d-Galacturonic acid can be obtained by hydrolyzing pectin, which is an abundant and low value raw material. By means of metabolic engineering, we constructed fungal strains for the conversion of d-galacturonate to meso-galactarate (mucate). Galactarate has applications in food, cosmetics, and pharmaceuticals and as a platform chemical. In fungi d-galacturonate is catabolized through a reductive pathway with a d-galacturonate reductase as the first enzyme. Deleting the corresponding gene in the fungi Hypocrea jecorina and Aspergillus niger resulted in strains unable to grow on d-galacturonate. The genes of the pathway for d-galacturonate catabolism were upregulated in the presence of d-galacturonate in A. niger, even when the gene for d-galacturonate reductase was deleted, indicating that d-galacturonate itself is an inducer for the pathway. A bacterial gene coding for a d-galacturonate dehydrogenase catalyzing the NAD-dependent oxidation of d-galacturonate to galactarate was introduced to both strains with disrupted d-galacturonate catabolism. Both strains converted d-galacturonate to galactarate. The resulting H. jecorina strain produced galactarate at high yield. The A. niger strain regained the ability to grow on d-galacturonate when the d-galacturonate dehydrogenase was introduced, suggesting that it has a pathway for galactarate catabolism.
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