Molecular mechanisms and highly functional development for stress tolerance of the yeast Saccharomyces cerevisiae

Metabolomics analysis of potential functional metabolites in synbiotic ice cream made with probiotic Saccharomyces cerevisiae var. boulardii CNCM I-745 and prebiotic inulin

Probiotic lactic acid bacteria have been widely studied, but much less was focused on probiotic yeasts in food systems. In this study, probiotic Saccharomyces cerevisiae var. boulardii CNCM I-745 was employed to prepare ice cream added with and without inulin (1%, w/v). Metabolomics analysis on the effect of inulin showed 84 and 147 differentially expressed metabolites identified in the ice cream samples from day 1 and day 30 of storage (-18 °C), respectively. Various potential functional metabolites were found, including citric acid, ornithine, D-glucuronic acid, sennoside A, stachyose, maltotetraose, maltopentaose, maltohexaose, maltoheptaose, cis-aconitic acid, gamma-aminobutyric acid, L-threonine, L-glutamic acid, tryptophan, benzoic acid, and trehalose. Higher expression of these metabolites suggested their possible roles through relevant metabolic pathways in improving survivability of the probiotic yeast and functionality of ice cream. This study provides further understanding on the metabolic characteristics of probiotic yeast that potentially affect the functionality of ice cream.

Med3-mediated NADPH generation to help Saccharomyces cerevisiae tolerate hyperosmotic stress

Hyperosmotic stress tolerance is crucial for Saccharomyces cerevisiae in producing value-added products from renewable feedstock. The limited understanding of its tolerance mechanism has impeded the application of these microbial cell factories. Previous studies have shown that Med3 plays a role in hyperosmotic stress in S. cerevisiae. However, the specific function of Med3 in hyperosmotic stress tolerance remains unclear. In this study, we showed that the deletion of the mediator Med3 impairs S. cerevisiae growth under hyperosmotic stress. Phenotypic analyses and yeast two-hybrid assays revealed that Med3 interacts with the transcription factor Stb5 to regulate the expression of the genes gnd1 and ald6, which are involved in NADPH production under hyperosmotic stress conditions. The deletion of med3 resulted in a decrease in intracellular NADPH content, leading to increased oxidative stress and elevated levels of intracellular reactive oxygen species under hyperosmotic stress, thereby impacting bud formation. These findings highlight the significant role of Med3 as a regulator in maintaining NADPH generation and redox homeostasis in S. cerevisiae during hyperosmotic stress.IMPORTANCEHyperosmotic stress tolerance in the host strain is a significant challenge for fermentation performance in industrial production. In this study, we showed that the S. cerevisiae mediator Med3 is essential for yeast growth under hyperosmotic conditions. Med3 interacts with the transcription factor Stb5 to regulate the expression of genes involved in the NADPH-generation system during hyperosmotic stress. Adequate NADPH ensures the timely removal of excess reactive oxygen species and supports bud formation under these conditions. This work highlights the crucial role of Med3 as a regulator in maintaining NADPH generation and redox homeostasis in S. cerevisiae during hyperosmotic stress.

Search for protein kinase(s) related to cell growth or viability maintenance in the presence of ethanol in budding and fission yeasts

Alcohol fermentation comprises two phases: phase 1, alcohol fermentation occurs while yeast cells proliferate; phase 2, growth stops and alcohol fermentation continues. We categorized genes related to proliferation in low ethanol (phase 1) and viability in high ethanol (phase 2) as Alcohol Growth Ability (AGA) and Alcohol Viability (ALV), respectively. Although genes required for phase 1 are examined in budding yeast, those for phase 2 are unknown. We set conditions for ALV screening, searched for protein kinases (PKs) related to ALV in budding yeast, and expanded two screenings to fission yeast. Bub1 kinase was important for proliferation in low ethanol but not for viability in high ethanol, suggesting that the important PKs differ between the two phases. It was indeed the case. Further, 3 common PKs were identified as AGA in both yeasts, suggesting that the important cellular mechanism in phase 1 is conserved in both yeasts, at least partially.

