Combinatorial engineering of betalain biosynthesis pathway in yeast Saccharomyces cerevisiae

Biotechnological production and emerging applications of betalains: A review

Betalains are food-grade hydrophilic pigments with antioxidant and biological activities, predominantly found in plants. Betanin is a red-violet betalain synthesized from tyrosine through L-DOPA formation, its subsequent aromatic ring-opening, spontaneous cyclization to betalamic acid, and then pH-dependent condensation with i) cyclo-DOPA-5-O-glucoside or ii) cyclo-DOPA followed by 5-O-glucosylation. This short pathway in plants for betanin biosynthesis has been heterologously expressed in other organisms (e.g. non-betalainic plants, yeasts, and fungi) using CYP76AD1, DOD1, and cDOPA5GT or B5GT, corresponding to the enzymatic steps mentioned above. For the red-violet color formation through heterologous expression of the pathway genes in non-betalainic plants, a simplified reporter gene called RUBY has been developed recently. Without any systems engineering, expression of RUBY in non-betalainic plants resulted in accumulation of up to 203 mg betalains/100 g fresh weight of peanut leaves. In yeasts, Saccharomyces cerevisiae and Yarrowia lipolytica, and fungus Fusarium venenatum, betanin production has been achieved through overexpression of the pathway genes, with productivity reaching up to 0.62 mg/L/h, 26 mg/L/h, and 26.4 mg/L/h from d-glucose as carbon source, respectively, after considerable systems engineering and gene copy number augmentation. This review critically analyzes recent biotechnological production of betalains to highlight the advancements and strategies for improvement in the technology. Also, emerging applications of betalain biosynthetic gene products or betalains as biosensors, fluorescent probes, meat analog colors, and others are discussed to strengthen the need for systems engineering and process optimization for large-scale industrial production of these pigments.

Screening of Plant UDP-Glycosyltransferases for Betanin Production in Yeast

To cover the rising demand for natural food dyes, new sources and production methods are needed. Microbial fermentation of nature-identical colours, such as the red pigment betanin, has the potential to be a cost-efficient alternative to plant extraction. The last step of betanin production is catalysed by a UDP-glycosyltransferase (UGT). To find a high-performing UGT, we screened 27 UGTs from different plant species and tested their ability to produce betanin in vivo in Saccharomyces cerevisiae. We identified two new UGTs likely involved in the betanin synthesis in the plant they derive from: CqGT2 (UGT73A37) from Chenopodium quinoa and BgGT2 (UGT92X1) from Bougainvillea glabra. The betanin-producing UGTs were also tested in Yarrowia lipolytica, where CqGT2 was the best-performing glycosyltransferase for betanin production. While it has previously been shown that the UGTs can glycosylate either betanidin or cyclo-DOPA to ultimately form betanin, the molecular mechanism behind the preference for the acceptor molecule has not been elucidated. Therefore, we performed in silico structural analysis to characterise the betanin-producing UGTs further, particularly by looking into their binding mechanism. The docking model suggested that a smaller binding site found in some UGTs only allows glycosylation of cDOPA, while a wider binding site allows glycosylation of both cyclo-DOPA and betanidin.

Natural pigments derived from plants and microorganisms: classification, biosynthesis, and applications

Pigments, as coloured secondary metabolites, endow the world with a rich palette of colours. They primarily originate from plants and microorganisms and play crucial roles in their survival and adaptation processes. In this article, we categorize pigments based on their chemical structure into flavonoids, carotenoids, pyrroles, quinones, azaphilones, melanins, betalains, flavins, and others. We further meticulously describe the colours, sources, and biosynthetic pathways, including key enzymatic steps and regulatory networks that control pigment production, in both plants and microorganisms. In particular, we highlight the role of transport proteins and transcription factors in fine-tuning these pathways. Finally, we introduce the use of pigments in practical production and research, aiming to provide new insights and directions for the application of coloured compounds in diverse fields, such as agriculture, industry, and medicine.

