Constitutive homologous expression of phosphoglucomutase and transaldolase increases the metabolic flux of Fusarium oxysporum.

BACKGROUND: Fusarium oxysporum is among the few filamentous fungi that have been reported of being able to directly ferment biomass to ethanol in a consolidated bioprocess. Understanding its metabolic pathways and their limitations can provide some insights on the genetic modifications required to enhance its growth and subsequent fermentation capability. In this study, we investigated the hypothesis reported previously that phosphoglucomutase and transaldolase are metabolic bottlenecks in the glycolysis and pentose phosphate pathway of the F. oxysporum metabolism. RESULTS: Both enzymes were homologously overexpressed in F. oxysporum F3 using the gpdA promoter of Aspergillus nidulans for constitutive expression. Transformants were screened for their phosphoglucomutase and transaldolase genes expression levels with northern blot. The selected transformant exhibited high mRNA levels for both genes, as well as higher specific activities of the corresponding enzymes, compared to the wild type. It also displayed more than 20 and 15% higher specific growth rate upon aerobic growth on glucose and xylose, respectively, as carbon sources and 30% higher biomass to xylose yield. The determination of the relative intracellular amino and non-amino organic acid concentrations at the end of growth on glucose revealed higher abundance of most determined metabolites between 1.5- and 3-times in the recombinant strain compared to the wild type. Lower abundance of the determined metabolites of the Krebs cycle and an 68-fold more glutamate were observed at the end of the cultivation, when xylose was used as carbon source. CONCLUSIONS: Homologous overexpression of phosphoglucomutase and transaldolase in F. oxysporum was shown to enhance the growth characteristics of the strain in both xylose and glucose in aerobic conditions. The intracellular metabolites profile indicated how the changes in the metabolome could have resulted in the observed growth characteristics.

Fungal multienzyme production on industrial by-products of the citrus-processing industry.

Orange peels is the principal solid by-product of the citrus processing industry and the disposal of the fresh peels is becoming a major problem to many factories. Dry citrus peels are rich in pectin, cellulose and hemicellulose and may be used as a fermentation substrate. Production of multienzyme preparations containing pectinolytic, cellulolytic and xylanolytic enzymes by the mesophilic fungi Aspergillus niger BTL, Fusarium oxysporum F3, Neurospora crassa DSM 1129 and Penicillium decumbens under solid-state fermentation (SSF) on dry orange peels was enhanced by optimization of initial pH of the culture medium and initial moisture level. Under optimal conditions A. niger BTL was by far the most potent strain in polygalacturonase and pectate lyase, production followed by F. oxysporum F3, N. crassa DSM 1129 and P. decumbens. N. crassa DSM 1129 produced the highest endoglucanase activity and P. decumbens the lowest one. Comparison of xylanase production revealed that A. niger BTL produced the highest activity followed by N. crassa DSM 1129, P. decumbens and F. oxysporum F3. N. crassa DSM 1129 and P. decumbens did not produce any beta-xylosidase activity, while A. niger BTL produced approximately 10 times more beta-xylosidase than F. oxysporum F3. The highest invertase activity was produced by A. niger BTL while the lowest ones by F. oxysporum F3 and P. decumbens. After SSF of the four fungi, under optimal conditions, the fermented substrate was either directly exposed to autohydrolysis or new material was added, and the in situ produced multienzyme systems were successfully used for the partial degradation of orange peels polysaccharides and the liberation of fermentable sugars.

Metabolic Engineering of Fusarium oxysporum to Improve Its Ethanol-Producing Capability.

Fusarium oxysporum is one of the few filamentous fungi capable of fermenting ethanol directly from plant cell wall biomass. It has the enzymatic toolbox necessary to break down biomass to its monosaccharides and, under anaerobic and microaerobic conditions, ferments them to ethanol. Although these traits could enable its use in consolidated processes and thus bypass some of the bottlenecks encountered in ethanol production from lignocellulosic material when Saccharomyces cerevisiae is used-namely its inability to degrade lignocellulose and to consume pentoses-two major disadvantages of F. oxysporum compared to the yeast-its low growth rate and low ethanol productivity-hinder the further development of this process. We had previously identified phosphoglucomutase and transaldolase, two major enzymes of glucose catabolism and the pentose phosphate pathway, as possible bottlenecks in the metabolism of the fungus and we had reported the effect of their constitutive production on the growth characteristics of the fungus. In this study, we investigated the effect of their constitutive production on ethanol productivity under anaerobic conditions. We report an increase in ethanol yield and a concomitant decrease in acetic acid production. Metabolomics analysis revealed that the genetic modifications applied did not simply accelerate the metabolic rate of the microorganism; they also affected the relative concentrations of the various metabolites suggesting an increased channeling toward the chorismate pathway, an activation of the γ-aminobutyric acid shunt, and an excess in NADPH regeneration.

Constitutive expression, purification and characterization of a phosphoglucomutase from Fusarium oxysporum.

The phosphoglucomutase gene from a wild type Fusarium oxysporum strain (F3), was homologously expressed, under the control of the constitutive promoter of gpdA of Aspergillus nidulans. The transformant produced elevated levels of phosphoglucomutase activity compared to the wild type, a fact that facilitated the subsequent purification procedure. The enzyme (FoPGM) was purified to homogeneity applying three anion exchange and one gel filtration chromatography steps. The native enzyme revealed a monomeric structure with a molecular mass of 60 kDa, while the isoelectric point was 3.5. FoPGM was active in pH ranged from 6.0 to 8.0, with an optimum using 3-(N-morpholino)propanesulfonic acid buffer at 7.0, while loss of activity was observed when phosphate buffer was used in the above mentioned pH range. The optimal temperature for activity was 45°C but the enzyme became unstable at temperatures above 40°C. FoPGM requires the presence of a divalent cation for its function with maximum activity being obtained with Co(2+). The apparent K(m) for Co(2+) was found to be 10 µM. The enzyme was also active with other divalent metal ions such as Mn(2+), Mg(2+), Ni(2+) and Ca(2+) but to a lesser extent. The following kinetic constants were determined: v(max), 0.74 µmol mg(protein)(-1)min(-1); k(cat), 44.2 min(-1); K(m)(G1P), 0.10mM; K(m)(G1,6 diP), 1.03 µM; k(cat)/K(m)(G1P), 443 mM(-1)min(-1) and k(cat)/K(m)(G1,6 diP), 42,860 mM(-1)min(-1). The enzyme was considered to follow a Ping Pong substituted enzyme or enzyme isomerization mechanism.