The Protein Response of Salt-Tolerant Zygosaccharomyces rouxii to High-Temperature Stress during the Lag Phase

Zygosaccharomyces rouxii used in soy sauce brewing is an osmotolerant and halotolerant yeast, but it is not tolerant to high temperatures and the underlying mechanisms remain poorly understood. Using a synthetic medium containing only Pro as a nitrogen source, the response of Z. rouxii in protein level to high-temperature stress (40 °C, HTS) during the lag phase was investigated. Within the first two h, the total intracellular protein concentration was significantly decreased from 220.99 ± 6.58 μg/mg DCW to 152.63 ± 10.49 μg/mg DCW. The analysis of the amino acid composition of the total protein through vacuum proteolysis technology and HPLC showed that new amino acids (Thr, Tyr, Ser, and His) were added to newborn protein over time during the lag phase under HTS. The nutritional conditions used in this study determined that the main source of amino acid supply for protein synthesis was through amino acid biosynthesis and ubiquitination-mediated protein degradation. Differential expression analysis of the amino acid biosynthesis-related genes in the transcriptome showed that most genes were upregulated under HTS, excluding ARO8, which was consistently repressed during the lag phase. RT-qPCR results showed that high-temperature stress significantly increased the upregulation of proteolysis genes, especially PSH1 (E3 ubiquitin ligase) by 13.23 ± 1.44 fold (p < 0.0001) within 4 h. Overall, these results indicated that Z. rouxii adapt to prolonged high temperatures stress by altering its basal protein composition. This protein renewal was related to the regulation of proteolysis and the biosynthesis of amino acids.

Engineering yeast cell factories to produce biodegradable plastics and their monomers: Current status and prospects

Traditional plastic products have caused serious environmental pollution due to difficulty to be degraded in the natural environment. In the recent years, biodegradable plastics are receiving increasing attention due to advantages in natural degradability and environmental friendliness. Biodegradable plastics have potential to be used in food, agriculture, industry, medicine and other fields. However, the high production cost of such plastics is the bottleneck that limits their commercialization and application. Yeasts, including budding yeast and non-conventional yeasts, are widely studied to produce biodegradable plastics and their organic acid monomers. Compared to bacteria, yeast strains are more tolerable to multiple stress conditions including low pH and high temperature, and also have other advantages such as generally regarded as safe, and no phage infection. In addition, synthetic biology and metabolic engineering of yeast have enabled its rapid and efficient engineering for bioproduction using various renewable feedstocks, especially lignocellulosic biomass. This review focuses on the recent progress in biosynthesis technology and strategies of monomeric organic acids for biodegradable polymers, including polylactic acid (PLA), polyhydroxyalkanoate (PHA), polybutylene succinate (PBS), and polybutylene adipate terephthalate (PBAT) using yeast cell factories. Improving the performance of yeast as a cell factory and strategies to improve yeast acid stress tolerance are also discussed. In addition, the critical challenges and future prospects for the production of biodegradable plastic monomer using yeast are also discussed.

Food yeasts: occurrence, functions, and stress tolerance in the brewing of fermented foods

With the rapid development of systems biology technology, there is a deeper understanding of the molecular biological mechanisms and physiological characteristics of microorganisms. Yeasts are widely used in the food industry with their excellent fermentation performances. While due to the complex environments of food production, yeasts have to suffer from various stress factors. Thus, elucidating the stress mechanisms of food yeasts and proposing potential strategies to improve tolerance have been widely concerned. This review summarized the recent signs of progress in the variety, functions, and stress tolerance of food yeasts. Firstly, the main food yeasts occurred in fermented foods, and the taxonomy levels are demonstrated. Then, the main functions of yeasts including aroma enhancer, safety performance enhancer, and fermentation period reducer are discussed. Finally, the stress response mechanisms of yeasts and the strategies to improve the stress tolerance of cells are reviewed. Based on sorting out these related recent researches systematically, we hope that this review can provide help and approaches to further exert the functions of food yeasts and improve food production efficiency.

Specific antioxidant enzymes are involved in the freeze-thawing response of industrial baker's yeasts

In this study, the biochemical basis of resistance to slow freezing and thawing (F-T) stress was explored in two baker yeast industrial strains of Saccharomyces cerevisiae that presented differential tolerance to freezing in order to be in the frozen bakery industry. Strain Y8, used commercially in sweet baking doughs, exhibited greater stress tolerance than Y9, a strain employed in regular doughs. Survival of Y8 was higher than that of Y9 (30% vs 12%) after F-T or other reactive oxygen species (ROS) inducing stresses compared to their non-stressed controls. The superior F-T tolerance of Y8 was related to its lower ROS accumulation capacity, determined by fluorometry in cell-free extracts and in vivo, by fluorescence microscopy upon F-T, being Y8 ROS accumulation 2-fold lower than that of Y9. That, in turn, could be positively associated with Y8's higher constitutive activities of cytosolic catalase (CAT) and superoxide dismutase by a significant activation (25%) of Y8 CAT after F-T. That would complement the protective effects of other protectant molecules like trehalose, present at high concentration in this strain.