BAHD acyltransferase from dragon fruit enables production of phyllocactin in engineered yeast

Microbial fermentation can provide a sustainable and cost-effective alternative to traditional plant extraction to produce natural food colours. Betalains are a class of yellow to red water-soluble pigments. Even though over 80 betalain variants are known, betanin is the only betalain available as a food colourant on the market. Many variants are acylated, which can enhance their stability and change the hue, but very few acyltransferases responsible for the acylation are known. Therefore, we mined the transcriptomes of Celosia argentea var. cristata and Hylocereus polyrhizus for BAHD acyltransferases, enzymes likely involved in betalain acylation. In vivo screening of the enzymes in betanin-producing Saccharomyces cerevisiae revealed that the acyltransferase HpBAHD3 from H. polyrhizus malonylates betanin, forming phyllocactin (6'-O-malonyl-betanin). This is the first identification of a BAHD acyltransferase involved in betalain biosynthesis. Expression of HpBAHD3 in a Yarrowia lipolytica strain engineered for high betanin production led to near-complete conversion of betanin to phyllocactin. In fed-batch fermentation, the strain produced 1.95 ± 0.024 g/l phyllocactin in 60 h. This study expands the range of natural food colourants produced through microbial fermentation and contributes to elucidating the biosynthesis pathway of acylated betalains.

Identification of R2R3-MYB Transcription Factor Family Based on Amaranthus tricolor Genome and AtrMYB72 Promoting Betalain Biosynthesis by Directly Activating AtrCYP76AD1 Expression

MYB (myeloblastosis) is one of the most abundant transcription factors in plants which regulates various biological processes. The molecular characteristics and function of R2R3-MYB transcription factors in amaranth remain unclear. In this study, 73 R2R3-MYB members were identified from the amaranth genome database and we further analyzed their chromosome position, conserved motifs, physiological and biochemical features, collinearity relationships, gene structure, phylogeny and cis-acting element. Based on the phylogenetic and expression pattern analysis, 14 candidate R2R3-MYB genes might be involved in the betalain synthesis. Amongst the 14 candidate R2R3-MYB genes, the expression level of AtrMYB72 was higher in 'Suxian No.1' than 'Suxian No.2', and also higher in the red section than in the green section of the same leaf in Amaranthus. The overexpression vector pCambia1301-AtrMYB72-GUS and VIGS (virus-induced gene silencing) vector pTRV2- AtrMYB72 were transferred into leaves of 'Suxian No.1' via an Agrobacterium-mediated method. The results showed that AtrMYB72 overexpression could promote betalain synthesis. A yeast one-hybrid assay and dual luciferase reporter gene assay demonstrated that AtrMYB72 could bind to the AtrCYP76AD1 promoter to promote betalain synthesis. These results indicated that AtrMYB72 promoted betalain biosynthesis in amaranth by activating the AtrCYP76AD1 transcription. Our results could provide new insights into the betalain biosynthesis in amaranth.

High-efficiency production of plant-derived pigment dopaxanthin in Escherichia coli by combination engineering

BACKGROUND

Dopaxanthin is a natural pigment betaxanthins family member with the highest antioxidant and free radical scavenging activities. However, its relatively low content in plants limited the wide range of applications. Cost-efficient microbial production, therefore, showed an attractive alternative.

RESULTS

Here, an Escherichia coli strain equipped with the de novo biosynthetic pathway for hyperproducing dopaxanthin was constructed by combining metabolic engineering and protein engineering. Firstly, a high-performance rate-limiting levodopa 4,5-dioxygenase (DODA) was mined and characterized based on sequence similarity searching followed by whole-cell catalysis and de novo synthesis strategy. Then, the catalytic efficiency of DODA was increased 34 times with directed evolution. The mutated DODA significantly facilitated the production of dopaxanthin, with an increase of 40.17% in plasmid expression and 64.11% in genome expression, respectively. Finally, through connecting the blocked pathway from 3-dehydroshikimate to levodopa (L-DOPAOPA) and enhancing the expression level of DODA, a titer of dopaxanthin of 22.87 g/L was achieved from glucose as feedstock, which is 286 times higher than that in the previous report.

