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aim (Li and Pfeifer 2014). Despite many advances, the choice of the proper host is a matter of balancing
advantages and challenges. As an alternative, a new host may be built – “bottom-up” – to optimise cellular
function and heterologous production. Microbial engineering of plant pathways, however, will also facilitate
the discovery of biosynthetic genes and the analysis of plant metabolite synthesis, and thus be an essential
step towards using the potential of synthetic biology (Facchini et al. 2012). Pathway assembly in yeast may, for
instance, contribute to the discovery of gene function, in particular when
in vitro enzyme characterisation is
not possible, to optimise pathway efficiency, and also as a combinatorial biochemistry platform to produce
novel molecules in plants.
Interest in plant metabolism is growing as plants represent an enormous repository of bioactive natural
products of pharmaceutical and biotechnological importance (Xu et al. 2013). These products are currently
mostly extracted from their native plant sources or semi-synthesised from extracted intermediates, both
processes that potentially suffer from low yield and complicated downstream purification processes. Synthesis
may involve toxic catalysts or require extreme reaction conditions. Microbial metabolic engineering is a way to
overcome these limitations, and offers the possibility to use a genetically tractable organism. As more and
more genetic parts and devices are characterised (e.g. synthetic promoter libraries or synthetic ribosome
binding sites are available) and the cost of DNA synthesis declines, tailor-made cell factories may be designed
and created for high-throughput, efficient production of natural products and fuels.
It is thus not surprising that one of the major potential application of synthetic biology using plant resources is
the reconstruction of plant biosynthetic pathways in heterologous hosts (Li and Pfeifer 2014). In parallel, for
example codons can be optimised or unnecessary sequences removed. This approach shows one of the most
important advantages of synthetic biology as compared to “simple” metabolic engineering, namely rendering
production more efficient. Alternative to already existing pathways that are optimised there is also the
possibility to build a pathway from genes/enzymes with known functionality. The ultimate aim is to provide
rapid and robust access to the desired compound. In this context, synthetic biology plays a crucial role in
improving expression and thus productivity.
Yeast (
Saccharomyces cerevisiae) is the organism of choice for the reconstruction of complex plant pathways
(Facchini et al. 2012). One reason for this is that genome-wide metabolic models and genetic resources are
available, and, more importantly, that optimal expression and activity of plant enzymes require a eukaryotic
cell environment. Microbial hosts may be engineered to produce sufficient levels of metabolic precursors and
are the starting point for the stepwise introduction of plant genes for the production of central intermediates
in the targeted pathway up to the creation of novel metabolic pathways. Challenges include finding the
optimal selection of regulatory elements for optimised pathway flux but also to overcome the limitations due
to the dynamic behaviour of complex biological systems, e.g. the maintenance of a constant level of essential
precursor metabolite flux that causes only limited performance improvement (Xu et al. 2013; Facchini et al.
2012). Despite such challenges, both
Escherichia coli and
Saccharomyces cerevisiae have been successfully
used
to produce fatty acids, terpenoids, flavonoids, polyketides and alkaloids (Xu et al. 2013).
Besides
Saccharomyces cerevisiae and
Escherichia coli,
Candida utilis,
Streptomyces avermitilis, and
Bacillus
subtilis have been used as heterologous hosts for the production of plant-derived isoprenoid products, like
lycopene from tomato, artemisin against malaria from
Artemisia annua, and paclitaxel (taxadiene) with anti-
cancer properties from
Taxus brevifolia (reviewed in Li and Pfeifer (2014)). Another example is the production
of isoprenoids in
Escherichia coli (Ajikumar et al. 2010).
Advanced applications of microorganisms in the biofuel sector
Yeast metabolism is converted such that the specific hydrocarbon or the target molecule farnesene is
produced. Yeast uses sugar as a nutrient and as a feedstock (Amyris 2014). It may be designed in a way that it
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directly converts sugar into the targeted end product bio-isobutane and consequently in high-octane gasoline
(PCSBI 2010; Bioenergies 2014).
LS9 has developed a platform technology that leverages the natural efficiency of microbial fatty acid
biosynthesis to produce a diversity of drop‐in fuels and chemicals. Microorganisms modified by synthetic
biology may perform a one‐step conversion of renewable carbohydrates (sugars) to two diesel alternatives, a
fatty acid methyl ester and an alkane (BIO 2013).To achieve this, alkane biosynthetic genes were engineered
into
Escherichia coli.
“Synthetic biology has been essential in engineering the LS9 microbial catalysts. The biosynthetic pathways to
produce finished fuel products do not exist in the native Escherichia coli
host, and prior to our efforts alkane
biosynthetic genes were unknown. LS9 designed the pathways, synthesised the genes encoding each enzyme in
the pathway, and constructed multigene biosynthetic operons enabling production. To improve yield,
productivity, and titer – the drivers of process economic efficiency – the biosynthetic pathways and host
metabolism have required significant genetic optimization. LS9 developed capabilities for the computational
design and automated parallel construction of gene, operon, and recombinant cell libraries that have enabled
the rapid construction and evaluation of thousands of rationally engineered microorganisms. This capability in
combination with state of the art screening, process development, and analytical methodologies has enabled
LS9 in only a few years to advance from concept to a process slated for commercial‐scale demonstration” (BIO
2013).
Figure 13: Producing biofuels and renewable chemicals by means of synthetic biology; adapted from BIO (2013);
TM….trade mark
Global Bioenergies (BIO 2013; Bioenergies 2014) modified several enzymes in a way so that enzymatic
catalysis
within artificial metabolic pathways was achieved. Subsequently, these enzymes were integrated into a
bacterium which converts sugar into bio-isobutane.
Some bacteria have the built-in enzymes to manufacture butanol, but the natural process is not very fast or
high-yield. Synthetic biologists have engineered the easy-to-manipulate bacterium
Escherichia coli to improve
this bacterial biochemical reaction to make butanol more industrially useful (PCSBI 2010).
Certain microorganisms have evolved to be proficient in converting lignocellulosic material to ethanol,
biobutanol and other biofuels. A longstanding challenge in metabolic and genetic engineering is determining
whether to improve the isolate host’s production capacity or whether to transplant the desired genes or
pathways into an industrial model host, such as
Escherichia coli or
Saccharomyces cerevisiae (Khalil and Collins
2010). For example, Calysta engineered, by means of synthetic biology, the metabolic pathways of