Pullulan biosynthesis in yeast-like fungal cells is regulated by the transcriptional activator Msn2 and cAMP-PKA signaling pathway

Advances in Aureobasidium research: Paving the path to industrial utilization

We here explore the potential of the fungal genus Aureobasidium as a prototype for a microbial chassis for industrial biotechnology in the context of a developing circular bioeconomy. The study emphasizes the physiological advantages of Aureobasidium, including its polyextremotolerance, broad substrate spectrum, and diverse product range, making it a promising candidate for cost-effective and sustainable industrial processes. In the second part, recent advances in genetic tool development, as well as approaches for up-scaled fermentation, are described. This review adds to the growing body of scientific literature on this remarkable fungus and reveals its potential for future use in the biotechnological industry.

Optimization of pullulan production by Aureobasidium pullulans using semi-solid-state fermentation and artificial neural networks: Characterization and antibacterial activity of pullulan impregnated with Ag-TiO2 nanocomposite

This study presents a novel and efficient approach for pullulan production using artificial neural networks (ANNs) to optimize semi-solid-state fermentation (S-SSF) on faba bean biomass (FBB). This method achieved a record-breaking pullulan yield of 36.81 mg/g within 10.82 days, significantly exceeding previous results. Furthermore, the study goes beyond yield optimization by characterizing the purified pullulan, revealing its unique properties including thermal stability, amorphous structure, and antioxidant activity. Energy-dispersive X-ray spectroscopy and scanning electron microscopy confirmed its chemical composition and distinct morphology. This research introduces a groundbreaking combination of ANNs and comprehensive characterization, paving the way for sustainable and cost-effective pullulan production on FBB under S-SSF conditions. Additionally, the study demonstrates the successful integration of pullulan with Ag@TiO2 nanoparticles during synthesis using Fusarium oxysporum. This novel approach significantly enhances the stability and efficacy of the nanoparticles by modifying their surface properties, leading to remarkably improved antibacterial activity against various human pathogens. These findings showcase the low-cost production medium, and extensive potential of pullulan not only for its intrinsic properties but also for its ability to significantly improve the performance of nanomaterials. This breakthrough opens doors to diverse applications in various fields.

Hyper-Production of Pullulan by a Novel Fungus of Aureobasidium melanogenum ZH27 through Batch Fermentation

Pullulan, which is a microbial exopolysaccharide, has found widespread applications in foods, biomedicines, and cosmetics. Despite its versatility, most wild-type strains tend to yield low levels of pullulan production, and their mutants present genetic instability, achieving a limited increase in pullulan production. Therefore, mining new wild strains with robust pullulan-producing abilities remains an urgent concern. In this study, we found a novel strain, namely, Aureobasidium melanogenum ZH27, that had a remarkable pullulan-producing capacity and optimized its cultivation conditions using the one-factor-at-a-time method. To elucidate the reasons that drove the hyper-production of pullulan, we scrutinized changes in cell morphology and gene expressions. The results reveal that strain ZH27 achieved 115.4 ± 1.82 g/L pullulan with a productivity of 0.87 g/L/h during batch fermentation within 132 h under the optimized condition (OC). This pullulan titer increased by 105% compared with the initial condition (IC). Intriguingly, under the OC, swollen cells featuring 1-2 large vacuoles predominated during a rapid pullulan accumulation, while these swollen cells with one large vacuole and several smaller ones were prevalent under the IC. Moreover, the expressions of genes associated with pullulan accumulation and by-product synthesis were almost all upregulated. These findings suggest that swollen cells and large vacuoles may play pivotal roles in the high level of pullulan production, and the accumulation of by-products also potentially contributes to pullulan synthesis. This study provides a novel and promising candidate for industrial pullulan production.

Improved 2,3-Butanediol Production Rate of Metabolically Engineered Saccharomyces cerevisiae by Deletion of RIM15 and Activation of Pyruvate Consumption Pathway

Saccharomyces cerevisiae is a promising host for the bioproduction of higher alcohols, such as 2,3-butanediol (2,3-BDO). Metabolically engineered S. cerevisiae strains that produce 2,3-BDO via glycolysis have been constructed. However, the specific 2,3-BDO production rates of engineered strains must be improved. To identify approaches to improving the 2,3-BDO production rate, we investigated the factors contributing to higher ethanol production rates in certain industrial strains of S. cerevisiae compared to laboratory strains. Sequence analysis of 11 industrial strains revealed the accumulation of many nonsynonymous substitutions in RIM15, a negative regulator of high fermentation capability. Comparative metabolome analysis suggested a positive correlation between the rate of ethanol production and the activity of the pyruvate-consuming pathway. Based on these findings, RIM15 was deleted, and the pyruvate-consuming pathway was activated in YHI030, a metabolically engineered S. cerevisiae strain that produces 2,3-BDO. The titer, specific production rate, and yield of 2,3-BDO in the test tube-scale culture using the YMS106 strain reached 66.4 ± 4.4 mM, 1.17 ± 0.017 mmol (g dry cell weight h)-1, and 0.70 ± 0.03 mol (mol glucose consumed)-1. These values were 2.14-, 2.92-, and 1.81-fold higher than those of the vector control, respectively. These results suggest that bioalcohol production via glycolysis can be enhanced in a metabolically engineered S. cerevisiae strain by deleting RIM15 and activating the pyruvate-consuming pathway.

