Construction of recombinant industrial brewer’s yeast with lower diacetyl production and proteinase A activity

GAT1 Gene, the GATA Transcription Activator, Regulates the Production of Higher Alcohol during Wheat Beer Fermentation by Saccharomyces cerevisiae

Uncoordinated carbon-nitrogen ratio in raw materials will lead to excessive contents of higher alcohols in alcoholic beverages. The effect of GAT1 gene, the GATA transcription activator, on higher alcohol biosynthesis was investigated to clarify the mechanism of Saccharomyces cerevisiae regulating higher alcohol metabolism under high concentrations of free amino nitrogen (FAN). The availability of FAN by strain SDT1K with a GAT1 double-copy deletion was 28.31% lower than that of parent strain S17, and the yield of higher alcohols was 33.91% lower. The transcript levels of the downstream target genes of GAT1 and higher alcohol production in the double-copy deletion mutant suggested that a part of the effect of GAT1 deletion on higher alcohol production was the downregulation of GAP1, ARO9, and ARO10. This study shows that GATA factors can effectively regulate the metabolism of higher alcohols in S. cerevisiae and provides valuable insights into higher alcohol biosynthesis, showing great significance for the wheat beer industry.

Enhanced Production of Ethyl Lactate in Saccharomyces cerevisiae by Genetic Modification

Ethyl lactate is an important flavor substance in baijiu, and it is also one of the common raw materials in the production of flavors and spices. In this study, we first established the ethyl lactate biosynthesis pathway in Saccharomyces cerevisiae α(L) by introducing propionyl coenzyme A transferase (Pct) and alcohol acyltransferase (AAT), and the results showed that strain α(L)-CP-Ae produced the most ethyl lactate 239.53 ± 5.45 mg/L. Subsequently, the copy number of the Pct_cp gene and AeAT9 gene was increased, and the modified strain α(L)-tCP-tAe produced 346.39 ± 3.99 mg/L ethyl lactate. Finally, the porin gene (por₂) and the mitochondrial pyruvate carrier gene (MPC2) were knocked to impede mitochondrial transport of pyruvate, and the final modified strain α(L)-tCP-tAeΔpor₂ produced ethyl lactate 420.48 ± 6.03 mg/L.

Identification of Down-Regulated Proteome in Saccharomyces cerevisiae with the Deletion of Yeast Cathepsin D in Response to Nitrogen Stress

Vacuolar proteinase A (Pep4p) is required for the post-translational precursor maturation of vacuolar proteinases in Saccharomyces cerevisiae, and important for protein turnover after oxidative damage. The presence of proteinase A in brewing yeast leads to the decline of beer foam stability, thus the deletion or inhibition of Pep4p is generally used. However, the influence of Pep4p deletion on cell metabolism in Saccharomyces cerevisiae is still unclear. Herein, we report the identification of differentially down-regulated metabolic proteins in the absence of Pep4p by a comparative proteomics approach. 2D-PAGE (two-dimensional polyacrylamide gel electrophoresis) presented that the number of significantly up-regulated spots (the Pep4p-deficient species versus the wild type) was 183, whereas the down-regulated spots numbered 111. Among them, 35 identified proteins were differentially down-regulated more than 10-fold in the Pep4p-deficient compared to the wild-type species. The data revealed that Pep4p was required for the synthesis and maturation of several glycolytic enzymes and stress proteins, including Eno2p, Fba1p, Pdc1p, Tpi1p, Ssa1, Hsp82p, and Trr1p. The transcription and post-translational modifications of glycolytic enzymes like Eno2p and Fba1p were sensitive to the absence of Pep4p; whereas the depletion of the pep4 gene had a negative impact on mitochondrial and other physiological functions. The finding of this study provides a systematic understanding that Pep4p may serve as a regulating factor for cell physiology and metabolic processes in S. cerevisiae under a nitrogen stress environment.

Beer Molecules and Its Sensory and Biological Properties: A Review

The production and consumption of beer plays a significant role in the social, political, and economic activities of many societies. During brewing fermentation step, many volatile and phenolic compounds are produced. They bring several organoleptic characteristics to beer and also provide an identity for regional producers. In this review, the beer compounds synthesis, and their role in the chemical and sensory properties of craft beers, and potential health benefits are described. This review also describes the importance of fermentation for the brewing process, since alcohol and many volatile esters are produced and metabolized in this step, thus requiring strict control. Phenolic compounds are also present in beer and are important for human health since it was proved that many of them have antitumor and antioxidant activities, which provides valuable data for moderate dietary beer inclusion studies.

