Nature-driven approaches to non-natural terpene analogues

The reactions catalysed by terpene synthases belong to the most complex and fascinating cascade-type transformations in Nature.

Biochemical and Mechanistic Characterization of the Fungal Reverse N-1-Dimethylallyltryptophan Synthase DMATS1Ff

Dimethylallyltryptophan synthases catalyze the regiospecific transfer of (oligo)prenylpyrophosphates to aromatic substrates like tryptophan derivatives. These reactions are key steps in many biosynthetic pathways of fungal and bacterial secondary metabolites. In vitro investigations on recombinant DMATS1_Ff from Fusarium fujikuroi identified the enzyme as the first selective reverse tryptophan-N-1 prenyltransferase of fungal origin. The enzyme was also able to catalyze the reverse N-geranylation of tryptophan. DMATS1_Ff was shown to be phylogenetically related to fungal tyrosine O-prenyltransferases and fungal 7-DMATS. Like these enzymes, DMATS1_Ff was able to convert tyrosine into its regularly O-prenylated derivative. Investigation of the binding sites of DMATS1_Ff by homology modeling and comparison to the crystal structure of 4-DMATS FgaPT2 showed an almost identical site for DMAPP binding but different residues for tryptophan coordination. Several putative active site residues were verified by site directed mutagenesis of DMATS1_Ff. Homology models of the phylogenetically related enzymes showed similar tryptophan binding residues, pointing to a unified substrate binding orientation of tryptophan and DMAPP, which is distinct from that in FgaPT2. Isotopic labeling experiments showed the reaction catalyzed by DMATS1_Ff to be nonstereospecific. Based on these data, a detailed mechanism for DMATS1_Ff catalysis is proposed.

An Unusual Skeletal Rearrangement in the Biosynthesis of the Sesquiterpene Trichobrasilenol from Trichoderma

The skeletons of some classes of terpenoids are unusual in that they contain a larger number of Me groups (or their biosynthetic equivalents such as olefinic methylene groups, hydroxymethyl groups, aldehydes, or carboxylic acids and their derivatives) than provided by their oligoprenyl diphosphate precursor. This is sometimes the result of an oxidative ring-opening reaction at a terpene-cyclase-derived molecule containing the regular number of Me group equivalents, as observed for picrotoxan sesquiterpenes. In this study a sesquiterpene cyclase from Trichoderma spp. is described that can convert farnesyl diphosphate (FPP) directly via a remarkable skeletal rearrangement into trichobrasilenol, a new brasilane sesquiterpene with one additional Me group equivalent compared to FPP. A mechanistic hypothesis for the formation of the brasilane skeleton is supported by extensive isotopic labelling studies.

Bacterial Diterpene Biosynthesis

This Minireview summarises recent developments in the biosynthesis of diterpenes by diterpene synthases in bacteria. It is structured by the class of enzyme involved in the first committed step towards diterpenes, starting with type I diterpene synthases, followed by type II enzymes and the more recently discovered UbiA-related diterpene synthases. A special emphasis lies on the reaction mechanisms of diterpene synthases that convert simple linear precursors through cationic cascades into structurally complex, usually polycyclic carbon skeletons with multiple stereogenic centres. A further main focus of this Minireview is a discussion of how these mechanisms can be unravelled. Downstream modifications to bioactive molecules are also covered.

Terpene Synthase Genes Originated from Bacteria through Horizontal Gene Transfer Contribute to Terpenoid Diversity in Fungi

Fungi are successful eukaryotes of wide distribution. They are known as rich producers of secondary metabolites, especially terpenoids, which are important for fungi-environment interactions. Horizontal gene transfer (HGT) is an important mechanism contributing to genetic innovation of fungi. However, it remains unclear whether HGT has played a role in creating the enormous chemical diversity of fungal terpenoids. Here we report that fungi have acquired terpene synthase genes (TPSs), which encode pivotal enzymes for terpenoid biosynthesis, from bacteria through HGT. Phylogenetic analysis placed the majority of fungal and bacterial TPS genes from diverse taxa into two clades, indicating ancient divergence. Nested in the bacterial TPS clade is a number of fungal TPS genes that are inferred as the outcome of HGT. These include a monophyletic clade of nine fungal TPS genes, designated as BTPSL for bacterial TPS-like genes, from eight species of related entomopathogenic fungi, including seven TPSs from six species in the genus Metarhizium. In vitro enzyme assays demonstrate that all seven BTPSL genes from the genus Metarhizium encode active enzymes with sesquiterpene synthase activities of two general product profiles. By analyzing the catalytic activity of two resurrected ancestral BTPSLs and one closely related bacterial TPS, the trajectory of functional evolution of BTPSLs after HGT from bacteria to fungi and functional divergence within Metarhizium could be traced. Using M. brunneum as a model species, both BTPSLs and typical fungal TPSs were demonstrated to be involved in the in vivo production of terpenoids, illustrating the general importance of HGT of TPS genes from bacteria as a mechanism contributing to terpenoid diversity in fungi.

