Malic acid is a C4 dicarboxylic acid which is used as an acidulant in food and beverages. It isalso considered as a bio-building block to replace petrochemically derived compounds in thepost oil era. This organic acid can be biotechnologically derived from fermentation usingrenewable feedstocks as carbon source. Aspergilli are among the best producers of organicacid and A. flavus/oryzae is the best natural producer of malic acid.The mechanism of malic acid production in A. oryzae was first assessed by transcriptomeanalysis. A nitrogen starvation response, probably regulated by a transcription factor relatedto the S. cerevisiae Msn2/4 transcriptional activator of stress related genes, was found toresult in high malic acid production. Furthermore the pyruvate carboxylase reaction wasidentified as a metabolic engineering target. This gene, together with the malatedehydrogenase and a malic acid exporter was overexpressed in the strain 2103a-68, whichwas characterized in a second project. The overexpression led to an 80% increase in yieldduring the starvation phase (1.49 mol (mol gluc)-1) and a triplication of the specificproduction rate. The increase in citric acid production in the engineered strain and itsevaluation through model simulations led to the curation of the A. oryzae GEM. The existingmodel was curated with special emphasis on the mitochondrial transport reactions and let toa more defined network around the production of organic acids. Furthermore, theperformance of the strain 2103a-68 on xylose as carbon source was evaluated as well andthe good results led to the final project of manipulating the carbon source utilization bydeleting the carbon catabolite repressor CreA.This work contributed to the understanding of the regulation of malic acid production. Thisknowledge was used for the development of A. oryzae as an organic acid producer throughmetabolic engineering. Furthermore, the evaluation of xylose as an alternative carbonsource paved the way towards the use of lignucellulosic feedstocks and showed thesuitability of A. oryzae for the biorefinery of the future
