AEM Accepts, published online ahead of print on 6 November 2009

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Appl. Environ. Microbiol. doi:10.1128/AEM.02273-09
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Metabolic engineering of fungal strains for the conversion of D-galacturonate to meso-galactarate

Dominik Mojzita, Marilyn Wiebe, Satu Hilditch, Harry Boer, Merja Penttilä, and Peter Richard^*

VTT Technical Research Centre of Finland, Espoo, Finland

^* To whom correspondence should be addressed. Email: Peter.Richard{at}vtt.fi.

D-Galacturonic acid can be obtained by hydrolysing 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 catabolised 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 catalysing 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.