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Geosmin precursor germacradienol is produced by a terpene cyclase from Aspergillus ustus

Affiliation
Institut für Pharmazeutische Biologie und Biotechnologie, Fachbereich Pharmazie, Philipps- Universität Marburg, 35037 Marburg, Germany
Peter, Marlies;
Affiliation
Institut für Pharmazeutische Biologie und Biotechnologie, Fachbereich Pharmazie, Philipps- Universität Marburg, 35037 Marburg, Germany
Yang, Yiling;
Affiliation
Institut für Pharmazeutische Biologie und Biotechnologie, Fachbereich Pharmazie, Philipps- Universität Marburg, 35037 Marburg, Germany
Li, Shu-Ming

The powerful odor geosmin is a non-canonical terpenoid with C12 skeleton which can be smelled after rain fall or in dug earth. Its biosynthesis is conserved in bacteria, a bifunctional sesquiterpene cyclase catalyses both the formation of its intermediate precursor germacradienol and the conversion to geosmin in two subsequent ionisation- and metal-dependent steps starting with the cyclisation of farnesyl diphosphate (FPP). In comparison to a variety of bacterial species, little is known about its biosynthesis in eukaryots like fungi and plants. Terpene cyclases function as geosmin synthases from basidiomycetes are reported but not for ascomycetes. In the genome of the ascomycetous fungus Aspergillus ustus, a putative sesquiterpene cyclase gene gdlS was identified. The respective protein shares 21 % identity on the amino acid level with the N-terminus of the germacradienol/geosmin synthase SCO6073 from Streptomyces coelicolor. Heterologous expression in Aspergillus nidulans proved gdlS coding for a germacradienol synthase, which was also confirmed biochemically. Germacradienol was identified in the enzyme assay as the sole product of GdlS in the presence of Mg2+ ions. Feeding experiments with germacradienol in A. ustus did not show any conversion to geosmin. Together with the results from phylogenetic analysis it can be proposed that the formation of germacradienol and its conversion to geosmin are catalysed in fungi by two distinct enzymes.

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