Gene expression and function involved in polyol biosynthesis of Trichosporonoides megachiliensis under hyper-osmotic stress

Strain improvement for enhanced erythritol production by Moniliella pollinis Mutant-58 using jaggery as a cost-effective substrate

Erythritol has been produced by various microorganisms including Yarrowia, Moniliella, Aureobasidium, and Candida strains. Due to its relatively high price, erythritol sweetener is used lesser than other polyols despite having many advantages. Therefore, in this study, Moniliella pollinis strain was improved for erythritol production by chemical mutagenesis and subsequently screening for cost-effective carbon sources for the enhanced erythritol yield. M. pollinis was subjected to N-methyl N-nitro N-nitroso guanidine (NTG), ethyl methyl sulfonate (EMS), and UV mutagenesis for improved erythritol production. The fmutant strains were evaluated for enhanced erythritol production medium optimization by using different carbon substrates at the shake flask level. To enhance the production of erythritol and statistical media, optimization was carried out using a central composite design (CCD). Among 198 isolated mutants, Mutant-58 strain generated by EMS mutagenesis was selected for further assessment. The Mutant-58 strain showed significant morphological changes as compared to the parent strain. Furthermore, statistically optimized media composition resulted in the higher production of erythritol (91.2 ± 3.4 g/L) with a yield of 40.7 ± 3.4 % in shake flask experiments. The optimized medium composition for erythritol production constitutes (g/L) 225 jaggery, 4.4 yeast extract (YE), 4.4 KH2PO4, 0.31 MgSO4, and pH 5.5. The present study demonstrated strain improvement, media, and process optimization resulting in a 30% increase in the erythritol production in the Mutant-58 as compared to the parent strain. This is also the first instance where jaggery has been used as a cost-effective carbon source alternative to glucose for industrial-scale erythritol production.

Symbiotic bacteria and fungi proliferate in diapause and may enhance overwintering survival in a solitary bee

Host-microbe interactions underlie the development and fitness of many macroorganisms, including bees. Whereas many social bees benefit from vertically transmitted gut bacteria, current data suggests that solitary bees, which comprise the vast majority of species diversity within bees, lack a highly specialized gut microbiome. Here, we examine the composition and abundance of bacteria and fungi throughout the complete life cycle of the ground-nesting solitary bee Anthophora bomboides standfordiana. In contrast to expectations, immature bee stages maintain a distinct core microbiome consisting of Actinobacterial genera (Streptomyces, Nocardiodes) and the fungus Moniliella spathulata. Dormant (diapausing) larval bees hosted the most abundant and distinctive bacteria and fungi, attaining 33 and 52 times their initial copy number, respectively. We tested two adaptive hypotheses regarding microbial functions for diapausing bees. First, using isolated bacteria and fungi, we found that Streptomyces from brood cells inhibited the growth of multiple pathogenic filamentous fungi, suggesting a role in pathogen protection during overwintering, when bees face high pathogen pressure. Second, sugar alcohol composition changed in tandem with major changes in fungal abundance, suggesting links with bee cold tolerance or overwintering biology. We find that A. bomboides hosts a conserved core microbiome that may provide key fitness advantages through larval development and diapause, which raises the question of how this microbiome is maintained and faithfully transmitted between generations. Our results suggest that focus on microbiomes of mature or active insect developmental stages may overlook stage-specific symbionts and microbial fitness contributions during host dormancy.

Understanding the role of GRE3 in the erythritol biosynthesis pathway in Saccharomyces uvarum and its implication in osmoregulation and redox homeostasis

Erythritol is produced in yeasts via the reduction of erythrose into erythritol by erythrose reductases (ERs). However, the genes codifying for the ERs involved in this reaction have not been described in any Saccharomyces species yet. In our laboratory, we recently showed that, during alcoholic fermentation, erythritol is differentially produced by Saccharomyces cerevisiae and S. uvarum species, the latter being the largest producer. In this study, by using BLAST analysis and phylogenetic approaches the genes GRE3, GCY1, YPR1, ARA1 and YJR096W were identified as putative ERs in Saccharomyces cerevisiae Then, these genes were knocked out in our S. uvarum strain (BMV58) with higher erythritol biosynthesis compared to control S. cerevisiae wine strain, to evaluate their impact on erythritol synthesis and global metabolism. Among the mutants, the single deletion of GRE3 markedly impacts erythritol production, although ΔYPR1ΔGCY1ΔGRE3 was the combination that most decreased erythritol synthesis. Consistent with the increased production of fermentative by-products involved in redox balance in the Saccharomyces uvarum strain BMV58, erythritol synthesis increases at higher sugar concentrations, hinting it might be a response to osmotic stress. However, the expression of GRE3 in the S. uvarum strain was found to peak just before the start of the stationary phase, being consistent with the observation that erythritol increases at the start of the stationary phase, when there is low sugar in the medium and nitrogen sources are depleted. This suggests that GRE3 plays its primary function to help the yeast cells to maintain the redox balance during the last phases of fermentation.

