Metabolic Engineering of Saccharomyces cerevisiae for Production of Canthaxanthin, Zeaxanthin, and Astaxanthin.

The sustainable production of natural compounds is increasingly important in today's industrial landscape. This study investigates the metabolic engineering of Saccharomyces cerevisiae for the efficient biosynthesis of valuable carotenoids: canthaxanthin, zeaxanthin, and astaxanthin. Utilizing a tailored parental yeast strain, Sp_Bc, we optimized the carotenoid pathway by screening and identifying CrtW and CrtZ enzymatic variants. The CrtW variant from Bradyrhizobium sp. achieved a canthaxanthin titer of 425.1 ± 69.1 µg/L, while the CrtZ variant from Pantoea ananatis achieved a zeaxanthin titer of 70.5 ± 10.8 µg/L. Additionally, we optimized carotenoid production by exploring enzyme fusion strategies for all three studied carotenoids and organelle compartmentalization specifically for enhancing astaxanthin synthesis. We further improved carotenoid production by integrating the optimal gene constructs into the yeast genome and deleting the GAL80 gene, enabling the use of sucrose as a carbon source. The engineered strain Sp_Bc-Can001 ∆gal80 was evaluated in a 5 L bioreactor fermentation, achieving a notable canthaxanthin titer of 60.36 ± 1.51 mg/L using sucrose. This research conclusively establishes S. cerevisiae as a viable platform for efficient carotenoid biosynthesis and, for the first time in this yeast system, illustrates sucrose's viability as a carbon source for canthaxanthin production. These findings pave the way for sustainable, cost-effective carotenoid production at an industrial scale.

Efficient ß-carotene production in engineered Saccharomyces cerevisiae using simple sugars and agricultural waste-based carbon and nitrogen sources.

ß-carotene, a precursor to vitamin A, holds significant promise for health and nutrition applications. This study introduces an optimized approach for ß-carotene production in Saccharomyces cerevisiae, leveraging metabolic engineering and a novel use of agricultural waste. The GAL80 gene deletion facilitated efficient ß-carotene synthesis from sucrose, avoiding the costly galactose induction, and achieved titers up to 727.8 ±â€¯68.0 mg/L with content levels of 71.8 ±â€¯0.4 mg/g dry cell weight (DCW). Furthermore, the application of agricultural by-products, specifically molasses and fish meal as carbon and nitrogen sources, was investigated. This approach yielded a substantial ß-carotene titer of 354.9 ±â€¯8.2 mg/L and a content of 60.5 ±â€¯4.3 mg/g DCW, showcasing the potential of these sustainable substrates for industrial-scale production. This study sets a new benchmark for cost-effective, green manufacturing of vital nutrients, demonstrating a scalable, eco-friendly alternative for ß-carotene production.

Increased production of isobutanol from xylose through metabolic engineering of Saccharomyces cerevisiae overexpressing transcription factor Znf1 and exogenous genes.

Only trace amount of isobutanol is produced by the native Saccharomyces cerevisiae via degradation of amino acids. Despite several attempts using engineered yeast strains expressing exogenous genes, catabolite repression of glucose must be maintained together with high activity of downstream enzymes, involving iron-sulfur assimilation and isobutanol production. Here, we examined novel roles of nonfermentable carbon transcription factor Znf1 in isobutanol production during xylose utilization. RNA-seq analysis showed that Znf1 activates genes in valine biosynthesis, Ehrlich pathway and iron-sulfur assimilation while coupled deletion or downregulated expression of BUD21 further increased isobutanol biosynthesis from xylose. Overexpression of ZNF1 and xylose-reductase/dehydrogenase (XR-XDH) variants, a xylose-specific sugar transporter, xylulokinase, and enzymes of isobutanol pathway in the engineered S. cerevisiae pho13gre3Δ strain resulted in the superb ZNXISO strain, capable of producing high levels of isobutanol from xylose. The isobutanol titer of 14.809 ± 0.400 g/L was achieved, following addition of 0.05 g/L FeSO4.7H2O in 5 L bioreactor. It corresponded to 155.88 mg/g xylose consumed and + 264.75% improvement in isobutanol yield. This work highlights a new regulatory control of alternative carbon sources by Znf1 on various metabolic pathways. Importantly, we provide a foundational step toward more sustainable production of advanced biofuels from the second most abundant carbon source xylose.

Development of a Novel D-Lactic Acid Production Platform Based on Lactobacillus saerimneri TBRC 5746.

