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Figure 7: Regulatory devices: a) oscillator, b) riboswitch, c) artificial scaffold protein. Figure from Purnick and Weiss
(2009)
Applications of regulatory devices
The devices shown in Figure 6 and Figure 7 and many other basic devices have been conceived in Escherichia
coli and yeast. These examples use standard inducers, such as IPTG, arabinose and anhydro-tetracycline and
control standard reporter genes like GFP. However, the same principles can be used to build genetic systems
that respond to all kinds of chemical or physical stimuli and control various cellular processes.
In numerous studies microorganisms have been equipped with devices that express reporter genes in
presence of environmental contaminants. Bacteria have also been programmed to recognise several tumour
characteristics over logical AND-gates and to invade correctly identified cancer cells. Artificial communication
between cell populations has been established via conditional expression of essential metabolites, toxins and
quorum sensing compounds such acyl homoserine lactone (Purnick and Weiss 2009). One of the rare examples
of a genetic device implemented in a plant is an artificial receptor for the environmental pollutant TNT in
Arabidopsis thaliana that signalises environmental pollution by chlorophyll loss (Bowen et al. 2008).
Importantly, genetic devices can also be integrated into biosynthetic pathways for dynamic process control in
biotechnology (see below). The experimental application possibilities of regulatory genetic devices are virtually
infinite. Table 2 lists selected projects that were presented at the international genetically engineered machine
(iGEM) competition to provide an idea of the application potential of regulatory genetic devices.
Global transcription machinery engineering (gTME), i.e. creating libraries of mutant transcription factors, is an
approach that has been implemented in both prokaryotes and eukaryotes (Seo et al. 2013). The connection
between mutant transcription factor sequences and function is difficult to predict de novo. Phenotypes are
selected and screened to identify and characterise the desired one.
Comprehensive lists of projects and references are available on the iGEM website (footnotes in Table 2).
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Year
Function
Reference
Environment – sensing and remediation
2007
E. coli expressing ß-galactosidase in presence of arsenate in water samples
leading to a pH drop
A
2007
A yeast sensor for real extra virgin olive oil
A
2007
A cellular lead sensor
A
2007
A biological radiation sensor
A
2013
E. coli and B. subtilis acting as microbial “mops” that detect algal toxins and
detoxify them by ligand release
B
Health and Medicine
2007
A biological system to sense environmental glucose concentration and decrease
it by insulin release
A
2007
An AHL sensor combined with a GFP reporter to detect biofilm formation in
catheters.
A
2013
E. coli cells that recognise and kill S. aureus by linking quorum sensing to the
release of antimicrobial peptides
B
Information processing and biocomputing
2007
Tri-stable toggle switch
A
2009
A “bio screen” of voltage activated yeast cells
A
2007
A divide-by-two circuit
A
2013
A system of two E. coli strains demonstrating the “Prisoners’ Dilemma” from
game theory
B
A
http://openwetware.org/wiki/IGEM:Projects_categorized
B
http://2013.igem.org/Jamboree/Team_Abstracts
Table 2: Genetic device projects presented at international genetically engineered machine (iGEM) competition
3.3.3
Design and optimisation of synthetic biological systems
Construction of synthetic devices and pathways involves a quantitative characterisation of the individual parts,
mathematical modelling, physical assembly, genetic modification, and systematic improvement of the system
based on experimental data and modelling.
Mathematical modelling
Mathematical models guiding the design of genetic modules, devices and pathways usually take the form of
differential equations describing the chemical reactions in the system. Figure 8 shows a simple genetic system
consisting of a constitutively expressed repressor, which can be inactivated by addition of an inducer molecule
and regulates the expression of a reporter protein. The reactions that have to be described are: promoter
binding of RNA polymerase, repressor-promoter interaction, inducer-repressor interaction, ribosome binding
to mRNA and degradation of mRNA and protein. The corresponding equations are shown next to the figure
and have to be filled with quantitative information (Arpino et al. 2013). Numerous software tools for design
and analysis of genetic networks are available (Purnick and Weiss 2009).
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Figure 8: A simple genetic system with model equations. Figure from Arpino et al. (2013)
Assembly of parts
A very common approach to assemble DNA parts is classical cloning based on restriction and ligation.
Restriction/ligation is implemented in the BioBrick™ standard (see below) and allows for stepwise and
combinatorial assembly. The major downside is that short scar sequences are left at every junction and that
the recognition sites of the involved restriction enzymes are forbidden within the parts. A set of alternative
methods are based on sequence overlaps between neighbouring parts, that can be connected by overlap
extension PCR, 'chew back and anneal' techniques such as sequence and ligation independent cloning (SLIC) or
the 'Gibson' isothermal assembly method. For a detailed review on current DNA assembly techniques see Ellis
et al. (2011).
Characterisation and “tuning” of parts
As the basic “parts” used in synthetic biology are derived from disparate sources, it is of central importance to
obtain quantitative information characterising their reaction rates and input-output thresholds (Arpino et al.
2013). In order to work synergistically in a synthetic system, parts and modules have to be adapted to each
other. Such system optimisation or “tuning” is typically carried out through iterative steps of mathematical
modelling, genetic manipulation, experimental testing and model refinement (Purnick and Weiss 2009).
State of the art molecular biology enables multifaceted modifications. At transcriptional level, promoter
strength can be influenced by modifying the sequence of the RNA-polymerase binding sites and thus binding
affinity. Regulated promoters can also be modified by variation of the operator sequence, which changes the
strength of the interaction between repressor and DNA. Mutations in the DNA sequences between important
binding motifs have also been shown to produce variation in promoter strength, presumably by altering local
DNA conformation (Arpino et al. 2013; Ellis et al. 2009).
At translational level, the efficiency of ribosome recruitment can be controlled by modifying the sequence of
the ribosome binding sites (RBS). Moreover, translation rates of protein coding genes can be improved by
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