Das Cellobioselipid Ustilaginsäure aus Ustilago maydis: Biosynthese und transkriptionelleRegulation

Under conditions of nitrogen starvation the basidiomycetous fungus Ustilago maydis produces large amounts of extracellular glycolipids, ustilagic acids and mannosylerythritollipids. In this work the biosynthetic pathway for Ustilagic acid has been elucidated. A gene cluster, encompassing 12 open reading frames is responsible for ustilagic acid production and is regulated by its own transcriptional regulator. The gene cluster contains two cytochrome P450 monooxygenases, an acyl- and an acetyl-transferase, a fatty acid synthase, a glycosyl transferase, two hydroxylases and an export protein of the ABC-transporter family. To analyze the biosynthetic route of ustilagic acid production, we generated knock-out mutants for all genes and characterized the intermediates of glycolipid biosynthesis by thin layer chromatography and mass spectrometric analysis. Further the transcriptional regulator Rua1 could be analyzed. Rua1 belongs to the family of Cys2His2 zinc finger proteins and is necessary for the regulation of the ustilagic acid gene cluster. Rua1 possesses at the C-terminal end two zinc finger domains which are necessary for DNA binding. Rua1 binds to a conserved DNA motive within the promoter regions of the different cluster genes and activates gene expression. Ustilagic acid has antibiotic activities and is toxic against different microorganisms. Incubation of budding yeast with culture supernatants of U. maydis wild-type and mutant strains showed which part of the molecule is essential for the toxic effect of ustilagic acid. Further we could demonstrate that U. maydis has biocontrol activities against the plant pathogenic fungus Botrytis cinerea which causes gray mold on tomato leaves. Detached tomato leaves were infected with spores of B. cinerea mixed with U. maydis cells. While infection with B. cinerea alone resulted in the formation of necrotic lesions on the tomato leaves, no symptoms of grey mold disease were observed in the presence of U. maydis wild-type cells. This indicates that U. maydis cells act antagonistically against B. cinerea spores. Most remarkably, U. maydis cells, which are unable to secrete UA, were not able to prevent infection. We take this as evidence that UA secreted by U. maydis exerts an antagonistic effect on B. cinerea. Therefore, U. maydis might be suited to serve as biocontrol agent against the plant pathogenic fungus B. cinerea.