Engineering acetyl-CoA metabolism to enhance stress tolerance of yeast by regulating membrane functionality

Zygosaccharomyces rouxii has excellent fermentation performance and good tolerance to osmotic stress. Acetyl-CoA is a crucial intermediate precursor in the central carbon metabolic pathway of yeast. This study investigated the effect of engineering acetyl-CoA metabolism on the membrane functionality and stress tolerance of yeast. Firstly, exogenous supplementation of acetyl-CoA improved the biomass and the ability of unsaturated fatty acid synthesis of Z. rouxii under salt stress. Q-PCR results suggested that the gene ACSS (coding acetyl-CoA synthetase) was significantly up-expressed. Subsequently, the gene ACSS from Z. rouxii was transformed and heterologously expressed in S. cerevisiae. The recombinant cells exhibited better multiple stress (salt, acid, heat, and cold) tolerance, higher fatty acid contents, membrane integrity, and fluidity. Our findings may provide a suitable means to enhance the stress tolerance and fermentation efficiency of yeast under harsh fermentation environments.

Crucial aspects of metabolism and cell biology relating to industrial production and processing of Saccharomyces biomass

The multitude of applications to which Saccharomyces spp. are put makes these yeasts the most prolific of industrial microorganisms. This review considers biological aspects pertaining to the manufacture of industrial yeast biomass. It is proposed that the production of yeast biomass can be considered in two distinct but interdependent phases. Firstly, there is a cell replication phase that involves reproduction of cells by their transitions through multiple budding and metabolic cycles. Secondly, there needs to be a cell conditioning phase that enables the accrued biomass to withstand the physicochemical challenges associated with downstream processing and storage. The production of yeast biomass is not simply a case of providing sugar, nutrients, and other growth conditions to enable multiple budding cycles to occur. In the latter stages of culturing, it is important that all cells are induced to complete their current budding cycle and subsequently enter into a quiescent state engendering robustness. Both the cell replication and conditioning phases need to be optimized and considered in concert to ensure good biomass production economics, and optimum performance of industrial yeasts in food and fermentation applications. Key features of metabolism and cell biology affecting replication and conditioning of industrial Saccharomyces are presented. Alternatives for growth substrates are discussed, along with the challenges and prospects associated with defining the genetic bases of industrially important phenotypes, and the generation of new yeast strains."I must be cruel only to be kind: Thus bad begins, and worse remains behind." William Shakespeare: Hamlet, Act 3, Scene 4.

Generation of a Yeast Cell Model Potentially Useful to Identify the Mammalian Mitochondrial N-Acetylglutamate Transporter

The human mitochondrial carrier family (MCF) consists of 53 members. Approximately one-fifth of them are still orphans of a function. Most mitochondrial transporters have been functionally characterized by reconstituting the bacterially expressed protein into liposomes and transport assays with radiolabeled compounds. The efficacy of this experimental approach is constrained to the commercial availability of the radiolabeled substrate to be used in the transport assays. A striking example is that of N-acetylglutamate (NAG), an essential regulator of the carbamoyl synthetase I activity and the entire urea cycle. Mammals cannot modulate mitochondrial NAG synthesis but can regulate the levels of NAG in the matrix by exporting it to the cytosol, where it is degraded. The mitochondrial NAG transporter is still unknown. Here, we report the generation of a yeast cell model suitable for identifying the putative mammalian mitochondrial NAG transporter. In yeast, the arginine biosynthesis starts in the mitochondria from NAG which is converted to ornithine that, once transported into cytosol, is metabolized to arginine. The deletion of ARG8 makes yeast cells unable to grow in the absence of arginine since they cannot synthetize ornithine but can still produce NAG. To make yeast cells dependent on a mitochondrial NAG exporter, we moved most of the yeast mitochondrial biosynthetic pathway to the cytosol by expressing four E. coli enzymes, argB-E, able to convert cytosolic NAG to ornithine. Although argB-E rescued the arginine auxotrophy of arg8∆ strain very poorly, the expression of the bacterial NAG synthase (argA), which would mimic the function of a putative NAG transporter increasing the cytosolic levels of NAG, fully rescued the growth defect of arg8∆ strain in the absence of arginine, demonstrating the potential suitability of the model generated.