D-Lactic acid is a chiral, three-carbon organic acid, that bolsters the thermostability of polylactic acid. In this study, we developed a microbial production platform for the high-titer production of D-lactic acid. We screened 600 isolates of lactic acid bacteria (LAB) and identified twelve strains that exclusively produced D-lactic acid in high titers. Of these strains, Lactobacillus saerimneri TBRC 5746 was selected for further development because of its homofermentative metabolism. We investigated the effects of high temperature and the use of cheap, renewable carbon sources on lactic acid production and observed a titer of 99.4 g/L and a yield of 0.90 g/g glucose (90% of the theoretical yield). However, we also observed L-lactic acid production, which reduced the product's optical purity. We then used CRISPR/dCas9-assisted transcriptional repression to repress the two Lldh genes in the genome of L. saerimneri TBRC 5746, resulting in a 38% increase in D-lactic acid production and an improvement in optical purity. This is the first demonstration of CRISPR/dCas9-assisted transcriptional repression in this microbial host and represents progress toward efficient microbial production of D-lactic acid.

Engineered Production of Isobutanol from Sugarcane Trash Hydrolysates in Pichia pastoris.

Concerns over climate change have led to increased interest in renewable fuels in recent years. Microbial production of advanced fuels from renewable and readily available carbon sources has emerged as an attractive alternative to the traditional production of transportation fuels. Here, we engineered the yeast Pichia pastoris, an industrial powerhouse in heterologous enzyme production, to produce the advanced biofuel isobutanol from sugarcane trash hydrolysates. Our strategy involved overexpressing a heterologous xylose isomerase and the endogenous xylulokinase to enable the yeast to consume both C5 and C6 sugars in biomass. To enable the yeast to produce isobutanol, we then overexpressed the endogenous amino acid biosynthetic pathway and the 2-keto acid degradation pathway. The engineered strains produced isobutanol at a titer of up to 48.2 ± 1.7 mg/L directly from a minimal medium containing sugarcane trash hydrolysates as the sole carbon source. To our knowledge, this is the first demonstration of advanced biofuel production using agricultural waste-derived hydrolysates in the yeast P. pastoris. We envision that our work will pave the way for a scalable route to this advanced biofuel and further establish P. pastoris as a versatile production platform for fuels and high-value chemicals.

Evaluation of Methylotrophic Yeast Ogataea thermomethanolica TBRC 656 as a Heterologous Host for Production of an Animal Vaccine Candidate.

Multiple yeast strains have been developed into versatile heterologous protein expression platforms. Earlier works showed that Ogataea thermomethanolica TBRC 656 (OT), a thermotolerant methylotrophic yeast, can efficiently produce several industrial enzymes. In this work, we demonstrated the potential of this platform for biopharmaceutical manufacturing. Using a swine vaccine candidate as a model, we showed that OT can be optimized to express and secrete the antigen based on porcine circovirus type 2d capsid protein at a respectable yield. Crucial steps for yield improvement include codon optimization and reduction of OT protease activities. The antigen produced in this system could be purified efficiently and induce robust antibody response in test animals. Improvements in this platform, especially more efficient secretion and reduced extracellular proteases, would extend its potential as a competitive platform for biopharmaceutical industries.

Improvement in D-xylose utilization and isobutanol production in S. cerevisiae by adaptive laboratory evolution and rational engineering.

As the effects of climate change become apparent, metabolic engineers and synthetic biologists are exploring sustainable sources for transportation fuels. The design and engineering of microorganisms to produce gasoline, diesel, and jet fuel compounds from renewable feedstocks can significantly reduce our dependence on fossil fuels as well as lower the emissions of greenhouse gases. Over the past 2 decades, a considerable amount of work has led to the development of microbial strains for the production of advanced fuel compounds from both C5 and C6 sugars. In this work, we combined two strategies-adaptive laboratory evolution and rational metabolic engineering-to improve the yeast Saccharomyces cerevisiae's ability to utilize D-xylose, a major C5 sugar in biomass, and produce the advanced biofuel isobutanol. Whole genome resequencing of several evolved strains followed by reverse engineering identified two single nucleotide mutations, one in CCR4 and another in TIF1, that improved the yeast's specific growth rate by 23% and 14%, respectively. Neither one of these genes has previously been implicated to play a role in utilization of D-xylose. Fine-tuning the expression levels of the bottleneck enzymes in the isobutanol pathway further improved the evolved strain's isobutanol titer to 92.9 ± 4.4 mg/L (specific isobutanol production of 50.2 ± 2.6 mg/g DCW), a 90% improvement in titer and a 110% improvement in specific production over the non-evolved strain. We hope that our work will set the stage for an economic route to the advanced biofuel isobutanol and enable efficient utilization of xylose-containing biomass.

Mating-type switching and mating-type gene array expression in the methylotrophic yeast Ogataea thermomethanolica TBRC656.

