The effect of pH on glucoamylase production, glycosylation and chemostat evolution of Aspergillus niger

Enhancement of fatty acid degradation pathway promoted glucoamylase synthesis in Aspergillus niger

Background

Our recent multi-omics analyses of glucoamylase biosynthesis in Aspergillus niger (A. niger) suggested that lipid catabolism was significantly up-regulated during high-yield period under oxygen limitation. Since the catabolism of fatty acids can provide energy compounds such as ATP and important precursors such as acetyl-CoA, we speculated that enhancement of this pathway might be beneficial to glucoamylase overproduction.

Results

Based on previous transcriptome data, we selected and individually overexpressed five candidate genes involved in fatty acid degradation under the control of the Tet-on gene switch in A. niger. Overexpression of the fadE, fadA and cyp genes increased the final specific enzyme activity and total secreted protein on shake flask by 21.3 ~ 31.3% and 16.0 ~ 24.2%, respectively. And a better inducible effect by doxycycline was obtained from early logarithmic growth phase (18 h) than stationary phase (42 h). Similar with flask-level results, the glucoamylase content and total extracellular protein in engineered strains OE-fadE (overexpressing fadE) and OE-fadA (overexpressing fadA) on maltose-limited chemostat cultivation were improved by 31.2 ~ 34.1% and 35.1 ~ 38.8% compared to parental strain B36. Meanwhile, intracellular free fatty acids were correspondingly decreased by 41.6 ~ 44.6%. The metabolomic analysis demonstrated intracellular amino acids pools increased 24.86% and 18.49% in two engineered strains OE-fadE and OE-fadA compared to B36. Flux simulation revealed that increased ATP, acetyl-CoA and NADH was supplied into TCA cycle to improve amino acids synthesis for glucoamylase overproduction.

Conclusion

This study suggested for the first time that glucoamylase production was significantly improved in A. niger by overexpression of genes fadE and fadA involved in fatty acids degradation pathway. Harnessing the intracellular fatty acids could be a strategy to improve enzyme production in Aspergillus niger cell factory.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01966-3.

Glycosylation of Cellulases: Engineering Better Enzymes for Biofuels

Cellulose in plant cell walls is the largest reservoir of renewable carbon on Earth. The saccharification of cellulose from plant biomass into soluble sugars can be achieved using fungal and bacterial cellulolytic enzymes, cellulases, and further converted into fuels and chemicals. Most fungal cellulases are both N- and O-glycosylated in their native form, yet the consequences of glycosylation on activity and structure are not fully understood. Studying protein glycosylation is challenging as glycans are extremely heterogeneous, stereochemically complex, and glycosylation is not under direct genetic control. Despite these limitations, many studies have begun to unveil the role of cellulase glycosylation, especially in the industrially relevant cellobiohydrolase from Trichoderma reesei, Cel7A. Glycosylation confers many beneficial properties to cellulases including enhanced activity, thermal and proteolytic stability, and structural stabilization. However, glycosylation must be controlled carefully as such positive effects can be dampened or reversed. Encouragingly, methods for the manipulation of glycan structures have been recently reported that employ genetic tuning of glycan-active enzymes expressed from homogeneous and heterologous fungal hosts. Taken together, these studies have enabled new strategies for the exploitation of protein glycosylation for the production of enhanced cellulases for biofuel production.

Protein O-glycosylation in fungi: diverse structures and multiple functions

Protein glycosylation is essential for eukaryotic cells from yeasts to humans. When compared to N-glycosylation, O-glycosylation is variable in sugar components and the mode of linkages connecting the sugars. In fungi, secretory proteins are commonly mannosylated by protein O-mannosyltransferase (PMT) in the endoplasmic reticulum, and subsequently glycosylated by several glycosyltransferases in the Golgi apparatus to form glycoproteins with diverse O-glycan structures. Protein O-glycosylation has roles in modulating the function of secretory proteins by enhancing the stability and solubility of the proteins, by affording protection from protease degradation, and by acting as a sorting determinant in yeasts. In filamentous fungi, protein O-glycosylation contributes to proper maintenance of fungal morphology, hyphal development, and differentiation. This review describes recent studies of the structure and function of protein O-glycosylation in industrially and medically important fungi.

In vitro release by Aspergillus fumigatus of galactofuranose antigens, 1,3-beta-D-glucan, and DNA, surrogate markers used for diagnosis of invasive aspergillosis

Aspergillus markers are becoming increasingly important for the early diagnosis of invasive aspergillosis. The kinetics of release of these surrogate markers, however, is largely unknown. We investigated the release of beta-(1-5)-galactofuranosyl (galf) antigens (Platelia Aspergillus), 1,3-beta-D-glucan (BG) (Fungitell), and DNA (PCR) in an in vitro model of Aspergillus fumigatus. The results showed that release is correlated to the growth phase of the fungus, which depends on available nutrients. Whereas galf antigens and BG are released during logarithmic growth, DNA is released only after mycelium breakdown. During early logarithmic growth, galf antigens seem to be released somewhat earlier than BG. Furthermore, galf antigen concentrations of more than 120,000 times the serum cutoff value (0.5 ng/ml) can be measured, while BG concentrations reach a value only 978 times the serum cutoff value (60 pg/ml). During lytical growth, release of galf antigens further increased to a maximum level, which depended on pH. After that, the concentration of galf antigens stayed high (pH 7.4) or decreased to zero within 4 days (pH 5.0). In contrast to galf antigens, BG concentration decreased after 1 day of growth. The decrease of galf components seems to be due to the enzyme beta-galactofuranosidase, which is able to destroy galf epitopes and whose activity fluctuates in the culture filtrates in parallel with galf antigen concentration. Fungal DNA seems to be released only due to autolysis caused by nutrient limitation. In conclusion, several factors clearly influence the release of surrogate markers in vitro. These same factors might also play a role at the infection site of Aspergillus disease in humans.

