Lena Barra

Conformational Analysis, Thermal Rearrangement, and EI-MS Fragmentation Mechanism of (1(10)E,4E,6S,7R)-Germacradien-6-ol by (13)C-Labeling Experiments.

An uncharacterized terpene cyclase from Streptomyces pratensis was identified as (+)-(1(10)E,4E,6S,7R)-germacradien-6-ol synthase. The enzyme product exists as two interconvertible conformers, resulting in complex NMR spectra. For the complete assignment of NMR data, all fifteen ((13)C1)FPP isotopomers (FPP=farnesyl diphosphate) and ((13)C15)FPP were synthesized and enzymatically converted. The products were analyzed using various NMR techniques, including (13)C, (13)C COSY experiments. The ((13)C)FPP isotopomers were also used to investigate the thermal rearrangement and EI fragmentation of the enzyme product.

Structural Revision and Elucidation of the Biosynthesis of Hypodoratoxide by 13C,13C COSY NMR Spectroscopy

Feeding of (2,3,4,5,6-(13) C5 )mevalonolactone to the fungus Hypomyces odoratus resulted in a completely labeled sesquiterpene ether. The connectivity of the carbon atoms was easily deduced from a (13) C,(13) C COSY spectrum, revealing a structure that was different from the previously reported structure of hypodoratoxide, even though the reported (13) C NMR data matched. A structural revision of hypodoratoxide is thus presented. Its absolute configuration was tentatively assigned from its co-metabolite cis-dihydroagarofuran. Its biosynthesis was investigated by feeding of (3-(13) C)- and (4,6-(13) C2 )mevalonolactone, which gave insights into the complex rearrangement of the carbon skeleton during terpene cyclization by analysis of the (13) C,(13) C couplings.

Pogostol Biosynthesis by the Endophytic Fungus Geniculosporium

Six 13C‐labelled isotopomers of mevalonolactone were synthesised and used in feeding experiments with the endophytic fungus Geniculosporium. The high incorporation rates of 13C‐label into a sesquiterpene that was found in headspace extracts of the fungus enabled unambiguous identification of this volatile as pogostol without the need for compound purification, simply by collecting the volatile fraction with a closed‐loop stripping apparatus followed by direct 13C NMR analysis (CLSA‐NMR). The feeding experiments also gave insights into the biosynthesis of pogostol, including stereochemical aspects of the terpene cyclisation reaction. The possible biological function of pogostol is discussed.

Discovery of a Mosaic-Like Biosynthetic Assembly Line with a Decarboxylative Off-Loading Mechanism through a Combination of Genome Mining and Imaging

The biosynthetic gene cluster for the antiplasmodial natural product siphonazole was identified by using a combination of genome mining, imaging, and expression studies in the natural producer Herpetosiphon sp. B060. The siphonazole backbone is assembled from an unusual starter unit from the shikimate pathway that is extended by the action of polyketide synthases and non-ribosomal peptide synthetases with unusual domain structures, including several split modules and a large number of duplicated domains and domains predicted to be inactive. Product release proceeds through decarboxylation and dehydration independent of the thioesterase SphJ and yields the diene terminus of siphonazole. High variation in terms of codon-usage within the gene cluster, together with the dislocated domain organization, suggest a recent emergence in evolutionary terms.

Evolution and Diversity of Biosynthetic Gene Clusters in Fusarium

Plant pathogenic fungi in the Fusarium genus cause severe damage to crops, resulting in great financial losses and health hazards. Specialized metabolites synthesized by these fungi are known to play key roles in the infection process, and to provide survival advantages inside and outside the host. However, systematic studies of the evolution of specialized metabolite-coding potential across Fusarium have been scarce. Here, we apply a combination of bioinformatic approaches to identify biosynthetic gene clusters (BGCs) across publicly available genomes from Fusarium, to group them into annotated families and to study gain/loss events of BGC families throughout the history of the genus. Comparison with MIBiG reference BGCs allowed assignment of 29 gene cluster families (GCFs) to pathways responsible for the production of known compounds, while for 57 GCFs, the molecular products remain unknown. Comparative analysis of BGC repertoires using ancestral state reconstruction raised several new hypotheses on how BGCs contribute to Fusarium pathogenicity or host specificity, sometimes surprisingly so: for example, a gene cluster for the biosynthesis of hexadehydro-astechrome was identified in the genome of the biocontrol strain Fusarium oxysporum Fo47, while being absent in that of the tomato pathogen F. oxysporum f.sp. lycopersici. Several BGCs were also identified on supernumerary chromosomes; heterologous expression of genes for three terpene synthases encoded on the Fusarium poae supernumerary chromosome and subsequent GC/MS analysis showed that these genes are functional and encode enzymes that each are able to synthesize koraiol; this observed functional redundancy supports the hypothesis that localization of copies of BGCs on supernumerary chromosomes provides freedom for evolutionary innovations to occur, while the original function remains conserved. Altogether, this systematic overview of biosynthetic diversity in Fusarium paves the way for targeted natural product discovery based on automated identification of species-specific pathways as well as for connecting species ecology to the taxonomic distributions of BGCs.

Harzianone Biosynthesis by the Biocontrol Fungus Trichoderma

Analysis of the volatile terpenes produced by seven fungal strains of the genus Trichoderma by use of a closed‐loop stripping apparatus (CLSA) revealed a common production of harzianone, a bioactive, structurally unique diterpenoid consisting of a fused tetracyclic 4,7,5,6‐membered ring system. The terpene cyclization mechanism was studied by feeding experiments using selectively 13C‐ and 2H‐labeled synthetic mevalonolactone isotopologues, followed by analysis of the incorporation patterns by 13C NMR spectroscopy and GC/MS. The structure of harzianone was further supported from a 13C,13C COSY experiment of the in‐vivo‐generated fully 13C‐labeled diterpene.