The methylotrophic yeast, Ogataea thermomethanolica TBRC656, is an attractive host organism for heterologous protein production owing to the availability of protein expression vectors and a genome-editing tool. In this study, we focused on mating-type switching and gene expression in order to elucidate its sexual life cycle and establish genetic approaches applicable for the strain. A putative mating-type gene cluster was identified in TBRC656 that is syntenic to the cluster in Ogataea parapolymorpha DL-1 (previously named Hansenula polymorpha). Like DL-1, TBRC656 possesses two mating loci, namely MATa and MATα, and also shows flip-flop mating-type switching. Interestingly, unlike any other methylotrophic yeast, TBRC656 robustly switched mating type during late growth in rich medium (YPD). Under nutrient depletion, mating-type switching was observed within one hour. Transcription from both MATa and MATα mating loci was detected during growth in YPD, and possibly induced upon nitrogen depletion. Gene expression from MATα was detected as a single co-transcript from a three-gene array (α2-α1-a1S). Deletion of a putative a1S ORF at the MATα locus had no observed effect on mating-type switching but demonstrated significant effect on mating-type gene expression at both MATa and MATα loci.

Systematic improvement of isobutanol production from D-xylose in engineered Saccharomyces cerevisiae.

As the importance of reducing carbon emissions as a means to limit the serious effects of global climate change becomes apparent, synthetic biologists and metabolic engineers are looking to develop renewable sources for transportation fuels and petroleum-derived chemicals. In recent years, microbial production of high-energy fuels has emerged as an attractive alternative to the traditional production of transportation fuels. In particular, the Baker's yeast Saccharomyces cerevisiae, a highly versatile microbial chassis, has been engineered to produce a wide array of biofuels. Nevertheless, a key limitation of S. cerevisiae is its inability to utilize xylose, the second most abundant sugar in lignocellulosic biomass, for both growth and chemical production. Therefore, the development of a robust S. cerevisiae strain that is able to use xylose is of great importance. Here, we engineered S. cerevisiae to efficiently utilize xylose as a carbon source and produce the advanced biofuel isobutanol. Specifically, we screened xylose reductase (XR) and xylose dehydrogenase (XDH) variants from different xylose-metabolizing yeast strains to identify the XR-XDH combination with the highest activity. Overexpression of the selected XR-XDH variants, a xylose-specific sugar transporter, xylulokinase, and isobutanol pathway enzymes in conjunction with the deletions of PHO13 and GRE3 resulted in an engineered strain that is capable of producing isobutanol at a titer of 48.4 ± 2.0 mg/L (yield of 7.0 mg/g D-xylose). This is a 36-fold increase from the previous report by Brat and Boles and, to our knowledge, is the highest isobutanol yield from D-xylose in a microbial system. We hope that our work will set the stage for an economic route for the production of advanced biofuel isobutanol and enable efficient utilization of lignocellulosic biomass.

Methanol-inducible promoter of thermotolerant methylotrophic yeast Ogataea thermomethanolica BCC16875 potential for production of heterologous protein at high temperatures.

Methanol-utilizing metabolism is generally found in methylotrophic yeasts. Several potential promoters regulating enzymes in this pathway have been extensively studied, especially alcohol oxidase. Here, we characterized the alcohol oxidase gene promoter from thermotolerant Ogataea thermomethanolica (OthAOX). This promoter can be induced by methanol, and was shown to regulate expression of phytase up to 45 °C. The pattern of heterologous phytase N-glycosylation depends on the induction temperature. Unlike the AOX promoter from Pichia pastoris, this OthAOX initially turns on the expression of the heterologous protein at the de-repression stage in the presence of glycerol. Full induction of protein is observed when methanol is present. With this methanol-inducible promoter, target protein can be initially produced prior to the induction phase, which would help shorten the time for protein production. Being able to drive protein expression at various temperatures prompts this newly identified AOX promoter to be potential tool for heterologous protein production in high temperature conditions.

Improvement of recombinant endoglucanase produced in Pichia pastoris KM71 through the use of synthetic medium for inoculum and pH control of proteolysis.

The long lag time in basal salts medium (BSM) and an occurrence of proteolysis are major problems for recombinant protein production in Pichia pastoris KM71. In this study, optimal conditions were explored for fed-batch cultivation of recombinant fungal endoglucanase in P. pastoris KM71. It was found that lag and process times were much reduced when the synthetic FM22 medium was used for the inoculum compared with enriched buffered glycerol complex (BMGY) medium. The highest endoglucanase activity was obtained at 30°C which was more than 10 fold higher than that produced from shake flask. At 30°C, the specific endoglucanase activity was dependent on culture pH and a higher specific activity was observed at pH 5.0 than at pH 6.0. The higher activity was likely due to lower rate of proteolysis, since a truncated protein species was apparent at pH 6.0, but not pH 5.0. Thus, production of endoglucanase at 30°C and pH 5.0 is the optimal condition suitable for economical production in large scale. The combination of using synthetic FM22 medium for inoculum and proteolysis control by growth at lower pH could be applied for production of other recombinant proteins in P. pastoris.

