The interplay between transport and metabolism in fungal itaconic acid production.

Besides enzymatic conversions, many eukaryotic metabolic pathways also involve transport proteins that shuttle molecules between subcellular compartments, or into the extracellular space. Fungal itaconate production involves two such transport steps, involving an itaconate transport protein (Itp), and a mitochondrial tricarboxylate transporter (Mtt). The filamentous ascomycete Aspergillus terreus and the unicellular basidiomycete Ustilago maydis both produce itaconate, but do so via very different molecular pathways, and under very different cultivation conditions. In contrast, the transport proteins of these two strains are assumed to have a similar function. This study aims to investigate the roles of both the extracellular and mitochondrial transporters from these two organisms by expressing them in the corresponding U. maydis knockouts and monitoring the extracellular product concentrations. Both transporters from A. terreus complemented their corresponding U. maydis knockouts in mediating itaconate production. Surprisingly, complementation with At_MfsA from A. terreus led to a partial switch from itaconate to (S)-2-hydroxyparaconate secretion. Apparently, the export protein from A. terreus has a higher affinity for (S)-2-hydroxyparaconate than for itaconate, even though this species is classically regarded as an itaconate producer. Complementation with At_MttA increased itaconate production by 2.3-fold compared to complementation with Um_Mtt1, indicating that the mitochondrial carrier from A. terreus supports a higher metabolic flux of itaconic acid precursors than its U. maydis counterpart. The biochemical implications of these differences are discussed in the context of the biotechnological application in U. maydis and A. terreus for the production of itaconate and (S)-2-hydroxyparaconate.

Genetic and biochemical insights into the itaconate pathway of Ustilago maydis enable enhanced production.

The Ustilaginaceae family of smut fungi, especially Ustilago maydis, gained biotechnological interest over the last years, amongst others due to its ability to naturally produce the versatile bio-based building block itaconate. Along with itaconate, U. maydis also produces 2-hydroxyparaconate. The latter was proposed to be derived from itaconate, but the underlying biochemistry and associated genes were thus far unknown. Here, we confirm that 2-hydroxyparaconate is a secondary metabolite of U. maydis and propose an extension of U. maydis' itaconate pathway from itaconate to 2-hydroxyparaconate. This conversion is catalyzed by the P450 monooxygenase Cyp3, encoded by cyp3, a gene, which is adjacent to the itaconate gene cluster of U. maydis. By deletion of cyp3 and simultaneous overexpression of the gene cluster regulator ria1, it was possible to generate an itaconate hyper producer strain, which produced up to 4.5-fold more itaconate in comparison to the wildtype without the by-product 2-hydroxyparaconate. By adjusting culture conditions in controlled pulsed fed-batch fermentations, a product to substrate yield of 67% of the theoretical maximum was achieved. In all, the titer, rate and yield of itaconate produced by U. maydis was considerably increased, thus contributing to the industrial application of this unicellular fungus for the biotechnological production of this valuable biomass derived chemical.