The filamentous fungus Aspergillus niger is a well-known hyper-producer of organic acids such as citric acid and oxalic acid. This fungus possesses the cyanide (CN)-insensitive respiration pathway consisting of alternative oxidase (EC 184.108.40.206; AOX), in addition to the cytochrome pathway. Since this CN-insensitive respiration pathway reoxidizes NADH without ATP production, it contributes to continuous glycolysis in A. niger. In this study, to show the availability of aoxA gene encoding AOX as a tool for metabolic engineering, we generated efficient oxalic acid (OA)-producers by genetic engineering of A. niger using aoxA gene. The OA-producing strain EOAH-1, generated by overexpression of the oxaloacetate hydrolase (EC 220.127.116.11; OAH) gene oahA in A. niger WU-2223L, produced 28 g/L OA from 30 g/L glucose during the 9-day cultivation period. Moreover, the strain EAOXOAH-1, generated by overexpression of both aoxA and oahA genes in strain WU-2223L, produced 28 g/L OA during the 7-day cultivation period. Strain EAOXOAH-1 showed higher glucose consumption rate than EOAH-1 did, indicating that overexpression of aoxA contributed to the acceleration of glucose consumption, and that the OA production period was shortened by 2 days. Thus, we clearly show that AOX gene must be an effective tool in metabolic engineering for efficient organic acids production from carbohydrates.
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