An overview of erythritol production by yeast strains

Erythritol is a 4-carbon polyol produced with the aid of microbes in presence of hyper-osmotic stress. It is the most effective sugar alcohol that is produced predominantly by fermentation. In comparison to various polyols, it has many precise functions and is used as a flavor enhancer, sequestrant, humectant, nutritive sweetener, stabilizer, formulation aid, thickener, and a texturizer. Erythritol production is a common trait in a number of the yeast genera viz., Trigonopsis, Candida, Pichia, Moniliella, Yarrowia, Pseudozyma, Trichosporonoides, Aureobasidium, and Trichoderma. Extensive work has been carried out on the biological production of erythritol through Yarrowia, Moniliella, Candida, and other yeast strains, and numerous strategies used to improve erythritol productivity through mutagenesis and genetic engineering are discussed in this review.

Utilizing yeasts for the conversion of renewable feedstocks to sugar alcohols - a review

Sugar alcohols are widely marketed compounds. They are useful building block chemicals and of particular value as low- or non-calorigenic sweeteners, serving as sugar substitutes in the food industry. To date most sugar alcohols are produced by chemical routes using pure sugars, but a transition towards the use of renewable, non-edible feedstocks is anticipated. Several yeasts are naturally able to convert renewable feedstocks, such as lignocellulosic substrates, glycerol and molasses, into sugar alcohols. These bioconversions often face difficulties to obtain sufficiently high yields and productivities necessary for industrialization. This review provides insight into the most recent studies on utilizing yeasts for the conversion of renewable feedstocks to diverse sugar alcohols, including xylitol, erythritol, mannitol and arabitol. Moreover, metabolic approaches are highlighted that specifically target shortcomings of sugar alcohol production by yeasts from these renewable substrates.

Osmolyte Signatures for the Protection of Aspergillus sydowii Cells under Halophilic Conditions and Osmotic Shock

Aspergillus sydowii is a moderate halophile fungus extensively studied for its biotechnological potential and halophile responses, which has also been reported as a coral reef pathogen. In a recent publication, the transcriptomic analysis of this fungus, when growing on wheat straw, showed that genes related to cell wall modification and cation transporters were upregulated under hypersaline conditions but not under 0.5 M NaCl, the optimal salinity for growth in this strain. This led us to study osmolyte accumulation as a mechanism to withstand moderate salinity. In this work, we show that A. sydowii accumulates trehalose, arabitol, mannitol, and glycerol with different temporal dynamics, which depend on whether the fungus is exposed to hypo- or hyperosmotic stress. The transcripts coding for enzymes responsible for polyalcohol synthesis were regulated in a stress-dependent manner. Interestingly, A. sydowii contains three homologs (Hog1, Hog2 and MpkC) of the Hog1 MAPK, the master regulator of hyperosmotic stress response in S. cerevisiae and other fungi. We show a differential regulation of these MAPKs under different salinity conditions, including sustained basal Hog1/Hog2 phosphorylation levels in the absence of NaCl or in the presence of 2.0 M NaCl, in contrast to what is observed in S. cerevisiae. These findings indicate that halophilic fungi such as A. sydowii utilize different osmoadaptation mechanisms to hypersaline conditions.

