Enhancement of malate-production and increase in sensitivity to dimethyl succinate by mutation of the VID24 gene in Saccharomyces cerevisiae

Response mechanisms of different Saccharomyces cerevisiae strains to succinic acid

BACKGROUND

The production of succinic acid (SA) from biomass has attracted worldwide interest. Saccharomyces cerevisiae is preferred for SA production due to its strong tolerance to low pH conditions, ease of genetic manipulation, and extensive application in industrial processes. However, when compared with bacterial producers, the SA titers and productivities achieved by engineered S. cerevisiae strains were relatively low. To develop efficient SA-producing strains, it's necessary to clearly understand how S. cerevisiae cells respond to SA.

RESULTS

In this study, we cultivated five S. cerevisiae strains with different genetic backgrounds under different concentrations of SA. Among them, KF7 and NBRC1958 demonstrated high tolerance to SA, whereas NBRC2018 displayed the least tolerance. Therefore, these three strains were chosen to study how S. cerevisiae responds to SA. Under a concentration of 20 g/L SA, only a few differentially expressed genes were observed in three strains. At the higher concentration of 60 g/L SA, the response mechanisms of the three strains diverged notably. For KF7, genes involved in the glyoxylate cycle were significantly downregulated, whereas genes involved in gluconeogenesis, the pentose phosphate pathway, protein folding, and meiosis were significantly upregulated. For NBRC1958, genes related to the biosynthesis of vitamin B6, thiamin, and purine were significantly downregulated, whereas genes related to protein folding, toxin efflux, and cell wall remodeling were significantly upregulated. For NBRC2018, there was a significant upregulation of genes connected to the pentose phosphate pathway, gluconeogenesis, fatty acid utilization, and protein folding, except for the small heat shock protein gene HSP26. Overexpression of HSP26 and HSP42 notably enhanced the cell growth of NBRC1958 both in the presence and absence of SA.

CONCLUSIONS

The inherent activities of small heat shock proteins, the levels of acetyl-CoA and the strains' potential capacity to consume SA all seem to affect the responses and tolerances of S. cerevisiae strains to SA. These factors should be taken into consideration when choosing host strains for SA production. This study provides a theoretical basis and identifies potential host strains for the development of robust and efficient SA-producing strains.

Japanese sake making using wild yeasts isolated from natural environments

Saccharomyces cerevisiae is one of the most important microorganisms for the food industry, including Japanese sake, beer, wine, bread, and other products. For sake making, Kyokai sake yeast strains are considered one of the best sake yeast strains because these strains possess fermentation properties that are suitable for the quality of sake required. In recent years, the momentum for the development of unique sake, which is distinct from conventional sake, has grown, and there is now a demand to develop unique sake yeasts that have different sake making properties than Kyokai sake yeast strains. In this minireview, we focus on "wild yeasts," which inhabit natural environments, and introduce basic research on the wild yeasts for sake making, such as their genetic and sake fermentation aspects. Finally, we also discuss the molecular breeding of wild yeast strains for sake fermentation and the possibility for sake making using wild yeasts.

Effect of yeast chromosome II aneuploidy on malate production in sake brewing

Chromosome aneuploidy is a common phenomenon in industrial yeast. Aneuploidy is considered one of the strategies to enhance the industrial properties of Saccharomyces cerevisiae strains. However, the effects of chromosomal aneuploidy on the brewing properties of sake have not been extensively studied. In this study, sake brewing was performed using a series of genome-wide segmental duplicated laboratory S. cerevisiae strains, and the effects of each segmentally duplicated region on sake brewing were investigated. We found that the duplication of specific chromosomal regions affected the production of organic acids and aromatic compounds in sake brewing. As organic acids significantly influence the taste of sake, we focused on the segmental duplication of chromosome II that alters malate levels. Sake yeast Kyokai No. 901 strains with segmental chromosome II duplication were constructed using a polymerase chain reaction-mediated chromosomal duplication method, and sake was brewed using the resultant aneuploid sake yeast strains. The results showed the possibility of developing sake yeast strains exhibiting low malate production without affecting ethanol production capacity. Our study revealed that aneuploidy in yeast alters the brewing properties; in particular, the aneuploidy of chromosome II alters malate production in sake brewing. In conclusion, aneuploidization can be a novel and useful tool to breed sake yeast strains with improved traits, possessing industrial significance.

