Engineering the production of major catechins by Escherichia coli carrying metabolite genes of Camellia sinensis

Advances in biochemistry and the biotechnological production of taxifolin and its derivatives

Taxifolin (dihydroquercetin) and its derivatives are medicinally important flavanonols with a wide distribution in plants. These compounds have been isolated from various plants, such as milk thistle, onions, french maritime, and tamarind. In general, they are commercially generated in semisynthetic forms. Taxifolin and related compounds are biosynthesized via the phenylpropanoid pathway, and most of the biosynthetic steps have been functionally characterized. The knowledge gained through the detailed investigation of their biosynthesis has provided the foundation for the reconstruction of biosynthetic pathways. Plant- and microbial-based platforms are utilized for the expression of such pathways for generating taxifolin-related compounds, either by whole-cell biotransformation or through reconfiguration of the genetic circuits. In this review, we summarize recent advances in the biotechnological production of taxifolin and its derivatives.

Metabolic Flux Analysis of Catechin Biosynthesis Pathways Using Nanosensor

(+)-Catechin is an important antioxidant of green tea (Camelia sinensis (L.) O. Kuntze). Catechin is known for its positive role in anticancerous activity, extracellular matrix degradation, cell death regulation, diabetes, and other related disorders. As a result of enormous interest in and great demand for catechin, its biosynthesis using metabolic engineering has become the subject of concentrated research with the aim of enhancing (+)-catechin production. Metabolic flux is an essential concept in the practice of metabolic engineering as it helps in the identification of the regulatory element of a biosynthetic pathway. In the present study, an attempt was made to analyze the metabolic flux of the (+)-catechin biosynthesis pathway in order to decipher the regulatory element of this pathway. Firstly, a genetically encoded fluorescence resonance energy transfer (FRET)-based nanosensor (FLIP-Cat, fluorescence indicator protein for (+)-catechin) was developed for real-time monitoring of (+)-catechin flux. In vitro characterization of the purified protein of the nanosensor showed that the nanosensor was pH stable and (+)-catechin specific. Its calculated K_d was 139 µM. The nanosensor also performed real-time monitoring of (+)-catechin in bacterial cells. In the second step of this study, an entire (+)-catechin biosynthesis pathway was constructed and expressed in E. coli in two sets of plasmid constructs: pET26b-PT7-rbs-PAL-PT7-rbs-4CL-PT7-rbs-CHS-PT7-rbs-CHI and pET26b-T7-rbs-F3H-PT7-rbs- DFR-PT7-rbs-LCR. The E. coli harboring the FLIP-Cat was transformed with these plasmid constructs. The metabolic flux analysis of (+)-catechin was carried out using the FLIP-Cat. The FLIP-Cat successfully monitored the flux of catechin after adding tyrosine, 4-coumaric acid, 4-coumaroyl CoA, naringenin chalcone, naringenin, dihydroquercetin, and leucocyanidin, individually, with the bacterial cells expressing the nanosensor as well as the genes of the (+)-catechin biosynthesis pathway. Dihydroflavonol reductase (DFR) was identified as the main regulatory element of the (+)-catechin biosynthesis pathway. Information about this regulatory element of the (+)-catechin biosynthesis pathway can be used for manipulating the (+)-catechin biosynthesis pathway using a metabolic engineering approach to enhance production of (+)-catechin.

Recent advancement of engineering microbial hosts for the biotechnological production of flavonoids

Flavonoids are polyphenols that are important organic chemicals in plants. The health benefits of flavonoids that result in high commercial values make them attractive targets for large-scale production through bioengineering. Strategies such as engineering a flavonoid biosynthetic pathway in microbial hosts provide an alternative way to produce these beneficial compounds. Escherichia coli, Saccharomyces cerevisiae and Streptomyces sp. are among the expression systems used to produce recombinant products, as well as for the production of flavonoid compounds through various bioengineering approaches including clustered regularly interspaced short palindromic repeats (CRISPR)-based genome engineering and genetically encoded biosensors to detect flavonoid biosynthesis. In this study, we review the recent advances in engineering model microbial hosts as being the factory to produce targeted flavonoid compounds.

Recent Advances in the Recombinant Biosynthesis of Polyphenols

Plants are the source of various natural compounds with pharmaceutical and nutraceutical importance which have shown numerous health benefits with relatively fewer side effects. However, extraction of these compounds from native producers cannot meet the ever-increasing demands of the growing population due to, among other things, the limited production of the active compound(s). Their production depends upon the metabolic demands of the plant and is also subjected to environmental conditions, abundance of crop species and seasonal variations. Moreover, their extraction from plants requires complex downstream processing and can also lead to the extinction of many useful plant varieties. Microbial engineering is one of the alternative approaches which can meet the global demand for natural products in an eco-friendly manner. Metabolic engineering of microbes or pathway reconstruction using synthetic biology tools and novel enzymes lead to the generation of a diversity of compounds (like flavonoids, stilbenes, anthocyanins etc.) and their natural and non-natural derivatives. Strain and pathway optimization, pathway regulation and tolerance engineering have produced microbial cell factories into which the metabolic pathway of plants can be introduced for the production of compounds of interest on an industrial scale in an economical and eco-friendly way. While microbial production of phytochemicals needs to further increase product titer if it is ever to become a commercial success. The present review covers the advancements made for the improvement of microbial cell factories in order to increase the product titer of recombinant polyphenolic compounds.