CONCLUSION

This work not only established a promising platform for the environmentally friendly production of dopaxanthin but also laid a foundation for the commercialization of other betalain.

Biotechnological approaches for producing natural pigments in yeasts

Pigments are widely used in the food, cosmetic, textile, pharmaceutical, and materials industries. Demand for natural pigments has been increasing due to concerns regarding potential health problems and environmental pollution from synthetic pigments. Microbial production of natural pigments is a promising alternative to chemical synthesis or extraction from natural sources. Here, we discuss yeasts as promising chassis for producing natural pigments with their advantageous traits such as genetic amenability, safety, rapid growth, metabolic diversity, and tolerance. Metabolic engineering strategies and optimizing strategies in downstream process to enhance production of natural pigments are thoroughly reviewed. We discuss the challenges, including expanding the range of natural pigments and improving their feasibility of industrial scale-up, as well as the potential strategies for future development.

Synthesis, characterization and application of microbial pigments in foods as natural colors

Colorants have played a crucial role in various applications, particularly in food processing, with natural sources such as mineral ores, plants, insects, and animals being commonly used. However, the nineteenth century saw the development of synthetic dyes, which replaced these natural colorants. In recent years, there has been a growing demand for natural products, driving an increased interest in natural colorants. Microbial pigments have emerged as promising sources of natural pigments due to their numerous health benefits. They can be produced in large quantities rapidly and from more affordable substrates, making them economically attractive. This review focuses on the current advancements in the low-cost synthesis of microbial pigments, exploring their biological activities and commercial applications. Microbial pigments offer a sustainable and economically viable alternative to natural and synthetic colorants, meeting the growing demand for natural products. These pigments are relatively nontoxic and exhibit significant health benefits, making them suitable for a wide range of applications. As interest in natural products continues to rise, microbial pigments hold great potential in shaping the future of colorant production across various sectors.

Combinatorial iterative method for metabolic engineering of Yarrowia lipolytica: Application for betanin biosynthesis

Combinatorial library-based metabolic engineering approaches allow lower cost and faster strain development. We developed a genetic toolbox EXPRESSYALI for combinatorial engineering of the oleaginous yeast Yarrowia lipolytica. The toolbox enables consecutive rounds of engineering, where up to three combinatorially assembled gene expression cassettes can be integrated into each yeast clone per round. The cassettes are integrated into distinct intergenic sites or an open reading frame of a target gene if a simultaneous gene knockout is desired. We demonstrate the application of the toolbox by optimizing the Y. lipolytica to produce the red beet color betanin via six consecutive rounds of genome editing and screening. The library size varied between 24 and 360. Library screening was facilitated by automated color-based colony picking. In the first round, betanin pathway genes were integrated, resulting in betanin titer of around 20 mg/L. Through the following five consecutive rounds, additional biosynthetic genes were integrated, and the precursor supply was optimized, resulting in a titer of 70 mg/L. Three beta-glucosidases were deleted to prevent betanin deglycosylation, which led to a betanin titer of 130 mg/L in a small scale and a titer of 1.4 g/L in fed-batch bioreactors. The EXPRESSYALI toolbox can facilitate metabolic engineering efforts in Y. lipolytica (available via AddGene Cat. Nr. 212682-212704, Addgene kit ID # 1000000245).

Synchronous Bioproduction of Betanin and Mycoprotein in the Engineered Edible Fungus Fusarium venenatum

Sustainable production of edible microbial proteins and red food colorants is an important demand for future food. Therefore, creation of a chassis strain that can efficiently synthesize both products is extremely necessary and meaningful. To realize this envision, a CRISPR/Cas9-based visual multicopy integration system was successfully developed in Fusarium venenatum. Subsequently, the de novo synthesis of the red food colorant betanin was achieved in the engineered F. venenatum using the above system. After fermentation optimization, the final yields of betanin and mycoprotein reached 1.91 and 9.53 g/L, respectively, when the constant pH naturally decreased from 6 to 4 without the addition of acid after 48 h of fermentation. These results determine a highly suitable chassis strain for the microbial biomanufacturing of betanin, and the obtained engineered strain here is expected to expand the application prospect and improve economic returns of F. venenatum in the field of future food.

