Aspergillus niger is an industrially approved citric acid cell factory. Very high product concentrations at very low pH are easily achieved and large scale production processes are in place. This organism appears therefore as ideal candidate for itaconic acid production, as citric acid is a precursor of itaconic acid. However, the respective metabolic engineering appears somewhat tricky in this case, as one has to deal with different cellular compartments. For example, expression of cis-aconitic acid decarboxylase (CAD) in the mitochondrium leads to improved itaconic acid production in comparison to cytosolic expression. Furthermore, it is unclear how the acids are transported across the mitochondrial and cellular membranes. The natural itaconic acid producer Aspergillus terreusmust express efficient transporters. Two candidate genes have been identified as part of an itaconic acid production related gene cluster. We analysed these transporters in more detail and could show a very significant effect of the mitochondrial transporter on itaconic acid production.
A further striking result is that engineering of different microbial cell factories for production of itaconic acid leads to strikingly different results. While, at first sight the central carbon metabolism of many organisms appears to be quite similar, the metabolic networks react quite different to the introduction of CAD. Robustness of the networks appears to be achieved by very different mechanisms.