The YBR056W-A and Its Ortholog YDR034W-B of S. cerevisiae Belonging to CYSTM Family Participate in Manganese Stress Overcoming

The CYSTM (cysteine-rich transmembrane module) protein family comprises small molecular cysteine-rich tail-anchored membrane proteins found in many eukaryotes. The Saccharomyces cerevisiae strains carrying the CYSTM genes YDRO34W-B and YBR056W-A (MNC1) fused with GFP were used to test the expression of these genes under different stresses. The YBR056W-A (MNC1) and YDR034W-B genes are expressed under stress conditions caused by the toxic concentrations of heavy metal ions, such as manganese, cobalt, nickel, zinc, cuprum, and 2.4-dinitrophenol uncoupler. The expression level of YDR034W-B was higher than that of YBR056W-A under alkali and cadmium stresses. The Ydr034w-b-GFP and Ybr056w-a-GFP proteins differ in the cellular localization: Ydr034w-b-GFP was mainly observed in the plasma membrane and vacuolar membrane, while Ybr056w-a-GFP was observed in the cytoplasm, probably in intracellular membranes. The null-mutants in both genes demonstrated decreased cell concentration and lytic phenotype when cultivated in the presence of excess manganese. This allows for speculations about the involvement of Mnc1 and Ydr034w-b proteins in manganese stress overcoming.

Peptide extract from spent yeast improves resistance of Saccharomyces cerevisiae to oxidative stress

Yeast cells face various stress factors during industrial fermentations, since they are exposed to harsh environmental conditions, which may impair biomolecules productivity and yield. In this work, the use of an antioxidant peptide extract obtained from industrial spent yeast was explored as supplement for Saccharomyces cerevisiae fermentation to prevent a common bottleneck: oxidative stress. For that, a recombinant yeast strain, producer of β-farnesene, was firstly incubated with 0.5 and 0.7 g/L peptide extract, in the presence and absence of hydrogen peroxide (an oxidative stress inducer), for 1-5 h, and then assayed for intracellular reactive oxygen species, and growth ability in agar spot assays. Results showed that under 2 mM H₂O_2, the peptide extract could improve cells growth and reduce reactive oxygen species production. Therefore, this antioxidant effect was further evaluated in shake-flasks and 2-L bioreactor batch fermentations. Peptide extract (0.7 g/L) was able to increase yeast resistance to the oxidative stress promoted by 2 mM H₂O₂, by reducing reactive oxygen species levels between 1.2- and 1.7-fold in bioreactor and between 1.2- and 3-fold in shake-flask fermentations. Moreover, improvements on yeast cell density of up to 1.5-fold and 2-fold, and on biomolecule concentration of up to 1.6-fold and 2.8-fold, in bioreactor and shake-flasks, respectively, were obtained. Thus, culture medium supplementation with antioxidant peptide extracted from industrial spent yeast is a promising strategy to improve fermentation performance while valuing biomass waste. This valorization can promote a sustainable and eco-friendly solution for the biotechnology industry by the implementation of a circular economy model. KEY POINTS: • Peptide extract from spent yeast applied for the first time on yeast fermentation. • Antioxidant peptide extract enhanced S. cerevisiae oxidative stress resistance. • Fermentation performance under stress improved by peptide extract supplementation.

Development of sake yeast breeding and analysis of genes related to its various phenotypes

Sake is a traditional Japanese alcoholic beverage made from rice and water, fermented by the filamentous fungi Aspergillus oryzae and the yeast Saccharomyces cerevisiae. Yeast strains, also called sake yeasts, with high alcohol yield and the ability to produce desired flavor compounds in the sake, have been isolated from the environment for more than a century. Furthermore, numerous methods to breed sake yeasts without genetic modification have been developed. The objectives of breeding include increasing the efficiency of production, improving the aroma and taste, enhancing safety, imparting functional properties, and altering the appearance of sake. With the recent development of molecular biology, the suitable sake brewing characteristics in sake yeasts, and the causes of acquisition of additional phenotypes in bred yeasts have been elucidated genetically. This mini-review summarizes the history and lineage of sake yeasts, their genetic characteristics, the major breeding methods used, and molecular biological analysis of the acquired strains. The data in this review on the metabolic mechanisms of sake yeasts and their genetic profiles will enable the development of future strains with superior phenotypes.

