods,
numerous NNAA have been incorporated into proteins, enabling the site-specific labelling of proteins with
biophysical probes, photo-crosslinking reagents, fluorescent groups, heavy atoms, and orthogonal reactive
groups (Filipovska and Rackham 2013).
Despite the many advances in expanding the genetic code, with over 70 NNAA successfully added to date,
these approaches have been almost entirely relied on recoding of the rare UAG stop codon. Because all 61
sense codons are actively used by existing tRNAs carrying the 20 canonical AA it has been challenging to add
multiple different NNAA to proteins within the same cells. One remedy would be to move beyond the existing
set of triplet codons and use tRNAs with extended anticodons that recognise quadruplet codons. However, the
efficiency of incorporation was very low, most likely due to the ribosome’s reduced ability to accommodate
the larger anticodon. To solve this problem, mutant-ribosomes were constructed which were able to use
quadruplet codons as efficiently as triplet codons (Filipovska and Rackham 2013).
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NNAAs can be synthesised with a great variety of side chains. These techniques were expanded to yeast,
mammalian cells, and multicellular organisms. One disadvantage is that incorporation of NNAA by replacing
natural codons necessarily competes with natural processes (Marcheschi et al. 2013).
Generally, NNAA introduce new chemical functionality to directly participate in the enzymatic mechanism.
Examples include metal-ion binding, photoreactive AA, and photocaged AA. Also relevant are AA that
represent a post-translationally modified version of a natural AA. Examples include sulfated, methylated,
nitrosylated, and recently phosphorylated AA (Marcheschi et al. 2013)
RNA modifications
Also RNA could be used to program synthetic cells, which seems far more complicated to build than a DNA
cell (Noireaux et al. 2011).
3.3
Building biological systems from parts
In line with the principles of classical engineering synthetic biology aims at building biological systems from
basic components. On DNA level, basic components are promoters, operators, upstream activation sequences
(UAS), ribosome binding sites (RBS), open reading frames (ORF), terminators etc. These “parts” are combined
to create genetic circuits of increasing complexity following a systematic design framework. Current synthetic
biology mainly focuses on the construction and optimisation of regulatory devices and metabolic pathways.
The creation of novel genomes from parts is an ultimate goal for the future (Arpino et al. 2013; Ellis et al.
2011).
3.3.1
Regulation of gene expression: parts and devices
A part is a distinct DNA sequence which performs a defined function in a genetic circuit and is compatible with
an assembly technique. A gene is an open reading frame (ORF) along with all regulatory elements required for
successful expression. A functional gene or “composite part” usually consists of a promoter, a translation start
site (the RBS in prokaryotes), the protein coding ORF and a terminator. Assembly of a gene from its individual
parts functionally requires ordered assembly. One of the foundational advances of synthetic biology was the
BioBrick™, a DNA unit with standardised flanking sequences that enables assembly to be achieved by a cheap,
simple and standardised restriction/ligation method. With BioBricks™ it became possible to store pots of
modular biological parts that could be shared and easily assembled in different combinations by a vibrant
community.
A pathway is a group of genes or operons (multi-ORF genes) which may perform related functions. Such
pathways, devices and regulatory networks could be used for integrated assembling to build synthetic
designer genomes or chromosomes for custom organisms. Research in this area is already active and
beginning to bear fruit (Ellis et al. 2011).
Regulatory elements are of pivotal importance for designing predictable systems (Boyle and Silver 2009).
Regulation is achieved at several levels in biological systems: transcription, RNA processing, translation,
protein-protein interactions, and protein-substrate interactions. Spatial and temporal organisation of these
control systems ensures their proper function. In particular proteins may interact with any suitable binding
partners, regardless of the cellular compartment. Synthetic proteins may be rearranged and thereby yield new
behaviour. In synthetic systems, the metabolism may be optimised at many scales (compartmentalisation,
proteins – substrate channelling, scaffolding of complexes, enzymes – electron transfer in mitochondria and
chloroplasts (Agapakis et al. 2012).
Many new biological parts, like organelle-targeting sequences, transmembrane transporters and bacterial
microcompartment (BMC) pores, need to be characterised for efficient intracellular engineering (Agapakis et