8-37: A point mutation in the xylose reductase gene increases citric acid production in Aspergillus carbonarius

The filamentous fungus Aspergillus carbonarius was identified as a good natural biocatalyst for production of organic acids as well as interesting secondary metabolites through a screening program and is selected for pathway engineering. This includes engineering of the central carbon metabolism to improve the conversion of the renewable carbon sources to pyruvate and improving the flux into selected organic acids. The present work focuses on changing the regulation of the C-5 sugar conversion as efficient simultaneous conversion of C-5 and C-6 sugars into high value products has a high interest for industrial utilization of lignocellulosic biomass.  A. carbonarius is able to utilize D-xylose as sole carbon source for the production of citric acid, and in eukaryotes in general, the conversion of D-xylose takes place in the Pentose Catabolic Pathway starting with the conversion of D-xylose to D-xylulose in a 2 step reaction that involve the NADPH dependent xylose reductase and the NAD dependent xylitol dehydrogenase. We have introduced a single amino acid substitution in the xylose reductase gene of a strain of A. carbonarius, which resulted in an increase in citric acid production. The mutant showed elevated citric acid production and reduced xylitol accumulation compared to the wild type strain in preliminary fermentation tests on D-xylose media. However, further investigation is required in order to conclude the effect of the point mutation in the xylose reductase of the fungus.