A review of yeast: High cell-density culture, molecular mechanisms of stress response and tolerance during fermentation

Yeast is widely used in the fermentation industry, and the major challenges in fermentation production system are high capital cost and low reaction rate. High cell-density culture is an effective method to increase the volumetric productivity of the fermentation process, thus making the fermentation process faster and more robust. During fermentation, yeast is subjected to various environmental stresses, including osmotic, ethanol, oxidation, and heat stress. To cope with these stresses, yeast cells need appropriate adaptive responses to acquire stress tolerances to prevent stress-induced cell damage. Since a single stressor can trigger multiple effects, both specific and nonspecific effects, general and specific stress responses are required to achieve comprehensive protection of cells. Since all these stresses disrupt protein structure, the upregulation of heat shock proteins and trehalose genes is induced when yeast cells are exposed to stress. A better understanding of the research status of yeast HCDC and its underlying response mechanism to various stresses during fermentation is essential for designing effective culture control strategies and improving the fermentation efficiency and stress resistance of yeast.

Metabolic profiles alteration of Southern Thailand traditional sweet pickled mango during the production process

Sweet pickled mango named Ma-Muang Bao Chae-Im (MBC), a delicacy from the Southern part of Thailand, has a unique aroma and taste. The employed immersion processes (brining 1, brining 2, and immersion in a hypertonic sugar solution, sequentially) in the MBC production process bring changes to the unripe mango, which indicate the occurrence of metabolic profiles alteration during the production process. This occurrence was never been explored. Thus, this study investigated metabolic profile alteration during the MBC production process. The untargeted metabolomics profiling method was used to reveal the changes in volatile and non-volatile metabolites. Headspace solid-phase micro-extraction tandem with gas chromatography quadrupole time of flight (GC/QTOF) was employed for the volatile analysis, while metabolites derivatization for non-volatile analysis. In conclusion, a total of 82 volatile and 41 non-volatile metabolites were identified during the production process. Terpenes, terpenoids, several non-volatile organic acids, and sugars were the major mango metabolites that presented throughout the process. Gamma-aminobutyric acid (GABA) was only observed during the brining processes, which suggested the microorganism’s stress response mechanism to an acidic environment and high chloride ions in brine. Esters and alcohols were abundant during the last immersion process, which had an important role in MBC flavor characteristics. The knowledge of metabolites development during the MBC production process would be beneficial for product development and optimization.

Metabolic characteristics of intracellular trehalose enrichment in salt-tolerant Zygosaccharomyces rouxii

The salt-tolerant flavor yeast Zygosaccharomyces rouxii is an important food flavor microorganism, but its intracellular stress-resistant trehalose synthesis efficiency has been shown to be low, resulting in its weak high-temperature resistance. The intracellular and extracellular levels of carbohydrates, organic acids, and amino acids of Z. rouxii in a 20-L mechanically stirred ventilated fermenter were analyzed using metabolomics research methods. Our results showed that glucose supplementation could promote the growth of yeast cells, but high temperatures (> 35°C) significantly prevented cell growth. Under three different growth strategies, extracellular glucose was continuously utilized and intracellular glucose was continuously metabolized, but glucose overflow metabolism was inhibited by high temperature, which showed that the level of intracellular/extracellular ethanol was stable. High temperature stimulated significant intracellular trehalose accumulation (c₂₀._5h = 80.78 mg/g Dry Cell Weight (DCW)) but not efflux, as well as xylitol accumulation (c₂₀._5h = 185.97 mg/g DCW) but with efflux (c₂₀._5h = 29.78 g/L). Moreover, heat resistance evaluation showed that xylitol and trehalose had heat-protective effects on Z. rouxii. In addition, a large amount of propionic acid and butyric acid accumulated inside and outside these cells, showing that the conversion of glucose to acid in yeast cells becomes the main pathway of glucose overflow metabolism in high temperatures. In addition, the increased demand of yeast cells for phenylalanine, threonine, and glycine at high temperatures suggested that these metabolites participated in the temperature adaptation of Z. rouxii in different ways. Valine and leucine/isoleucine [branched-chain amino acids (BCAAs)] were mainly affected by the addition of glucose, while glucose, sucrose, aspartic acid/asparagine, and glutamate/glutamine were not affected by this temperature regulation as a whole. This study could help deepen our understanding of the high-temperature adaptation mechanism of salt-tolerant Z. rouxii, and has theoretical significance for the application of highly tolerant yeast to food brewing.

