95
yield in A. niger. The fermentation conditions of IA can be enhanced by using nitrogen supplementation that reduces IA-induced weak-acid stress.
Along with the editing of specific genes, a variety of promoters have also been
utilized to improve gene expression and increase the production of IA [10]. In 2011,
Steiger et al. developed six novel promoters for A. niger. They obtained a 0.6 g L
−1
titer of IA with the PmbfA promoter, while other promoters were not active for such
transformation [46]. Li et al. introduced gpdA promoter to control cadA gene expression. The recombinant strain A. niger produced a relatively low titer (0.6 g L
−1
) of
IA [40]. To achieve a higher IA titer, Blumhoff et al. applied Ppki and PicdA promoters to A. niger. Both promoters increased the expression of genes entailed in the
fermentation process and increased the titer of IA to 1.4 g L
−1
[43]. In 2017, Yin
et al. designed a low-pH-induced promoter Pgas that dynamically controls gene
expression in A. niger and produced IA at a titer of 5 g L
−1
[47]. This promoter
worked well even at very low pH (2.0), and the strength of the promoter was not
dependent on acid ion concentration or type of acid used.
2.2.2 IA Production by Bacteria
Escherichia coli Escherichia coli (E. coli) is one of the most versatile organisms
used as a host for the industrial synthesis of various chemicals [48]. E. coli strains
have relatively shorter cultivation times for IA production compared to fungi. It is a
stable bacterial strain that thrives under different growth conditions. Moreover, its
flexibility towards genetic manipulation is excellent. Because of these advantages
over fungi, several molecular cloning and genetic modification techniques have
been introduced for the synthesis of IA. The biosynthesis of IA from E. coli was first
achieved in 2011 by Li et al. [40]. They produced 0.08 g L
−1
IA in Luria-Bertani
(LB) medium with expression of cad gene under its inducible T7 promoter in culture of E. coli. The low activity of E. coli is associated with the fact that the key
enzyme that is encoded by the cad gene and catalyzes this transformation was primarily present in inclusion bodies. To increase the titers of IA, Vuoristo et al. engineered E. coli by employing citrate synthase and aconitase from Corynebacterium
glutamicum and by eliminating the genes encoding phosphate acetyltransferase and
lactate dehydrogenase [49]. However such modifications only managed to increase
IA production up to 0.69 g L
−1
. Following a similar approach, Okamoto et al.
observed that the inactivation of isocitrate dehydrogenase gene (icd) is beneficial
for IA production and can enhance the yield of IA. E. coli produced 4.34 g L
−1
I A
from glucose by expression of T7-inducible cad gene along with icd inactivation
after 105 h cultivation in LB medium [50]. The intracellular metabolism of E. coli
strain was further engineered by applying a synthetic protein scaffold that improved
metabolite synthesis with low expression levels of pathway enzymes [51]. The engineered strain produced 6.57 g L
−1
IA in basic media (pH 8.5) from glucose.
Metabolic modelling is a promising approach to confirm that the engineered
strains are capable to produce a desired product [52]. Harder et al. successfully
Bio-Catalytic Itaconic Acid and Bio-Based Vinyl Monomer Production Processes
yield in A. niger. The fermentation conditions of IA can be enhanced by using nitrogen supplementation that reduces IA-induced weak-acid stress.
Along with the editing of specific genes, a variety of promoters have also been
utilized to improve gene expression and increase the production of IA [10]. In 2011,
Steiger et al. developed six novel promoters for A. niger. They obtained a 0.6 g L
−1
titer of IA with the PmbfA promoter, while other promoters were not active for such
transformation [46]. Li et al. introduced gpdA promoter to control cadA gene expression. The recombinant strain A. niger produced a relatively low titer (0.6 g L
−1
) of
IA [40]. To achieve a higher IA titer, Blumhoff et al. applied Ppki and PicdA promoters to A. niger. Both promoters increased the expression of genes entailed in the
fermentation process and increased the titer of IA to 1.4 g L
−1
[43]. In 2017, Yin
et al. designed a low-pH-induced promoter Pgas that dynamically controls gene
expression in A. niger and produced IA at a titer of 5 g L
−1
[47]. This promoter
worked well even at very low pH (2.0), and the strength of the promoter was not
dependent on acid ion concentration or type of acid used.
2.2.2 IA Production by Bacteria
Escherichia coli Escherichia coli (E. coli) is one of the most versatile organisms
used as a host for the industrial synthesis of various chemicals [48]. E. coli strains
have relatively shorter cultivation times for IA production compared to fungi. It is a
stable bacterial strain that thrives under different growth conditions. Moreover, its
flexibility towards genetic manipulation is excellent. Because of these advantages
over fungi, several molecular cloning and genetic modification techniques have
been introduced for the synthesis of IA. The biosynthesis of IA from E. coli was first
achieved in 2011 by Li et al. [40]. They produced 0.08 g L
−1
IA in Luria-Bertani
(LB) medium with expression of cad gene under its inducible T7 promoter in culture of E. coli. The low activity of E. coli is associated with the fact that the key
enzyme that is encoded by the cad gene and catalyzes this transformation was primarily present in inclusion bodies. To increase the titers of IA, Vuoristo et al. engineered E. coli by employing citrate synthase and aconitase from Corynebacterium
glutamicum and by eliminating the genes encoding phosphate acetyltransferase and
lactate dehydrogenase [49]. However such modifications only managed to increase
IA production up to 0.69 g L
−1
. Following a similar approach, Okamoto et al.
observed that the inactivation of isocitrate dehydrogenase gene (icd) is beneficial
for IA production and can enhance the yield of IA. E. coli produced 4.34 g L
−1
I A
from glucose by expression of T7-inducible cad gene along with icd inactivation
after 105 h cultivation in LB medium [50]. The intracellular metabolism of E. coli
strain was further engineered by applying a synthetic protein scaffold that improved
metabolite synthesis with low expression levels of pathway enzymes [51]. The engineered strain produced 6.57 g L
−1
IA in basic media (pH 8.5) from glucose.
Metabolic modelling is a promising approach to confirm that the engineered
strains are capable to produce a desired product [52]. Harder et al. successfully
Bio-Catalytic Itaconic Acid and Bio-Based Vinyl Monomer Production Processes