Regulating the Golgi apparatus sorting of proteinase A to decrease its excretion in Saccharomyces cerevisiae

Beer foam stability, a key factor in evaluating overall beer quality, is influenced by proteinase A (PrA). Actin-severing protein cofilin and Golgi apparatus-localized Ca²⁺ ATPase Pmr1 are involved in protein sorting at the trans-Golgi network (TGN) in yeast Curwin et al. (Mol Biol Cell 23:2327-2338, 2012). To reduce PrA excretion into the beer fermentation broth, we regulated the Golgi apparatus sorting of PrA, thereby facilitating the delivery of more PrA to the vacuoles in the yeast cells. In the present study, the cofilin-coding gene COF1 and the Pmr1-coding gene PMR1 were overexpressed in the parental strain W303-1A and designated as W + COF1 and W + PMR1, respectively. The relative expression levels of COF1 in W + COF1 and PMR1 in W + PMR1 were 5.26- and 19.76-fold higher than those in the parental strain. After increases in the expression levels of cofilin and Pmr1 were confirmed, the PrA activities in the wort broth fermented with W + COF1, W + PMR1, and W303-1A were measured. Results showed that the extracellular PrA activities of W + COF1 and W + PMR1 were decreased by 9.24% and 13.83%, respectively, at the end of the main fermentation compared with that of W303-1A. Meanwhile, no apparent differences were found on the fermentation performance of recombinant and parental strains. The research uncovers an effective strategy for decreasing PrA excretion in Saccharomyces cerevisiae.

Diacetyl control during brewery fermentation via adaptive laboratory engineering of the lager yeast Saccharomyces pastorianus

Diacetyl contributes to the flavor profile of many fermented products. Its typical buttery flavor is considered as an off flavor in lager-style beers, and its removal has a major impact on time and energy expenditure in breweries. Here, we investigated the possibility of lowering beer diacetyl levels through evolutionary engineering of lager yeast for altered synthesis of α-acetolactate, the precursor of diacetyl. Cells were exposed repeatedly to a sub-lethal level of chlorsulfuron, which inhibits the acetohydroxy acid synthase responsible for α-acetolactate production. Initial screening of 7 adapted isolates showed a lower level of diacetyl during wort fermentation and no apparent negative influence on fermentation rate or alcohol yield. Pilot-scale fermentation was carried out with one isolate and results confirmed the positive effect of chlorsulfuron adaptation. Diacetyl levels were over 60% lower at the end of primary fermentation relative to the non-adapted lager yeast and no significant change in fermentation performance or volatile flavor profile was observed due to the adaptation. Whole-genome sequencing revealed a non-synonymous SNP in the ILV2 gene of the adapted isolate. This mutation is known to confer general tolerance to sulfonylurea compounds, and is the most likely cause of the improved tolerance. Adaptive laboratory evolution appears to be a natural, simple and cost-effective strategy for diacetyl control in brewing.

Enhancing the performance of brewing yeasts

Beer production is one of the oldest known traditional biotechnological processes, but is nowadays facing increasing demands not only for enhanced product quality, but also for improved production economics. Targeted genetic modification of a yeast strain is one way to increase beer quality and to improve the economics of beer production. In this review we will present current knowledge on traditional approaches for improving brewing strains and for rational metabolic engineering. These research efforts will, in the near future, lead to the development of a wider range of industrial strains that should increase the diversity of commercial beers.

Reduced Production of Higher Alcohols by Saccharomyces cerevisiae in Red Wine Fermentation by Simultaneously Overexpressing BAT1 and Deleting BAT2

In red wine, the contents of higher alcohols and ethyl carbamate (EC) are two significant health concerns. To reduce the production of higher alcohols by wine yeast YZ22 with low production of EC, one BAT2 was replaced by a BAT1 expression cassette first and then another BAT2 was deleted to obtain the mutant SYBB3. Real-time quantitative PCR showed that the relative expression level of BAT1 in SYBB3 improved 28 times compared with that in YZ22. The yields of isobutanol and 3-methyl-1-butanol produced by mutant SYBB3 reduced by 39.41% and 37.18% compared to those by the original strain YZ22, and the total production of higher alcohols decreased from 463.82 mg/L to 292.83 mg/L in must fermentation of Cabernet Sauvignon. Meanwhile, there were no obvious differences on fermentation characteristics of the mutant and parental strain. This research has suggested an effective strategy for decreasing production of higher alcohols in Saccharomyces cerevisiae.