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.

Mutation in gene coding for glucose-induced degradation-deficient protein contributes to high malate production in yeast strain No. 28 and No. 77 used for industrial brewing of sake

Saccharomyces cerevisiae produces organic acids including malate during alcohol fermentation. Since malate contributes to the pleasant flavor of sake, high-malate-producing yeast strain No. 28 and No. 77 have been developed by the Brewing Society of Japan. In this study, the genes responsible for the high malate phenotype in these strains were investigated. We had previously found that the deletion of components of the glucose-induced degradation-deficient (GID) complex led to high malate production in yeast. Upon examining GID protein-coding genes in yeast strain No. 28 and No. 77, a nonsense homozygous mutation of GID4 in strain No. 28 and of GID2 in strain No. 77 were identified as the cause of high malate production. Furthermore, complementary tests of these mutations indicated that the heterozygous nonsense mutation in GID2 was recessive. In contrast, the heterozygous nonsense mutation in GID4 was considered semidominant.

Effects of a novel variant of the yeast γ-glutamyl kinase Pro1 on its enzymatic activity and sake brewing

Sake is a traditional Japanese alcoholic beverage brewed with the yeast Saccharomyces cerevisiae. Sake taste is affected by sugars, organic acids, and amino acids. We previously isolated mutants resistant to the proline analogue azetidine-2-carboxylate derived from a diploid sake yeast strain. Some of the mutants produced a greater amount of proline in the brewed sake. One of them (strain K-9-AZC) carried a novel mutation in the PRO1 gene encoding the Gln79His variant of the γ-glutamyl kinase Pro1, a key enzyme in proline biosynthesis in S. cerevisiae. This mutation resulted in extreme desensitization to feedback inhibition by proline, leading to proline overproduction. Interestingly, sake brewed with K-9-AZC contained 3.7-fold more proline, but only 25% less succinate than sake brewed with the parent strain. Metabolome analysis suggests that the decrease in succinate was attributable to a lower level of 2-oxoglutarate, which is converted into glutamate. The approach here could be a practical method for breeding of yeast strains involved in the diversity of sake taste.

Development of sake yeast haploid set with diverse brewing properties using sake yeast strain Hiroshima no. 6 exhibiting sexual reproduction

Among sake yeast strains, Kyokai no. 7 (K7) and its closely related strains (K7 group) are predominantly used because of their excellent brewing properties. In the sake industrial sector, the need for various types of yeast strains is high. Although crossbreeding is an effective method for generating genetic diversity that should result in diverse characteristics, most K7 group strains lack normal sporulation ability, including the ability to undergo meiotic chromosomal recombination, which leads to difficulties in crossbreeding. Accordingly, the improvement of sake yeast strains primarily depends on mutagenesis and suitable selection in a stepwise manner. Our recent study revealed that the long-preserved sake yeast strain Hiroshima no. 6 (H6) does not belong to the K7 group despite genetically being extremely similar. In addition, H6 exhibited normal sporulation. Thus, we isolated haploid cells from H6 and mated them with previously isolated haploid cells of K7 group strains. The crossbred diploid strains had normal sporulation ability; hence, we performed tetrad analysis. The brewing characteristics of the obtained haploid set were extremely diverse. Principal component analysis based on the volatile and organic acid components measured using small-scale sake brewing tests revealed that the haploid strains derived from each diploid strain displayed a characteristic distribution. Thus, we demonstrated the availability of genetic crossbreeding using H6 with sporulation ability to facilitate both the development of novel sake yeast strains with many desirable characteristics and analyses of the function of sake yeast.