Pyramiding of tea Dihydroflavonol reductase and Anthocyanidin reductase increases flavan-3-ols and improves protective ability under stress conditions in tobacco

Tea (Camellia sinensis) is one of the richest sources of flavan-3-ols, an important class of flavonoids. The expression level of gene-encoded key regulatory enzymes of flavan-3-ol/anthocyanin biosynthetic pathway, dihydroflavonol 4-reductase (DFR) and anthocyanidin reductase (ANR), has been highly correlated with the flavan-3-ol contents and antioxidant activity in tea plant. In the present study, pyramiding of CsDFR and CsANR in tobacco was achieved. However, single transgenic tobacco overexpressing either CsDFR or CsANR was documented earlier. In continuation, pyramided transgenic lines were evaluated for the possible, either same or beyond, effect on flavan-3-ol accumulation and protective ability against biotic and abiotic stresses. The pyramided transgenic lines showed early flowering and improved seed yield. The transcript levels of flavan-3-ol/anthocyanin biosynthetic pathway and related genes in pyramided transgenic lines were upregulated as compared to control tobacco plants. The accumulations of flavan-3-ols were also found to be higher in pyramided transgenic lines than control tobacco plants. In contrast, anthocyanin content was observed to be decreased in pyramided transgenic lines, while DPPH activity was higher in pyramided transgenic lines. In pyramided transgenic lines, strong protective ability against feeding by Spodoptera litura was documented. The seeds of pyramided transgenic lines were also found to have better germination rate under aluminum toxicity as compared to control tobacco plants. Interestingly, the synergistic effect of these two selected genes are not beyond from transgenic lines expressing either CsDFR and CsANR alone as published earlier in terms of flavan-3-ols accumulation. However, the unique flower color and better seed germination rate are some interestingly comparable differences that were reported in pyramided lines in relation to individual transgenic plants. In conclusion, the present results reveal an interesting dynamic between CsDFR and CsANR in modulating flavan-3-ol/anthocyanin levels and functional analysis of stacked CsDFR and CsANR transgenic tobacco lines.

Cs-miR156 is involved in the nitrogen form regulation of catechins accumulation in tea plant (Camellia sinensis L.)

The nitrogen source affects the growth of tea plants and regulates the accumulation of catechins in the leaves. In this report, we assessed the influences of NH4(+) and NO3(-) on plant growth, catechins accumulation and associated gene expression. Compared with the preferential nitrogen source NH4(+), when NO3(-) was supplied as the sole nitrogen source, tea plants showed similar symptoms with the nitrogen-free treatments and showed lower nitrogen, free amino acid accumulation, chlorophyll content and biomass gain, indicating NO3(-) was not efficiently used by these plants. However, the total shoot catechins content was significantly higher for NO3(-) treatments than that for NH4(+) treatment or combined NH4(+)+NO3(-) treatment, suggesting that, in addition to its influence on plant growth, the nitrogen form regulated the accumulation of catechins in tea. The expression of catechins biosynthesis-related genes was associated with the regulation of catechins accumulation and composition changes mediated by nitrogen form. PAL, CHS, CHI, and DFR genes exhibited higher expression levels in plants supplied with NO3(-), in which the transcript level of DFR in the shoots was significantly correlated with the catechins content. In the end, we identified a new function for the Cs-miR156, which was drastically induced through NH4(+). Moreover, a potential mechanism of the Cs-miR156 pathway in regulating catechins biosynthesis in tea plants has been suggested, with particular respect to nitrogen forms. Cs-miR156 might repress the expression of the target gene SPL to regulate the DFR gene, which plays a vital role in catechins biosynthesis.

Improvement of catechin production in Escherichia coli through combinatorial metabolic engineering

Reconstruction of highly efficient biosynthesis pathways is essential for the production of valuable plant secondary metabolites in recombinant microorganisms. In order to improve the titer of green tea catechins in Escherichia coli, combinatorial strategies were employed using the ePathBrick vectors to express the committed catechin pathway: flavanone 3β-hydroxylase (F3H), dihydroflavonol 4-reductase (DFR), and leucoanthocyanidin reductase (LAR). Three F3H, three DFR, and two LAR genes originating from different plant species were selected and synthesized, to create 18 pathway variants to be screened in E. coli. Constructs containing F3H(syn) originally from Camellia sinensis, DFR(syn) from Anthurium andraeanum, C. sinensis, or Fragaria ananass, and LAR(syn) from Desmodium uncinatum (p148, p158 and p168) demonstrated high conversion efficiency with either eriodictyol or naringenin as substrate. A highly efficient construct was created by assembling additional copies of DFR(syn) and LAR(syn) enabling a titer of 374.6 ± 43.6 mg/L of (+)-catechin. Improving the NADPH availability via the ΔpgiΔppc mutation, BLΔpgiΔppc-p148 produced the highest titer of catechin at 760.9 ± 84.3 mg/L. After utilizing a library of scaffolding proteins, the strain BLΔpgiΔppc-p168-759 reached the highest titer of (+)-catechin of 910.9 ± 61.3 mg/L from 1.0 g/L of eriodictyol in batch culture with M9 minimal media. The impact of oxygen availability on the biosynthesis of catechin was also investigated.