Regulation of Saccharomyces cerevisiae genetic engineering on the production of acetate esters and higher alcohols during Chinese Baijiu fermentation

Acetate esters and higher alcohols greatly influence the quality and flavor profiles of Chinese Baijiu (Chinese liquor). Various mutants have been constructed to investigate the interactions of ATF1 overexpression, IAH1 deletion, and BAT2 deletion on the production of acetate esters and higher alcohols. The results showed that the overexpression of ATF1 under the control of the PGK1 promoter with BAT2 and IAH1 double-gene deletion led to a higher production of acetate esters and a lower production of higher alcohols than the overexpression of ATF1 with IAH1 deletion or overexpression of ATF1 with BAT2 deletion. Moreover, deletion of IAH1 in ATF1 overexpression strains effectively increased the production of isobutyl acetate and isoamyl acetate by reducing the hydrolysis of acetate esters. The decline in the production of higher alcohol by the ATF1 overexpression strains with BAT2 deletion is due to the interaction of ATF1 overexpression and BAT2 deletion. Mutants with varying abilities of producing acetate esters and higher alcohols were developed by genetic engineering. These strains have great potential for industrial application.

Decreased proteinase A excretion by strengthening its vacuolar sorting and weakening its constitutive secretion in Saccharomyces cerevisiae

Proteinase A (PrA), encoded by PEP4 gene, is detrimental to beer foam stability. There are two transport pathways for the new synthesized PrA in yeast, sorting to the vacuole normally, or excreting out of the cells under stress conditions. They were designated as the Golgi-to-vacuole pathway and the constitutive secretory pathway, respectively. To reduce PrA excretion in some new way instead of its coding gene deletion, which had a negative effect on cell metabolism and beer fermentation, we modified the PrA transport based on these above two pathways. In the Golgi-to-vacuole pathway, after the verification that Vps10p is the dominant sorting receptor for PrA Golgi-to-vacuolar transportation by VPS10 deletion, VPS10 was then overexpressed. Furthermore, SEC5, encoding exocyst complexes’ central subunit (Sec5p) in the constitutive secretory pathway, was deleted. The results show that PrA activity in the broth fermented with WGV10 (VPS10 overexpressing strain) and W∆SEC5 (SEC5 deletion strain) was lowered by 76.96 and 32.39%, compared with the parental strain W303-1A, at the end of main fermentation. There are negligible changes in fermentation performance between W∆SEC5 and W303-1A, whereas, surprisingly, WGV10 had a significantly improved fermentation performance compared with W303-1A. WGV10 has an increased growth rate, resulting in higher biomass and faster fermentation speed; finally, wort fermentation is performed thoroughly. The results show that the biomass production of WGV10 is always higher than that of W∆SEC5 and W303-1A at all stages of fermentation, and that ethanol production of WGV10 is 1.41-fold higher than that of W303-1A. Obviously, VPS10 overexpression is beneficial for yeast and is a more promising method for reduction of PrA excretion.

Enhanced leavening properties of baker's yeast by reducing sucrase activity in sweet dough

Leavening ability in sweet dough is required for the commercial applications of baker's yeast. This property depends on many factors, such as glycolytic activity, sucrase activity, and osmotolerance. This study explored the importance of sucrase level on the leavening ability of baker's yeast in sweet dough. Furthermore, the baker's yeast strains with varying sucrase activities were constructed by deleting SUC2, which encodes sucrase or replacing the SUC2 promoter with the VPS8/TEF1 promoter. The results verify that the sucrase activity negatively affects the leavening ability of baker's yeast strains under high-sucrose conditions. Based on a certain level of osmotolerance, sucrase level plays a significant role in the fermentation performance of baker's yeast, and appropriate sucrase activity is an important determinant for the leavening property of baker's yeast in sweet dough. Therefore, modification on sucrase activity is an effective method for improving the leavening properties of baker's yeast in sweet dough. This finding provides guidance for the breeding of industrial baker's yeast strains for sweet dough leavening. The transformants BS1 with deleted SUC2 genetic background provided decreased sucrase activity (a decrease of 39.3 %) and exhibited enhanced leavening property (an increase of 12.4 %). Such a strain could be useful for industrial applications.