Regulation of cell differentiation to promote pullulan synthesis in Aureobasidium pullulans NG

Pullulan is a polymer produced by Aureobasidium spp. The yield of pullulan production can be impacted by the cellular differentiation of Aureobasidium spp., which changes with alterations in the growth environment. To improve pullulan yield, identifying key factors that regulate cellular differentiation is crucial. In this study, the main form of pullulan synthesis in Aureobasidium pullulans NG was through swollen cells (SC). The results showed that citric acid (CA) can regulate the cellular differentiation of Aureobasidium pullulans NG by accumulating higher levels of CA in the cells to maintain growth in SC form and increase pullulan production. The addition of 1.0% CA to Aureobasidium pullulans NG for 96 h resulted in a significant increase in pullulan production, producing 18.32 g/l compared to the control group which produced 10.23 g/l. Our findings suggest that controlling cellular differentiation using CA is a promising approach for enhancing pullulan production in Aureobasidium pullulans. KEY POINTS: • The regulation of cell differentiation in Aureobasidium pullulans NG is demonstrated to be influenced by citric acid. • Intracellular citric acid levels in Aureobasidium pullulans NG have been shown to support the growth of swollen cells. • Citric acid has been found to increase pullulan production in Aureobasidium pullulans NG.

Glucose Starvation Stimulates the Promoting Strength of a Novel Evolved Suc2 Promoter

Promoters are essential for designing Saccharomyces cerevisiae cell factories. Identifying novel promoters tuned to produce specific metabolites under increasingly diverse industrial stresses is required to improve the economic feasibility of whole fermentation processes. In this study, a positively evolved Suc2 promoter (SUC 2p) with promoter activity stronger than that of the wild-type Suc2 promoter (SUC 2wtp) was obtained. Quantitative real-time PCR, fluorescence analysis, Western blotting, and a β-galactosidase activity assay revealed that SUC 2p is a medium-strength promoter compared with eight reported promoters at a medium glucose concentration (2% (w/v)). Different glucose concentrations modulated the strength of SUC 2p. Low glucose concentrations (0.05 and 0.5% (w/v)) enhanced the promoter strength of SUC 2p dramatically, with promoter activity higher than that of reported strong promoters. Glucose starvation resulted in the formation of a new Msn2/4 binding site on SUC 2p. Our work should facilitate the development of promoters with novel fine-tuning properties and the construction of S. cerevisiae cell factories suitable for the industrial production of essential chemicals under glucose-deprived conditions.

Identification of the Key Enzymes in WL Gum Biosynthesis and Critical Composition in Viscosity Control

As an important microbial exopolysaccharide, the sphingan WL gum could be widely used in petroleum, food, and many other fields. However, its lower production is still limiting its wider application. Therefore, to gain insights into the bottlenecks of WL gum production by identifying the key enzymes in the WL gum biosynthesis pathway, more than 20 genes were over-expressed in Sphingomonas sp. WG and their effects on WL gum production and structure were investigated. Compared to the control strain, the WL gum production of welB over-expression strain was increased by 19.0 and 21.0% at 36 and 84 h, respectively. The WL gum production of both atrB and atrD over-expression strains reached 47 g/L, which was approximately 34.5% higher than that of the control strain at 36 h. Therefore, WelB, AtrB, and AtrD may be the key enzymes in WL production. Interestingly, the broth viscosity of most over-expression strains decreased, especially the welJ over-expression strain whose viscosity decreased by 99.3% at 84 h. Polysaccharides' structural features were investigated to find the critical components in viscosity control. The uronic acid content and total sugar content was affected by only a few genes, therefore, uronic acid and total sugar content may be not the key composition. In comparison, the acetyl degrees were enhanced by over-expression of most genes, which meant that acetyl content may be the critical factor and negatively correlated with the apparent viscosity of WL gum. This work provides useful information on the understanding of the bottlenecks of WL gum biosynthesis and will be helpful for the construction of high WL gum-yielding strains and rheological property controlling in different industries.