Screening novel genes by a comprehensive strategy to construct multiple stress-tolerant industrial Saccharomyces cerevisiae with prominent bioethanol production

BACKGROUND

Strong multiple stress-tolerance is a desirable characteristic for Saccharomyces cerevisiae when different feedstocks are used for economical industrial ethanol production. Random mutagenesis or genome shuffling has been applied for improving multiple stress-tolerance, however, these techniques are generally time-consuming and labor cost-intensive and their molecular mechanisms are unclear. Genetic engineering, as an efficient technology, is poorly applied to construct multiple stress-tolerant industrial S. cerevisiae due to lack of clear genetic targets. Therefore, constructing multiple stress-tolerant industrial S. cerevisiae is challenging. In this study, some target genes were mined by comparative transcriptomics analysis and applied for the construction of multiple stress-tolerant industrial S. cerevisiae strains with prominent bioethanol production.

RESULTS

Twenty-eight shared differentially expressed genes (DEGs) were identified by comparative analysis of the transcriptomes of a multiple stress-tolerant strain E-158 and its original strain KF-7 under five stress conditions (high ethanol, high temperature, high glucose, high salt, etc.). Six of the shared DEGs which may have strong relationship with multiple stresses, including functional genes (ASP3, ENA5), genes of unknown function (YOL162W, YOR012W), and transcription factors (Crz1p, Tos8p), were selected by a comprehensive strategy from multiple aspects. Through genetic editing based on the CRISPR/Case9 technology, it was demonstrated that expression regulation of each of these six DEGs improved the multiple stress-tolerance and ethanol production of strain KF-7. In particular, the overexpression of ENA5 significantly enhanced the multiple stress-tolerance of not only KF-7 but also E-158. The resulting engineered strain, E-158-ENA5, achieved higher accumulation of ethanol. The ethanol concentrations were 101.67% and 27.31% higher than those of the E-158 when YPD media and industrial feedstocks (straw, molasses, cassava) were fermented, respectively, under stress conditions.

CONCLUSION

Six genes that could be used as the gene targets to improve multiple stress-tolerance and ethanol production capacities of S. cerevisiae were identified for the first time. Compared to the other five DEGs, ENA5 has a more vital function in regulating the multiple stress-tolerance of S. cerevisiae. These findings provide novel insights into the efficient construction of multiple stress-tolerant industrial S. cerevisiae suitable for the fermentation of different raw materials.

Role of Gln79 in Feedback Inhibition of the Yeast γ-Glutamyl Kinase by Proline

Awamori, the traditional distilled alcoholic beverage of Okinawa, Japan, is brewed with the yeast Saccharomyces cerevisiae. During the distillation process after the fermentation, enormous quantities of distillation residues containing yeast cells must be disposed of, and this has recently been recognized as a major problem both environmentally and economically. Proline, a multifunctional amino acid, has the highest water retention capacity among amino acids. Therefore, distillation residues with large amounts of proline could be useful in cosmetics. Here, we isolated a yeast mutant with high levels of intracellular proline and found a missense mutation (Gln79His) on the PRO1 gene encoding the γ-glutamyl kinase Pro1, a limiting enzyme in proline biosynthesis. The amino acid change of Gln79 to His in Pro1 resulted in desensitization to the proline-mediated feedback inhibition of GK activity, leading to the accumulation of proline in cells. Biochemical and in silico analyses showed that the amino acid residue at position 79 is involved in the stabilization of the proline binding pocket in Pro1 via a hydrogen-bonding network, which plays an important role in feedback inhibition. Our current study, therefore, proposed a possible mechanism underlying the feedback inhibition of γ-glutamyl kinase activity. This mechanism can be applied to construct proline-accumulating yeast strains to effectively utilize distillation residues.