Reduced production of ethyl carbamate for wine fermentation by deleting CAR1 in Saccharomyces cerevisiae

Ethyl carbamate (EC), a pluripotent carcinogen, is mainly formed by a spontaneous chemical reaction of ethanol with urea in wine. The arginine, one of the major amino acids in grape musts, is metabolized by arginase (encoded by CAR1) to ornithine and urea. To reduce the production of urea and EC, an arginase-deficient recombinant strain YZ22 (Δcarl/Δcarl) was constructed from a diploid wine yeast, WY1, by successive deletion of two CAR1 alleles to block the pathway of urea production. The RT-qPCR results indicated that the YZ22 almost did not express CAR1 gene and the specific arginase activity of strain YZ22 was 12.64 times lower than that of parent strain WY1. The fermentation results showed that the content of urea and EC in wine decreased by 77.89 and 73.78 %, respectively. Furthermore, EC was forming in a much lower speed with the lower urea during wine storage. Moreover, the two CAR1 allele deletion strain YZ22 was substantially equivalent to parental strain in terms of growth and fermentation characteristics. Our research also suggested that EC in wine originates mainly from urea that is produced by the arginine.

Effects of SNF1 on Maltose Metabolism and Leavening Ability of Baker's Yeast in Lean Dough

Maltose metabolism of baker's yeast (Saccharomyces cerevisiae) in lean dough is negatively influenced by glucose repression, thereby delaying the dough fermentation. To improve maltose metabolism and leavening ability, it is necessary to alleviate glucose repression. The Snf1 protein kinase is well known to be essential for the response to glucose repression and required for transcription of glucose-repressed genes including the maltose-utilization genes (MAL). In this study, the SNF1 overexpression and deletion industrial baker's yeast strains were constructed and characterized in terms of maltose utilization, growth and fermentation characteristics, mRNA levels of MAL genes (MAL62 encoding the maltase and MAL61 encoding the maltose permease) and maltase and maltose permease activities. Our results suggest that overexpression of SNF1 was effective to glucose derepression for enhancing MAL expression levels and enzymes (maltase and maltose permease) activities. These enhancements could result in an 18% increase in maltose metabolism of industrial baker's yeast in LSMLD medium (the low sugar model liquid dough fermentation medium) containing glucose and maltose and a 15% increase in leavening ability in lean dough. These findings provide a valuable insight of breeding industrial baker's yeast for rapid fermentation.

Reduction of biogenic amines production by eliminating the PEP4 gene in Saccharomyces cerevisiae during fermentation of Chinese rice wine

Biogenic amines in Chinese rice wine have a potential threat of toxicity to human health. In this study, PEP4 gene in Saccharomyces cerevisiae was knocked out in order to evaluate its effect on biogenic amines production; the enzyme encodes proteinase A (PrA), an enzyme that is responsible for the production of free amino acids. It was found that compared to the wild type strain, the PrA activity and amino acid concentration decreased significantly, and the production of biogenic amines in this knockout strain decreased by 25.5%, from 180.1mg/L to 134.2mg/L. Especially, tyramine, cadaverine and histamine concentrations were also decreased by 57.5%, 24.6% and 54.3%, respectively. The main reason for the decrease of biogenic amines may be due to the low concentration of free amino acids. Our results provide a new strategy to minimize the biogenic amine production during fermentation of Chinese rice wine.

Effects of GLC7 and REG1 deletion on maltose metabolism and leavening ability of baker's yeast in lean dough

Maltose metabolism and leavening ability of baker's yeast (Saccharomyces cerevisiae) in lean dough is negatively influenced by glucose repression. To improve maltose metabolism and leavening ability, it is necessary to alleviate glucose repression. In this study, we focus on the effects of regulators (GLC7 encoding the catalytic and REG1 encoding the regulatory subunits of protein phosphatase type 1) of glucose repression on maltose metabolism and leavening ability of baker's yeast in lean dough. To this end, GLC7 and/or REG1 deletions were constructed and characterized in terms of the growth characteristics, maltose metabolism, leavening ability, and enzyme activities. The results suggest that GLC7 and/or REG1 deletions increased maltose metabolism and leavening ability at different level with glucose derepression and increased enzymes (maltase and maltose permease) activities. In a medium containing glucose and maltose, at the point of glucose exhaustion the maltose metabolized and the leavening ability were increased 59.3% and 23.1%, respectively, in the case of a REG1 single gene deletion.

Enhanced leavening ability of baker’s yeast by overexpression of SNR84 with PGM2 deletion

Dough-leavening ability is one of the main aspects considered when selecting a baker’s yeast strain for baking industry. Generally, modification of maltose metabolic pathway and known regulatory networks of maltose metabolism were used to increase maltose metabolism to improve leavening ability in lean dough. In this study, we focus on the effects of PGM2 (encoding for the phosphoglucomutase) and SNR84 (encoding for the H/ACA snoRNA) that are not directly related to both the maltose metabolic pathway and known regulatory networks of maltose metabolism on the leavening ability of baker’s yeast in lean dough. The results show that the modifications on PGM2 and/or SNR84 are effective ways in improving leavening ability of baker’s yeast in lean dough. Deletion of PGM2 decreased cellular glucose-1-phosphate and overexpression of SNR84 increased the maltose permease activity. These changes resulted in 11, 19 and 21 % increases of the leavening ability for PGM2 deletion, SNR84 overexpression and SNR84 overexpression combining deleted PGM2, respectively.