The signaling pathways involved in metabolic regulation and stress responses of the yeast-like fungi Aureobasidium spp

Aureobasidium spp. can use a wide range of substrates and are widely distributed in different environments, suggesting that they can sense and response to various extracellular signals and be adapted to different environments. It is true that their pullulan, lipid and liamocin biosynthesis and cell growth are regulated by the cAMP-PKA signaling pathway; Polymalate (PMA) and pullulan biosynthesis is controlled by the Ca²⁺ and TORC1 signaling pathways; the HOG1 signaling pathway determines high osmotic tolerance and high pullulan and liamocin biosynthesis; the Snf1/Mig1 pathway controls glucose repression on pullulan and liamocin biosynthesis; DHN-melanin biosynthesis and stress resistance are regulated by the CWI signaling pathway and TORC1 signaling pathway. In addition, the HSF1 pathway may control cell growth of some novel strains of A. melanogenum at 37 °C. However, the detailed molecular mechanisms of high temperature growth and thermotolerance of some novel strains of A. melanogenum and glucose derepression in A. melanogenum TN3-1 are still unclear.

The GATA type transcriptional factors regulate pullulan biosynthesis in Aureobasidium melanogenum P16

Aureobasidium melanogenum P16, the high pullulan producer, had only one GATA type transcriptional activator AreA and one GATA type transcriptional repressor AreB. It was found that 2.4 g/L of (NH₄)₂SO₄ had obvious nitrogen repression on pullulan biosynthesis by A. melanogenum P16. Removal of the AreB gene could make the disruptant DA6 produce 34.8 g/L pullulan while the P16 strain only produced 28.8 g/L pullulan at the efficient nitrogen condition. Further both removal of the native AreA gene and overexpression of the mutated AreA_S628-S678 gene with non-phosphorylatable residues could render the transformant DEA12 to produce 39.8 g/L pullulan. The transcriptional levels of most of the genes related to pullulan biosynthesis in the transformant DEA12 were greatly enhanced. The mutated AreA_S628-S678 was localized in the nuclei of the transformant DEA12 while the native AreA was distributed in the cytoplasm in A. melanogenum P16. This meant that nitrogen repression on pullulan biosynthesis in the transformant DEA12 was indeed significantly relieved. This was the first time to report that the GATA type transcriptional factors of nitrogen catabolite repression system could regulate pullulan biosynthesis in Aureobasidium spp.

Transcription Factor MaMsn2 Regulates Conidiation Pattern Shift under the Control of MaH1 through Homeobox Domain in Metarhizium acridum

The growth pattern of filamentous fungi can switch between hyphal radial polar growth and non-polar yeast-like cell growth depending on the environmental conditions. Asexual conidiation after radial polar growth is called normal conidiation (NC), while yeast-like cell growth is called microcycle conidiation (MC). Previous research found that the disruption of MaH1 in Metarhizium acridum led to a conidiation shift from NC to MC. However, the regulation mechanism is not clear. Here, we found MaMsn2, an Msn2 homologous gene in M. acridum, was greatly downregulated when MaH1 was disrupted (ΔMaH1). Loss of MaMsn2 also caused a conidiation shift from NC to MC on a nutrient-rich medium. Yeast one-hybrid (Y1H) and electrophoretic mobility shift assay (EMSA) showed that MaH1 could bind to the promoter region of the MaMsn2 gene. Disrupting the interaction between MaH1 and the promoter region of MaMsn2 significantly downregulated the transcription level of MaMsn2, and the overexpression of MaMsn2 in ΔMaH1 could restore NC from MC of ΔMaH1. Our findings demonstrated that MaMsn2 played a role in maintaining the NC pattern directly under the control of MaH1, which revealed the molecular mechanisms that regulated the conidiation pattern shift in filamentous fungi for the first time.

Liamocins biosynthesis, its regulation in Aureobasidium spp., and their bioactivities

Liamocins synthesized by Aureobasidium spp. are glycolipids composed of a single mannitol or arabitol headgroup linked to either three, four or even six 3,5-dihydroxydecanoic ester tail-groups. The highest titer of liamocin achieved was over 40.0 g/L. The substrates for liamocins synthesis include glucose, sucrose, xylose, mannitol, and others. The Pks1 is responsible for the biosynthesis of the tail-group 3,5-dihydroxydecanoic acid, both mannitol dehydrogenase (MDH) and mannitol 1-phosphate 5-dehydrogenase (MPDH) catalyze the mannitol biosynthesis and the arabitol biosynthesis is controlled by arabitol dehydrogenase (ArDH). The ester bond formation between 3,5-dihydroxydecanoic acid and mannitol or arabitol is catalyzed by the esterase (Est1). Liamocin biosynthesis is regulated by the specific transcriptional activator (Gal1), global transcriptional activator (Msn2), various signaling pathways, acetyl-CoA flux while Pks1 activity is controlled by PPTase activity. The synthesized liamocins have high bioactivity against the pathogenic bacteria Streptococcus spp. and some kinds of cancer cells while Massoia lactone released liamocins which exhibited obvious antifungal and anticancer activities. Therefore, liamocins and Massoia lactone have many applications in various sectors of biotechnology.