High-level production of thermotolerant ß-xylosidase of Aspergillus sp. BCC125 in Pichia pastoris: characterization and its application in ethanol production.

A gene coding for thermotolerant ß-xylosidase from Aspergillus sp. BCC125 was characterized. The recombinant enzyme was expressed in methylotrophic yeast Pichia pastoris KM71 and especially high yield of secreted enzyme was obtained. ß-xylosidase possessed high enzyme efficiency (Kcat/Km=198.8mM(-1)s(-1)) toward pNP-ß-D-xylopyranoside (pNPßX) with optimal temperature and pH for activity of 60°C and pH 4.0-5.0, respectively. The identified ß-xylosidase showed clear synergism with previously identified xylanase for hydrolysis of xylan in vitro as well as simultaneous saccharification and fermentation process (SSF) in vivo with Pichia stipitis.

Heterologous protein expression in Pichia thermomethanolica BCC16875, a thermotolerant methylotrophic yeast and characterization of N-linked glycosylation in secreted protein.

This study describes Pichia thermomethanolica BCC16875, a new methylotrophic yeast host for heterologous expression. Both methanol-inducible alcohol oxidase (AOX1) and constitutive glyceraldehyde-3-phosphate dehydrogenase (GAP) promoters from Pichia pastoris were shown to drive efficient gene expression in this host. Recombinant phytase and xylanase were expressed from both promoters as secreted proteins, with the former showing different patterns of N-glycosylation dependent on the promoter used and culture medium. In addition, growth temperature also had an effect on N-glycan modification of cell wall mannoproteins. The major glycoprotein oligosaccharide species produced from P. thermomethanolica BCC16875 is Man(8-12) GlcNAc(2) , which is similar to that from other methylotrophs. Moreover, mannosylphosphate and α-1,6- and α-1,2-linked mannose modifications of heterologous secreted protein were also detected. The attainably high level of protein production in complement to distinctive thermotolerance rarely found in other industrial yeasts makes this microorganism an attractive host for large-scale fermentation.

Coexpression of fungal phytase and xylanase utilizing the cis-acting hydrolase element in Pichia pastoris.

Plant-based animal feed contains antinutritive agents, necessitating the addition of digestive enzymes in commercial feeds. Enzyme additives are costly because they are currently produced separately from recombinant sources. The coexpression of digestive enzymes in a single recombinant cell system would thus be advantageous. A coexpression system for the extracellular production of phytase and xylanase was established in Pichia pastoris yeast. The genes for each enzyme were fused in-frame with the α-factor secretion signal and linked by the 2A-peptide-encoding sequence. Each enzyme was expressed extracellularly as individual functional proteins. The specific activities of 2A-expressed phytase (PhyA-2A) and 2A-expressed xylanase (XylB-2A) were 9.3 and 97.3 U mg(-1) , respectively. Optimal PhyA-2A activity was observed at 55 degreesC and pH 5.0. PhyA-2A also exhibited broad pH stability from 2.5 to 7.0 and retained approximately 70% activity after heating at 90 degreesC for 5 min. Meanwhile, XylB-2A exhibited optimal activity at 50 degreesC and pH 5.5 and showed pH stability from 5.0 to 8.0. It retained >50% activity after incubation at 50 degreesC for 10 min. These enzyme properties are similar to those of individually expressed recombinant enzymes. In vitro digestibility test showed that PhyA-2A and XylB-2A are as efficient as individually expressed enzymes for hydrolyzing phytate and crude fiber in feedstuff, respectively.

High-level expression in Escherichia coli, purification and mosquito-larvicidal activity of the binary toxin from Bacillus sphaericus.

The mosquito-larvicidal binary toxin produced by Bacillus sphaericus consists of two polypeptides: BinA and BinB. Both proteins function together, and maximum toxicity is obtained when both are present in equimolar ratio. Cloning and expression of each component separately in heterologous hosts led to low toxicity of the crystal proteins. To improve the expression level, the purification process, and the activity of the binary toxin, the binA and binB genes were separately cloned in Escherichia coli. Each gene was fused in frame to the glutathione S-transferase (GST) gene to be expressed as GST-fusion protein (GST-BinA and GST-BinB). A high expression level was observed from both constructs, and the fusion proteins exhibited high toxicity to Culex quinquefasciatus larvae. High-purity toxin could be obtained by affinity chromatography. The result suggests that GST moiety facilitates high protein production and enables better solubility of the toxin inclusions inside the larval gut, leading to higher toxicity of the fusion protein.