Effects of MIG1, TUP1 and SSN6 deletion on maltose metabolism and leavening ability of baker's yeast in lean dough

Background

Glucose repression is a global regulatory system in baker’s yeast. Maltose metabolism in baker’s yeast strains is negatively influenced by glucose, thereby affecting metabolite productivity (leavening ability in lean dough). Even if the general repression system constituted by MIG1, TUP1 and SSN6 factors has already been reported, the functions of these three genes in maltose metabolism remain unclear. In this work, we explored the effects of MIG1 and/or TUP1 and/or SSN6 deletion on the alleviation of glucose-repression to promote maltose metabolism and leavening ability of baker’s yeast.

Results

Results strongly suggest that the deletion of MIG1 and/or TUP1 and/or SSN6 can exert various effects on glucose repression for maltose metabolism. The deletion of TUP1 was negative for glucose derepression to facilitate the maltose metabolism. By contrast, the deletion of MIG1 and/or SSN6, rather than other double-gene or triple-gene mutations could partly relieve glucose repression, thereby promoting maltose metabolism and the leavening ability of baker’s yeast in lean dough.

Conclusions

The mutants of industrial baker’s yeast with enhanced maltose metabolism and leavening ability in lean dough were developed by genetic engineering. These baker’s yeast strains had excellent potential industrial applications.

Variation in α-acetolactate production within the hybrid lager yeast group Saccharomyces pastorianus and affirmation of the central role of the ILV6 gene

A screen of 14 S. pastorianus lager-brewing strains showed as much as a nine-fold difference in wort total diacetyl concentration at equivalent stages of fermentation of 15°Plato brewer's wort. Two strains (A153 and W34), with relatively low and high diacetyl production, respectively, but which did not otherwise differ in fermentation performance, growth or flavour production, were selected for further investigation. Transcriptional analysis of key genes involved in valine biosynthesis showed differences between the two strains that were consistent with the differences in wort diacetyl concentration. In particular, the ILV6 gene, encoding a regulatory subunit of acetohydroxy acid synthase, showed early transcription (only 6 h after inoculation) and up to five-fold greater expression in W34 compared to A153. This earlier transcription was observed for both orthologues of ILV6 in the S. pastorianus hybrid (S. cerevisiae × S. eubayanus), although the S. cerevisiae form of ILV6 in W34 also showed a consistently higher transcript level throughout fermentation relative to the same gene in A153. Overexpression of either form of ILV6 (by placing it under the control of the PGK1 promoter) resulted in an identical two-fold increase in wort total diacetyl concentration relative to a control. The results confirm the role of the Ilv6 subunit in controlling α-acetolactate/diacetyl concentration and indicate no functional divergence between the two forms of Ilv6. The greater contribution of the S. cerevisiae ILV6 to acetolactate production in natural brewing yeast hybrids appears rather to be due to higher levels of transcription relative to the S. eubayanus form.

Influence of valine and other amino acids on total diacetyl and 2,3-pentanedione levels during fermentation of brewer's wort

Undesirable butter-tasting vicinal diketones are produced as by-products of valine and isoleucine biosynthesis during wort fermentation. One promising method of decreasing diacetyl production is through control of wort valine content since valine is involved in feedback inhibition of enzymes controlling the formation of diacetyl precursors. Here, the influence of valine supplementation, wort amino acid profile and free amino nitrogen content on diacetyl formation during wort fermentation with the lager yeast Saccharomyces pastorianus was investigated. Valine supplementation (100 to 300 mg L⁻¹) resulted in decreased maximum diacetyl concentrations (up to 37 % lower) and diacetyl concentrations at the end of fermentation (up to 33 % lower) in all trials. Composition of the amino acid spectrum of the wort also had an impact on diacetyl and 2,3-pentanedione production during fermentation. No direct correlation between the wort amino acid concentrations and diacetyl production was found, but rather a negative correlation between the uptake rate of valine (and also other branched-chain amino acids) and diacetyl production. Fermentation performance and yeast growth were unaffected by supplementations. Amino acid addition had a minor effect on higher alcohol and ester composition, suggesting that high levels of supplementation could affect the flavour profile of the beer. Modifying amino acid profile of wort, especially with respect to valine and the other branched-chain amino acids, may be an effective way of decreasing the amount of diacetyl formed during fermentation.