Fungal glucoamylases

Fungi are employed to produce industrially important glucoamylases. Most glucoamylases are glycosylated. Glycosylation enhances the enzyme stability. Glucoamylases contain both starch binding and catalytic binding domains, the former being responsible for activity on raw (insoluble) starch. Proteases may act on this domain causing the enzyme to lose its activity on insoluble starch. Optimal activity is observed at pH 4.5 to 6.5 and 50 to 70 degrees C. Glucoamylases contain up to 7 sub-sites with highly varying affinity. They can be produced by different methods including submerged, solid state and semi-solid state fermentation processes.

Metabolically independent and accurately adjustable Aspergillus sp. expression system

Filamentous fungi are well-established expression hosts often used to produce extracellular proteins of use in the food and pharmaceutical industries. The expression systems presently used in Aspergillus species rely on either strong constitutive promoters, e.g., that for glyceraldehyde-3-phosphate dehydrogenase, or inducible systems derived from metabolic pathways, e.g., glaA (glucoamylase) or alc (alcohol dehydrogenase). We describe for Aspergillus nidulans and Aspergillus niger a novel expression system that utilizes the transcriptional activation of the human estrogen receptor by estrogenic substances. The system functions independently from metabolic signals and therefore can be used with low-cost, complex media. A combination of positive and negative regulatory elements in the promoter drives the expression of a reporter gene, yielding a linear dose response to the inducer. The off status is completely tight, yet the system responds within minutes to induction and reaches a level of expression of up to 15% of total cell protein after 8 h. Both Aspergillus species are very sensitive to estrogenic substances, and low-cost inducers function in the picomolar concentration range, at which estrogenic substances also can be found in the environment. Given this high sensitivity to estrogens, Aspergillus cells carrying estrogen-responsive units could be used to detect xenoestrogens in food or in the environment.

Detection of circulating galactomannan for the diagnosis and management of invasive aspergillosis

The availability of the Platelia Aspergillus, a sandwich ELISA kit that detects circulating galactomannan, has been a major advance for managing patients at risk for invasive aspergillosis because of the early detection of the antigen. The assay is now widely used throughout the world, including the USA. Although initial studies that assessed the performance characteristics of this assay reported high sensitivity and specificity, more recent studies show significant variation in performance. The causes of this variability are multifactorial and, in large part, cannot be explained because there is insufficient understanding of the kinetics of galactomannan in vivo. We explored some of the factors that affect the release of the aspergillus antigen that bears the epitope that reacts with the monoclonal antibody used in the ELISA, its leakage from the site of infection into the blood, and its binding to substances present in the blood. Factors that affect the detection of antigen in blood are also discussed, most notably the pretreatment procedure aimed at liberating the antigen from immune complexes. Understanding the biology of galactomannan release by aspergillus will greatly enhance our understanding of the kinetics of this and other surrogate markers and allow their optimum use in the management of invasive aspergillosis.

Isolation of a fluffy mutant ofAspergillus niger from chemostat culture and its potential use as a morphologically stable host for protein production

Chemostat cultivation of Aspergillus niger and other filamentous fungi is often hindered by the spontaneous appearance of morphologic mutants. Using the Variomixing bioreactor and applying different chemostat conditions we tried to optimize morphologic stability in both ammonium- and glucose-limited cultures. In most cultivations mutants with fluffy (aconidial) morphology became dominant. From an ammonium-limited culture, a fluffy mutant was isolated and genetically characterized using the parasexual cycle. The mutant contained a single morphological mutation, causing an increased colony radial growth rate. The fluffy mutant was subjected to transformation and finally conidiospores from a forced heterokaryon were shown to be a proper inoculum for fluffy strain cultivation.

Regulation of gene expression by ambient pH in filamentous fungi and yeasts

Life, as we know it, is water based. Exposure to hydroxonium and hydroxide ions is constant and ubiquitous, and the evolutionary pressure to respond appropriately to these ions is likely to be intense. Fungi respond to their environments by tailoring their output of activities destined for the cell surface or beyond to the ambient pH. We are beginning to glimpse how they sense ambient pH and transmit this information to the transcription factor, whose roles ensure that a suitable collection of gene products will be made. Although relatively little is known about pH signal transduction itself, its consequences for the cognate transcription factor are much clearer. Intriguingly, homologues of components of this system mediating the regulation of fungal gene expression by ambient pH are to be found in the animal kingdom. The potential applied importance of this regulatory system lies in its key role in fungal pathogenicity of animals and plants and in its control of fungal production of toxins, antibiotics, and secreted enzymes.