photoautotrophic alternative, has a quicker generation rate and no dependency on
arable land; however, the cultivation cost of algae are excessively high (Scott et al.
2010). Consequently, there is a need to find a suitable host organism in which both
indigenous genes and the ones introduced from other organisms work in the coordinated fashion for the commercial production of high-value products (Moses et al.
2017).
Microbes can be considered as classic platform to rationally design new-to-nature
metabolites because they have a rapid generation rate and broad engineering toolkits,
are vigorous and readily scalable, and offer a simplified product purification technology. Most importantly production process in microbes is cost-effective as inexpensive carbon sources (e.g. agricultural wastes) are converted to high-value
compounds. The most broadly used heterologous host organisms are the bacterium
E. coli and the yeast S. cerevisiae, due to their extremely well-understood cell
biology, capacity to hold bulky genetic constructs as well as ability to tolerate
industrial growth conditions (Awan et al. 2016; Goold et al. 2018). Yeast Pichia
pastoris, able to grown on low-cost carbon sources, and the fungus Aspergillus
oryzae, with metabolism appropriate for the manufacture of polyketides and
non-ribosomal peptides can also be utilized as heterologous hosts (Keasling 2010;
Awan et al. 2016). Besides host selection, genes encoding enzymes catalysing
synthesis of natural product and their regulation to optimally balance the yield of
high-value metabolites and growth of the host must be selected. Depending on the
host organism selected, for a particular natural product, one or more biosynthetic
routes can be involved (Keasling 2010) and the proportionate contribution of native
and heterologous genes can vary (Awan et al. 2016). Moses et al. (2017) reviewed a
road map of the most recent SynBio tools under consideration and listed the
unicellular microbial hosts accessible for rational engineering of valuable plant
products.
The World Health Organization (WHO) has recommended artemisinin-based
combination therapies (ACTs) as an antimalarial treatment against the parasite
Plasmodium falciparum. Artemisinin (a sesquiterpenoid endoperoxide) is originally
sourced from Sweet wormwood (Artemisia annua) (Miller and Su 2011). However,
the delivery of plant-derived artemisinin is uneven, ensuing in frequent shortages
and cost instability. Superior production of artemisinic acid, a precursor of
artemisinin, in yeast was attained using numerous synthetic pathways:
(1) overexpression of a truncated 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (tHMGR) and downregulation of squalene synthase (ERG9) by a methionine
repressible promoter to increase farnesyl pyrophosphate (FPP) production and
decrease its use for sterols; (2) combination of a semidominant mutant allele of a
Zn(II)2Cys6 binuclear cluster transcription factor (upc2-1) with a downregulating
ERG9 and the addition of the amorphadiene synthase gene (ADS) from the Artemisia annua and an additional copy of tHMGR for the conversion of FPP to
amorphadiene; and (3) expression of a cytochrome P450 (CYP71AV1) and its
redox partner CPR under the control of galactose-inducible promoters for
artemisinic acid construction from amorphadiene (Ro et al. 2006). Paddon et al.
(2013) introduced the entire biosynthetic pathway (including plant dehydrogenase
15 Plant Synthetic Biology: A Paradigm Shift Targeting Stress Mitigation,. . .
469
arable land; however, the cultivation cost of algae are excessively high (Scott et al.
2010). Consequently, there is a need to find a suitable host organism in which both
indigenous genes and the ones introduced from other organisms work in the coordinated fashion for the commercial production of high-value products (Moses et al.
2017).
Microbes can be considered as classic platform to rationally design new-to-nature
metabolites because they have a rapid generation rate and broad engineering toolkits,
are vigorous and readily scalable, and offer a simplified product purification technology. Most importantly production process in microbes is cost-effective as inexpensive carbon sources (e.g. agricultural wastes) are converted to high-value
compounds. The most broadly used heterologous host organisms are the bacterium
E. coli and the yeast S. cerevisiae, due to their extremely well-understood cell
biology, capacity to hold bulky genetic constructs as well as ability to tolerate
industrial growth conditions (Awan et al. 2016; Goold et al. 2018). Yeast Pichia
pastoris, able to grown on low-cost carbon sources, and the fungus Aspergillus
oryzae, with metabolism appropriate for the manufacture of polyketides and
non-ribosomal peptides can also be utilized as heterologous hosts (Keasling 2010;
Awan et al. 2016). Besides host selection, genes encoding enzymes catalysing
synthesis of natural product and their regulation to optimally balance the yield of
high-value metabolites and growth of the host must be selected. Depending on the
host organism selected, for a particular natural product, one or more biosynthetic
routes can be involved (Keasling 2010) and the proportionate contribution of native
and heterologous genes can vary (Awan et al. 2016). Moses et al. (2017) reviewed a
road map of the most recent SynBio tools under consideration and listed the
unicellular microbial hosts accessible for rational engineering of valuable plant
products.
The World Health Organization (WHO) has recommended artemisinin-based
combination therapies (ACTs) as an antimalarial treatment against the parasite
Plasmodium falciparum. Artemisinin (a sesquiterpenoid endoperoxide) is originally
sourced from Sweet wormwood (Artemisia annua) (Miller and Su 2011). However,
the delivery of plant-derived artemisinin is uneven, ensuing in frequent shortages
and cost instability. Superior production of artemisinic acid, a precursor of
artemisinin, in yeast was attained using numerous synthetic pathways:
(1) overexpression of a truncated 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (tHMGR) and downregulation of squalene synthase (ERG9) by a methionine
repressible promoter to increase farnesyl pyrophosphate (FPP) production and
decrease its use for sterols; (2) combination of a semidominant mutant allele of a
Zn(II)2Cys6 binuclear cluster transcription factor (upc2-1) with a downregulating
ERG9 and the addition of the amorphadiene synthase gene (ADS) from the Artemisia annua and an additional copy of tHMGR for the conversion of FPP to
amorphadiene; and (3) expression of a cytochrome P450 (CYP71AV1) and its
redox partner CPR under the control of galactose-inducible promoters for
artemisinic acid construction from amorphadiene (Ro et al. 2006). Paddon et al.
(2013) introduced the entire biosynthetic pathway (including plant dehydrogenase
15 Plant Synthetic Biology: A Paradigm Shift Targeting Stress Mitigation,. . .
469