Total barley maiya alkaloids inhibit prolactin secretion by acting on dopamine D2 receptor and protein kinase A targets

ETHNOPHARMACOLOGICAL RELEVANCE

Barley maiya from gramineous plants (Hordeum vulgare L.) is obtained from ripe fruits through germination and drying. It is often used to treat diseases associated with high prolactin levels.

OBJECTIVE

To investigate the anti-hyperprolactinemia (anti-HPRL) mechanisms of total barley maiya alkaloids (TBMA) and hordenine.

METHODS

This experiment included 9 groups: Normal group, TBMA group, hordenine group, TBMA + haloperidol group, TBMA + forskolin group, TBMA + 8-bromo-cAMP group, hordenine + haloperidol group, hordenine + forskolin group, and hordenine + 8-bromo-cAMP group. The prolactin (PRL) concentration in the supernatant and the total cAMP concentration in the cells were detected by ELISA. The expression levels of PRL, dopamine D2 receptor (DRD2) and cAMP/PKA/CREB protein were measured by Western Blot.

RESULTS

In the TBMA group and the hordenine group, the PRL level in MMQ cells was significantly decreased, but in GH3 cells there was no change. DRD2 expression level was markedly increased, cAMP concentration was decreased, and the activity of PKA and CREB declined in MMQ cells. Compared with the TBMA group, there was a significant decrease of DRD2 expression level, a remarkable increase of PRL secretion and an increase of cAMP/PKA/CREB expression in MMQ cells within the TBMA + haloperidol group. Compared with the forskolin group, there was no significant change in PRL secretion and cAMP/PKA/CREB expression level in MMQ cells within the TBMA + forskolin group. There was a decrease in PRL secretion and cAMP/PKA/CREB expression level in MMQ cells within the TBMA + 8-bromo-cAMP group compared with the 8-bromo-cAMP group. Compared with the hordenine group, DRD2 expression level was significantly decreased, PRL secretion was markedly increased, and cAMP/PKA/CREB expression level was increased in MMQ cells within the hordenine + haloperidol group. There was no significant change in PRL secretion and cAMP/PKA/CREB expression level in MMQ cells within the hordenine + forskolin group compared with the forskolin group and within the hordenine + 8-bromo-cAMP group compared with the 8-bromo-cAMP group.

CONCLUSION

TBMA and hordenine can both play an anti-HPRL role via DRD2, and TBMA can also act on PKA targets to exert its anti-HPRL effect. TBMA and hordenine may be potential treatment strategies for HPRL.

cAMP-PKA and HOG1 signaling pathways regulate liamocin production by different ways via the transcriptional activator Msn2 in Aureobasidium melanogenum

Liamocins, as the secondary metabolites synthesized and secreted by Aureobasidium spp., consist of a single mannitol or a single arabitol head group partially O-acylated with three 3,5-dihydroxydecanoic ester groups or directly esterified with three or four 3,5-dihydroxydecanoic ester tails. Very recently, the whole synthetic pathway of liamocins in A. melanogenum 6-1-2 has been elucidated. It was found that the promoter sequences of all the genes related to liamocin synthesis in A. melanogenum 6-1-2 had stress regulatory elements with core sequences of AGGGG or CCCCT. Therefore, expression of all the genes would be regulated by the Msn2. In this study, it was found that removal of the single one MSN2 gene in A. melanogenum 6-1-2 made the mutant decrease yield of extracellular liamocin by 92.28 %, while complementation of the MSN2 gene in the mutant rendered liamocin synthesis to be restored. When A. melanogenum 6-1-2 was cultured in the liamocin fermentation medium with high glucose and low nitrogen, the Msn2 was localized in the nucleus and positively regulated the expression of the genes related to liamocin biosynthesis. Furthermore, when the key BCY1 gene encoding regulatory subunit of the cAMP-PKA signaling pathway in A. melanogenum 6-1-2 was knocked out, the amount of extracellular liamocins synthesized by the mutant was decreased by 96.73 % and the Msn2 was localized in the cytoplasm. Similarly, when the key HOG1 gene in the HOG1 signaling pathway was deleted, liamocin biosynthesis in the knockout strain was decreased by 98.09 %. However, it was found that the Hog1 may be one part of the general transcription complex to regulate the transcription of the MSN2 gene, leading to the reduced Msn2 and liamocin synthesis in the mutant. In addition, the key TOR1 gene and SNF1 gene in the TOR1 signaling pathway and the SNF1 signaling pathway were not involved in the regulation of the Msn2 activity and liamocin synthesis. It was concluded that the transcriptional activator Msn2, the HOG1 signaling pathway and the cAMP-PKA signaling pathway were involved in the regulation of liamocin biosynthesis and production.