Recent advances in the biotechnological production of erythritol and mannitol

Dietary habits that include an excess of added sugars have been strongly associated with an increased risk of obesity, heart disease, diabetes, and tooth decay. With this association in view, modern food systems aim to replace added sugars with low calorie sweeteners, such as polyols. Polyols are generally not carcinogenic and do not trigger a glycemic response. Furthermore, owing to the absence of the carbonyl group, they are more stable compared to monosaccharides and do not participate in Maillard reactions. As such, since polyols are stable at high temperatures, and they do not brown or caramelize when heated. Therefore, polyols are widely used in the diets of hypocaloric and diabetic patients, as well as other specific cases where controlled caloric intake is required. In recent years, erythritol and mannitol have gained increased importance, especially in the food and pharmaceutical industries. In these areas, research efforts have been made to improve the productivity and yield of the two polyols, relying on biotechnological manufacturing methods. The present review highlights the recent advances in the biotechnological production of erythritol and mannitol and summarizes the benefits of using the two polyols in the food and pharmaceutical industries.

Erythritol: Another C4 Platform Chemical in Biomass Refinery

The potential of erythritol as a platform chemical in biomass refinery is discussed in terms of erythritol production and utilization. Regarding erythritol production, fermentation of sugar or starch has been already commercialized. The shift of the carbon source from glucose to inexpensive inedible waste glycerol is being investigated, which will decrease the price of erythritol. The carbon-based yield of erythritol from glycerol is comparable to or even higher than that from glucose. The metabolic pathway of erythritol biosynthesis has become clarified: erythrose-4-phosphate, which is one of the intermediates in the pentose phosphate pathway, is dephosphorylated and reduced to erythritol. The information about the metabolic pathway may give insights to improve the productivity by bleeding. Regarding erythritol utilization, chemical conversions of erythritol, especially deoxygenation, have been investigated in these days. Erythritol is easily dehydrated to 1,4-anhydroerythritol, which can be also used as the substrate for production of useful C4 chemicals. C-O hydrogenolysis and deoxydehydration using heterogeneous catalysts are effective reactions for erythritol/1,4-anhydroerythritol conversion.

Microbial conversion of xylose into useful bioproducts

Microorganisms can produce a number of different bioproducts from the sugars in plant biomass. One challenge is devising processes that utilize all of the sugars in lignocellulosic hydrolysates. D-xylose is the second most abundant sugar in these hydrolysates. The microbial conversion of D-xylose to ethanol has been studied extensively; only recently, however, has conversion to bioproducts other than ethanol been explored. Moreover, in the case of yeast, D-xylose may provide a better feedstock for the production of bioproducts other than ethanol, because the relevant pathways are not subject to glucose-dependent repression. In this review, we discuss how different microorganisms are being used to produce novel bioproducts from D-xylose. We also discuss how D-xylose could be potentially used instead of glucose for the production of value-added bioproducts.

Enhancement of erythritol production by Trichosporonoides oedocephalis ATCC 16958 through regulating key enzyme activity and the NADPH/NADP ratio with metal ion supplementation

Erythritol, a well-known natural sweetener, is mainly produced by microbial fermentation. Various metal ions (Al³⁺, Cu²⁺, Mn²⁺, and Ni²⁺) were added to the culture medium of Trichosporonoides oedocephalis ATCC 16958 at 30 mg/L in shake flask cultures. Compared with controls, Cu²⁺ increased the erythritol content by 86% and decreased the glycerol by-product by 31%. After 48 hr of shake flask culture, sodium dodecyl sulfate polyacrylamide gel electrophoresis showed that expression levels of erythrose reductase (ER) in the presence of 30 mg/L CuSO₄ · 5H₂O were higher than those obtained after treatment with other examined metal ions. Furthermore, after 108 hr of batch culture in a 5-L bioreactor, supplementation with 30 mg/L of CuSO₄ · 5H₂O increased the specific erythritol content by 27%. Further studies demonstrated that ER activity under 30 mg/L CuSO₄ · 5H₂O supplementation in a fermentor was overtly increased compared with the control after 60 hr, while glycerol-3-phosphate dehydrogenase activity was clearly reduced in most of the fermentation process. Furthermore, the NADPH/NADP ratio was slightly lower in T. oedocephalis cells treated with Cu²⁺ compared with control cells. These results provide further insights into Cu²⁺ effects on erythritol biosynthesis in T. oedocephalis and should improve the industrial production of erythritol by biological processes.