Recent advances in engineering microorganisms for the production of natural food colorants

Food colorants are frequently added to processed foods since color is an important tool in the marketing of food products, influencing consumer perceptions, preferences, and purchasing behavior. While synthetic dyes currently dominate the food colorant market, consumer concern regarding their safety and sustainability is driving a demand for their replacement with naturally derived alternatives. However, natural colorants are costly compared to their synthetic counterparts as the pigment content in the native sources is usually very low and extraction can be challenging. Recent advances in the engineering of microbial metabolism have sparked interest in the development of cell factories capable of producing natural colorants from renewable resources. This review summarizes major developments within metabolic engineering for the production of nature-identical food colorants by fermentation. Additionally, it highlights common applications, formulations, and physicochemical characteristics of prevalent pigment classes. Lastly, it outlines a workflow for accelerating the optimization of cell factories for the production or derivatization of nature-identical food colorants.

Enzyme and Pathway Engineering for Improved Betanin Production in Saccharomyces cerevisiae

Betanin is a water-soluble red-violet pigment belonging to the betacyanins family. It has become more and more attractive for its natural food colorant properties and health benefits. However, the commercial production of betanin, typically extracted from red beetroot, faces economic and sustainability challenges. Microbial heterologous production therefore offers a promising alternative. Here, we performed combinatorial engineering of plant P450 enzymes and precursor metabolisms to improve the de novo production of betanin in Saccharomyces cerevisiae. Semirational design by computer simulation and molecular docking was used to improve the catalytic activity of CYP76AD. Alanine substitution and site-directed saturation mutants were screened, with a combination mutant showing an approximately 7-fold increase in betanin titer compared to the wild type. Subsequently, betanin production was improved by enhancing the l-tyrosine pathway flux and UDP-glucose supply. Finally, after optimization of the fermentation process, the engineered strain BEW10 produced 134.1 mg/L of betanin from sucrose, achieving the highest reported titer of betanin in a shake flask by microbes. This work shows the P450 enzyme and metabolic engineering strategies for the efficient microbial production of natural complex products.

Emerging trends in production of plant natural products and new-to-nature biopharmaceuticals in yeast

Natural products represent an inestimable source of valuable compounds for human health. Notably, those produced by plants remain challenging to access due to their low production. Potential shortages of plant-derived biopharmaceuticals caused by climate change or pandemics also regularly tense the market trends. Thus, biotechnological alternatives of supply based on synthetic biology have emerged. These innovative strategies mostly rely on the use of engineered microbial systems for compound synthesis. In this regard, yeasts remain the easiest-tractable eukaryotic models and a convenient chassis for reconstructing whole biosynthetic routes for the heterologous production of plant-derived metabolites. Here, we highlight the recent discoveries dedicated to the bioproduction of new-to-nature compounds in yeasts and provide an overview of emerging strategies for optimising bioproduction.

Beet red food colourant can be produced more sustainably with engineered Yarrowia lipolytica

Synthetic food colourants are widely used in the food industry, but consumer concerns about safety and sustainability are driving a need for natural food-colour alternatives. Betanin, which is extracted from red beetroots, is a commonly used natural red food colour. However, the betanin content of beetroot is very low (~0.2% wet weight), which means that the extraction of betanin is incredibly wasteful in terms of land use, processing costs and vegetable waste. Here we developed a sustainability-driven biotechnological process for producing red beet betalains, namely, betanin and its isomer isobetanin, by engineering the oleaginous yeast Yarrowia lipolytica. Metabolic engineering and fermentation optimization enabled production of 1,271 ± 141 mg l-1 betanin and 55 ± 7 mg l-1 isobetanin in 51 h using glucose as carbon source in controlled fed-batch fermentations. According to a life cycle assessment, at industrial scale (550 t yr-1), our fermentation process would require significantly less land, energy and resources compared with the traditional extraction of betanin from beetroot crops. Finally, we apply techno-economic assessment to show that betanin production by fermentation could be economically feasible in the existing market conditions.