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al. 2012). Metabolic networks function through different strategies that bring together appropriate cells,
enzymes and substrates in time and space.
Control of gene expression is achieved on the levels of transcription (regulated promoters, transcriptional
riboswitches) or translation/post-translation, which fosters faster dynamics. Components (
i.e. genetic parts,
devices and systems) and tools to regulate these components in a predictable and quantitatively controllable
manner have to be designed (Seo et al. 2013).
3.3.2
Genetic devices
Genetic devices are combinations of parts that implement a defined function. Hallmarks in this field are
several designs inspired by electronic circuitry, such as genetic toggle-switches, timers, oscillators and logic
evaluators (Purnick and Weiss 2009; Arpino et al. 2013). Their functioning is based on the control of
transcription, translation or post-translational processing. Complex “devices” are assembled from well-defined
modular parts but it is challenging to fully predict their function (Boyle and Silver 2009). They may also be less
robust than natural systems, and endogenous regulatory systems may interfere with the function of synthetic
biology devices. It is difficult to predict what the assembled parts will do, even when much is known about
them individually (Arkin and Fletcher 2006). Engineering new proteins or gene circuits may lead to direct
interaction among new components or to indirect interactions via their effects on the organism (“chassis”) into
which they are introduced. Due to lacking co-evolution unforeseen cross-reactions may occur. It may be
expected that engineered organisms survive poorly in real-world environments (Arkin and Fletcher 2006).
Figure 5 and 6 show several types of genetic devices.
Transcriptional control devices
Transcriptional control of gene expression is used to optimise biological systems (modification of the spacer
region between DNA sequences of native promoters and utilisation of polymerase chain reaction (PCR) to
introduce mutations into the entire promoter region. Also, synthetic hybrid promoter approaches that
combine core promoters with enhancer elements consisting of combinations or tandem repeats of upstream
activating sequences. Promoter strength can differ depending on flanking sequences upstream and
downstream of the consensus boxes and promoter copy number. A recent study demonstrated the
importance of precisely balancing metabolic fluxes by controlling transcription efficiency for the optimal
production of a target molecule.
In addition, rapid prototyping services like BIOFAB (International
Open Facility
Advancing Biotechnology,
http://biofab.synberc.org/
) are developed that provide libraries of characterised
regulatory elements and facilitate prototyping of synthetic devices
via high-throughput cloning and testing
(Boyle and Silver 2012).
A prototype of transcriptional control is the
bistable toggle switch (Gardner et al. 2000). It is composed of two
promoters that transcribe mutually inhibitory repressor proteins and can be inactivated by different inducers
(Figure 6). One promoter also transcribes a reporter gene, such as the green fluorescent protein (GFP). The
expression of the reporter can be switched on by transient addition of the first inducer and switched off by
transient addition of the second inducer.
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Figure 6: Bistable genetic toggle switch. Figure from Gardner et al. (2000)
More complex arrangements of promoters and repressors are for example transcriptional oscillators and
timers.
Oscillators (Figure 7a) increase and decrease the expression of a reporter gene in a rhythm that
depends on the concentrations of two inducer compounds present in the medium (Purnick and Weiss 2009).
With a genetic
timer, the expression of a reporter gene can be switched on or off by transient addition of an
inducer and is reset to the initial state after a given time span (Ellis et al. 2009).
Translational control devices
Typical elements of translational control are
synthetic ribozymes (Arpino et al. 2013; Purnick and Weiss 2009).
In these RNA devices the transcript of a gene of interest carries an autocatalytic region and aptamers to sense
specific ligands. Binding of a ligand to the RNA aptamer leads to a structural change in the autocatalytic region.
The activated autocatalytic region inhibits translation of the transcript. RNA devices that respond to various
low molecular metabolites are called
riboswitches. RNA devices that respond to small interfering RNAs
(siRNAs)
are called riboregulators.
Riboswitches have been used to create
logic evaluators that report the presence or absence of several ligands
simultaneously using AND/OR/NOT/NOR Boolean logic (Figure 7b). Such devices are also referred to as logical
gates (Purnick and Weiss 2009).
Post-translational control devices
An example of a post-translational control device is the
scaffold protein phosphorylation system shown in
Figure 7c scaffold proteins recruit proteins required for specific cellular processes and tether them together. In
the
presented device, the scaffold is activated by phosphorylation in
presence of specific inducers, and scaffold
activation initiates expression of the reporter gene GFP (Purnick and Weiss 2009).