Lactose and D-galactose catabolism in the filamentous fungus Aspergillus nidulans

Studying microbial triglyceride production from corn stover saccharides unveils insights into the galactose metabolism of Ustilago maydis

The global demand for plant oil has reached unprecedented levels and is relevant in all industrial sectors. Driven by the growing awareness for environmental issues of traditional plant oils and the need for eco-friendly alternatives, microbial oil emerges as a promising product with significant potential. Harnessing the capabilities of oleaginous microorganisms is an innovative approach for achieving sustainable oil production. To increase economic feasibility, it is crucial to explore feedstocks such as agricultural waste streams as renewable resource for microbial bioprocesses. The fungal model Ustilago maydis is one promising organism in the field of microbial triglyceride production. It has the ability to metabolize a wide variety of carbon sources for cell growth and accumulates high amounts of triglycerides intracellularly. In this study we asked whether this large variety of usable carbon sources can also be utilized for triglyceride production, using corn stover saccharides as a showcase.Our experiments revealed metabolization of the major saccharide building blocks present in corn stover, demonstrating the remarkable potential of U. maydis. The microorganism exhibited the capacity to synthesize triglycerides using the saccharides glucose, fructose, sucrose, xylose, arabinose, and galactose as carbon source. Notably, while galactose has been formerly considered as toxic to U. maydis, we found that the fungus can metabolize this saccharide, albeit with an extended lag phase of around 100 hours. We identified two distinct methods to significantly reduce or even prevent this lag phase, challenging previous assumptions and expanding the understanding of U. maydis metabolism.Our findings suggest that the two tested methods can prevent long lag phases on feedstocks with high galactose content and that U. maydis can produce microbial triglycerides very efficiently on many different carbon sources. Looking forward, exploring the metabolic capabilities of U. maydis on additional polymeric components of corn stover and beyond holds promise for innovative applications, marking a significant step toward environmentally sustainable bioprocessing technologies.

GalR, GalX and AraR co-regulate d-galactose and l-arabinose utilization in Aspergillus nidulans

Filamentous fungi produce a wide variety of enzymes in order to efficiently degrade plant cell wall polysaccharides. The production of these enzymes is controlled by transcriptional regulators, which also control the catabolic pathways that convert the released monosaccharides. Two transcriptional regulators, GalX and GalR, control d-galactose utilization in the model filamentous fungus Aspergillus nidulans, while the arabinanolytic regulator AraR regulates l-arabinose catabolism. d-Galactose and l-arabinose are commonly found together in polysaccharides, such as arabinogalactan, xylan and rhamnogalacturonan I. Therefore, the catabolic pathways that convert d-galactose and l-arabinose are often also likely to be active simultaneously. In this study, we investigated the interaction between GalX, GalR and AraR in d-galactose and l-arabinose catabolism. For this, we generated single, double and triple mutants of the three regulators, and analysed their growth and enzyme and gene expression profiles. Our results clearly demonstrated that GalX, GalR and AraR co-regulate d-galactose catabolism in A. nidulans. GalX has a prominent role on the regulation of genes of d-galactose oxido-reductive pathway, while AraR can compensate for the absence of GalR and/or GalX.

Utilization of agro-industrial waste for β-galactosidase production under solid state fermentation using halotolerant Aspergillus tubingensis GR1 isolate

A halotolerant fungal isolate Aspergillus tubingensis GR1 was isolated from the man-made solar saltern located at Khambhat, Gujarat, India, and identified using 28S rDNA partial genome sequencing. This isolate was studied for β-galactosidase production under solid state fermentation using wheat bran and deproteinized acid cheese whey. The influence of various agro-industrial wastes, nitrogen source and other growth conditions on β-galactosidase production was investigated using 'one-factor-at-a-time' approach. Among various variables screened along with wheat bran and deproteinized acid cheese whey as major growth substrate, corn steep liquor and MgSO₄ were found to be most significant. The optimum concentrations of these significant parameters were determined employing the response surface central composite design, revealing corn steep liquor concentration (2 mL) and magnesium sulphate (50 mg) per 5 g of wheat bran and 20 mL of deproteinized acid cheese whey for highest enzyme production (15,936 U/gds). These results suggest the feasibility of industrial large-scale production of β-galactosidase known to be valuable in whey hydrolysis and removal of galactosyl residue from polysaccharide.

L-xylo-3-hexulose reductase is the missing link in the oxidoreductive pathway for D-galactose catabolism in filamentous fungi

In addition to the well established Leloir pathway for the catabolism of d-galactose in fungi, the oxidoreductive pathway has been recently identified. In this oxidoreductive pathway, D-galactose is converted via a series of NADPH-dependent reductions and NAD(+)-dependent oxidations into D-fructose. The pathway intermediates include galactitol, L-xylo-3-hexulose, and d-sorbitol. This study identified the missing link in the pathway, the L-xylo-3-hexulose reductase that catalyzes the conversion of L-xylo-3-hexulose to D-sorbitol. In Trichoderma reesei (Hypocrea jecorina) and Aspergillus niger, we identified the genes lxr4 and xhrA, respectively, that encode the l-xylo-3-hexulose reductases. The deletion of these genes resulted in no growth on galactitol and in reduced growth on D-galactose. The LXR4 was heterologously expressed, and the purified protein showed high specificity for L-xylo-3-hexulose with a K(m) = 2.0 ± 0.5 mm and a V(max) = 5.5 ± 1.0 units/mg. We also confirmed that the product of the LXR4 reaction is D-sorbitol.