Complementary function of two transketolase isoforms from Moniliella megachiliensis in relation to stress response

Two transketolase isogenes, MmTKL1 and MmTKL2, isolated from Moniliella megachiliensis were investigated for their roles in stress response and erythritol biosynthesis. The encoded proteins were highly homologous in amino acid sequence and domain structure. Two stress response elements (STREs) were found upstream of MmTKL1, while no STRE was found upstream of MmTKL2. In contrast, two Ap-1 elements were present upstream of MmTKL2, but none were detected upstream of MmTKL1. MmTKL2 partially complemented the aromatic amino acid auxotrophy of a Saccharomyces cerevisiae tkl1 deletion mutant, suggesting that at least one of the MmTKLs functioned as a transketolase in vivo. In response to short-term osmotic stress (20% glucose or 1.2 M NaCl) in Moniliella cells, MmTKL1 expression increased rapidly through the first 40 min before subsequently decreasing gradually, while MmTKL2 expression showed no significant change. In contrast, short-term oxidative stress (0.15 mM menadione) induced considerable increases in MmTKL2, while MmTKL1 expression remained low under the same conditions. Long-term osmotic stress (20% glucose) yielded increased expression of both genes starting at 12 h and continuing through 72 h. During either osmotic or oxidative stress, intracellular erythritol accumulation could clearly be correlated with the pattern of expression of either MmTKL1 or MmTKL2. These results strongly suggested that MmTKL1 is responsible primarily for the response to osmotic stress, while MmTKL2 is responsible primarily for the response to oxidative stress. Thus, we postulate that the two transketolase isoforms of M. megachiliensis play distinct and complementary roles in coordinating erythritol production in response to distinct environmental stresses.

Transformation of the yeast Trichosporonoides oedocephalis

The osmotolerant yeast, Trichosporonoides oedocephalis, is an excellent producer of erythritol, which has wide industrial applications. In this study, we developed an efficient transformation method for T. oedocephalis. To evaluate the T. oedocephalis transformation, we constructed a DNA fragment (loxP-Kan-loxP/Cre system) that was targeted to the mitogen-activated protein kinase HOG1 gene. Transformants were selected on plates containing G418 and response surface methodology was employed to obtain optimum transformation conditions. Optimal transformation could be achieved at an incubation time of 40 min, when the concentration of zymolyase-100T was 30 µg/mL, and when 100 mM CaCl2 was added to the mixture. The predicted optimal transformation efficiency was 133 transformants per µg of DNA. This novel method will facilitate studies in synthetic biology and metabolic engineering of T. oedocephalis.

Erythritol production by Moniliella megachiliensis using nonrefined glycerol waste as carbon source

The number of naphtha plants is being reduced due to a worldwide shift in energy sources. Consequently, a shortage of chemical materials heavily dependent on naphtha-oil, especially C4 compounds such as butene and butane-diol, is an urgent issue in chemical manufacturing. Erythritol is a rare C4 compound produced by fermentation processes using glucose as the carbon source. Since erythritol is considerably more expensive than hydrocarbons derived from naphtha-oil, a reduction in its cost is critical. We found that Moniliella megachiliensis, a highly osmotolerant yeast strain, can utilize nonrefined glycerol waste derived from palm oil or beef tallow and convert it to erythritol. Cell growth on glycerol was almost the same as on glucose, and the cells could grow in up to 300 mg ml(-1) glycerol. When 200 mg ml(-1) nonrefined glycerol was supplied, the yield of erythritol from the glycerol was approx. 60%, which is slightly higher than that obtained using glucose. The cost of glycerol waste is considerably lower than that of glucose. Thus, the conversion of glycerol waste into valuable erythritol, proposed here, is attractive and promising from the viewpoint of ensuring a supply of C4 hydrocarbons and utilizing a waste natural resource.

SIGNIFICANCE AND IMPACT OF THE STUDY

A shortage of C4 hydrocarbon depending much on naptha-oil has become urgent problem due to rapid reduction of naphtha plants together with global energy revolution. Erythritol, obtained by fermentation, is a rare C4 polyol that can be converted to C4 hydrocarbons. Erythritol is considerably expensive than hydrocarbons, a reduction in cost is critical issue. To meet this, we proposed to utilize low-cost glycerol waste from bio-diesel fuel as a carbon source. Moniliella megachiliensis successfully converted glycerol waste to erythritol. This proposal is promising to obtain C4 hydrocarbon substitute, and concomitantly to dispose a large amount of glycerol waste discharged.