Progress in the genomics and genome-wide study of sake yeast

Completion of the whole genome sequence of a laboratory yeast strain Saccharomyces cerevisiae in 1996 ushered in the development of genome-wide experimental tools and accelerated subsequent genetic study of S. cerevisiae. The study of sake yeast also shared the benefit of such tools as DNA microarrays, gene disruption-mutant collections, and others. Moreover, whole genome analysis of representative sake yeast strain Kyokai no. 7 was performed in the late 2000s, and enabled comparative genomics between sake yeast and laboratory yeast, resulting in some notable finding for of sake yeast genetics. Development of next-generation DNA sequencing and bioinformatics also drastically changed the field of the genetics, including for sake yeast. Genomics and the genome-wide study of sake yeast have progressed under these circumstances during the last two decades, and are summarized in this article. Abbreviations: AFLP: amplified fragment length polymorphism; CGH: comparative genomic hybridization; CNV: copy number variation; DMS: dimethyl succinate; DSW: deep sea water; LOH: loss of heterozygosity; NGS: next generation sequencer; QTL: quantitative trait loci; QTN: quantitative trait nucleotide; SAM: S-adenosyl methionine; SNV: single nucleotide variation.

Agmatine Production by Aspergillus oryzae Is Elevated by Low pH during Solid-State Cultivation

Sake (rice wine) produced by multiple parallel fermentation (MPF) involving Aspergillus oryzae (strain RW) and Saccharomyces cerevisiae under solid-state cultivation conditions contained 3.5 mM agmatine, while that produced from enzymatically saccharified rice syrup by S. cerevisiae contained <0.01 mM agmatine. Agmatine was also produced in ethanol-free rice syrup prepared with A. oryzae under solid-state cultivation (3.1 mM) but not under submerged cultivation, demonstrating that A. oryzae in solid-state culture produces agmatine. The effect of cultivation conditions on agmatine production was examined. Agmatine production was boosted at 30°C and reached the highest level (6.3 mM) at pH 5.3. The addition of l-lactic, succinic, and citric acids reduced the initial culture pHs to 3.0, 3.5, and 3.2, respectively, resulting in a further increase in agmatine accumulation (8.2, 8.7, and 8.3 mM, respectively). Homogenate from a solid-state culture exhibited a maximum l-arginine decarboxylase (ADC) activity (74 pmol · min^-1 · μg^-1) at pH 3.0 at 30°C; homogenate from a submerged culture exhibited an extremely low activity (<0.3 pmol · min^-1 · μg^-1) under all conditions tested. These observations indicated that efficient agmatine production in ethanol-free rice syrup is achieved by an unidentified low-pH-dependent ADC induced during solid-state cultivation of A. oryzae, even though A. oryzae lacks ADC orthologs and instead possesses four ornithine decarboxylases (ODC1 to ODC4). Recombinant ODC1 and ODC2 exhibited no ADC activity at acidic pH (pH < 4.0), suggesting that other decarboxylases or an unidentified ADC is involved in agmatine production.IMPORTANCE It has been speculated that, in general, fungi do not synthesize agmatine from l-arginine because they do not possess genes encoding arginine decarboxylase. Numerous preclinical studies have shown that agmatine exerts pleiotropic effects on various molecular targets, leading to an improved quality of life. In the present study, we first demonstrated that l-arginine was a feasible substrate for agmatine production by the fungus Aspergillus oryzae RW. We observed that the productivity of agmatine by A. oryzae RW was elevated at low pH only during solid-state cultivation. A. oryzae is utilized in the production of various Asian fermented foods. The saccharification conditions optimized in the current study could be employed not only in the production of an agmatine-containing ethanol-free rice syrup but also in the production of many types of fermented foods, such as soy sauce (shoyu), rice vinegar, etc., as well as for use as novel therapeutic agents and nutraceuticals.

Current advance in biological production of malic acid using wild type and metabolic engineered strains

Malic acid (2-hydroxybutanedioic acid) is a four-carbon dicarboxylic acid, which has attracted great interest due to its wide usage as a precursor of many industrially important chemicals in the food, chemicals, and pharmaceutical industries. Several mature routes for malic acid production have been developed, such as chemical synthesis, enzymatic conversion and biological fermentation. With depletion of fossil fuels and concerns regarding environmental issues, biological production of malic acid has attracted more attention, which mainly consists of three pathways, namely non-oxidative pathway, oxidative pathway and glyoxylate cycle. In recent decades, metabolic engineering of model strains, and process optimization for malic acid production have been rapidly developed. Hence, this review comprehensively introduces an overview of malic acid producers and highlight some of the successful metabolic engineering approaches.