Phylogenomic analyses and distribution of terpene synthases among Streptomyces

Terpene synthases are widely distributed among microorganisms and have been mainly studied in members of the genus Streptomyces. However, little is known about the distribution and evolution of the genes for terpene synthases. Here, we performed whole-genome based phylogenetic analysis of Streptomyces species, and compared the distribution of terpene synthase genes among them. Overall, our study revealed that ten major types of terpene synthases are present within the genus Streptomyces, namely those for geosmin, 2-methylisoborneol, epi-isozizaene, 7-epi-α-eudesmol, epi-cubenol, caryolan-1-ol, cyclooctat-9-en-7-ol, isoafricanol, pentalenene and α-amorphene. The Streptomyces species divide in three phylogenetic groups based on their whole genomes for which the distribution of the ten terpene synthases was analysed. Geosmin synthases were the most widely distributed and were found to be evolutionary positively selected. Other terpene synthases were found to be specific for one of the three clades or a subclade within the genus Streptomyces. A phylogenetic analysis of the most widely distributed classes of Streptomyces terpene synthases in comparison to the phylogenomic analysis of this genus is discussed.

Diterpene Biosynthesis in Actinomycetes: Studies on Cattleyene Synthase and Phomopsene Synthase

Three diterpene synthases from actinomycetes have been studied. The first enzyme from Streptomyces cattleya produced the novel compound cattleyene. The other two enzymes from Nocardia testacea and Nocardia rhamnosiphila were identified as phomopsene synthases. The cyclisation mechanism of cattleyene synthase and the EIMS fragmentation mechanism of its product were extensively studied by incubation experiments with isotopically labelled precursors. Oxidative transformations expanded the chemical space of these unique diterpenes.

A terpene synthase-cytochrome P450 cluster in Dictyostelium discoideum produces a novel trisnorsesquiterpene

Terpenoids are enormously diverse, but our knowledge of their biosynthesis and functions is limited. Here we report on a terpene synthase (DdTPS8)-cytochrome P450 (CYP521A1) gene cluster that produces a novel C12 trisnorsesquiterpene and affects the development of Dictyostelium discoideum. DdTPS8 catalyzes the formation of a sesquiterpene discoidol, which is undetectable from the volatile bouquet of wild type D. discoideum. Interestingly, a DdTPS8 knockout mutant lacks not only discoidol, but also a putative trisnorsesquiterpene. This compound was hypothesized to be derived from discoidol via cytochrome P450 (CYP)-catalyzed oxidative cleavage. CYP521A1, which is clustered with DdTPS8, was identified as a top candidate. Biochemical assays demonstrated that CYP521A1 catalyzes the conversion of discoidol to a novel trisnorsesquiterpene named discodiene. The DdTPS8 knockout mutant exhibited slow progression in development. This study points to the untapped diversity of natural products made by D. discoideum, which may have diverse roles in its development and chemical ecology.

Mechanistic investigations on multiproduct β-himachalene synthase from Cryptosporangium arvum

A bacterial terpene synthase from Cryptosporangium arvum was characterised as a multiproduct β-himachalene synthase. In vitro studies showed not only a high promiscuity with respect to its numerous sesquiterpene products, including the structurally demanding terpenes longicyclene, longifolene and α-longipinene, but also to its substrates, as additional activity was observed with geranyl- and geranylgeranyl diphosphate. In-depth mechanistic investigations using isotopically labelled precursors regarding the stereochemical course of both 1,11-cyclisation and 1,3-hydride shift furnished a detailed catalytic model suggesting the molecular basis of the observed low product selectivity. The enzyme’s synthetic potential was also exploited in the preparation of sesquiterpene isotopomers, which provided insights into their EIMS fragmentation mechanisms.

Volatiles from the ascomycete Daldinia cf. childiae (Hypoxylaceae), originating from China

The volatiles from an isolate of the fungus Daldinia cf. childiae, obtained from a specimen collected in China, were collected by use of a closed-loop stripping apparatus and analysed by GC-MS. A total number of 33 compounds from different classes were rigorously identified by comparison of mass spectra to library spectra and of retention indices to tabulated data from the literature. For unknown compounds structural suggestions were delineated from the mass spectra and verified by chemical synthesis of reference materials. Through this approach two 2-alkylated furan derivatives were identified, demonstrating that the genus Daldinia continues to be an interesting source for the discovery of novel secondary metabolites. Feeding experiments with sodium (1,2-13C2)acetate were performed to investigate the biosynthesis of the polyketide 5-hydroxy-2-methyl-4-chromanone that are in favour of a non-enzymatic cyclisation step.

Deciphering the genome and secondary metabolome of the plant pathogen Fusarium culmorum

Fusarium culmorum is one of the most important fungal plant pathogens that causes diseases on a wide diversity of cereal and non-cereal crops. We report herein for the first time the genome sequence of F. culmorum strain PV and its associated secondary metabolome that plays a role in the interaction with other microorganisms and contributes to its pathogenicity on plants. The genome revealed the presence of two terpene synthases, trichodiene and longiborneol synthase, which generate an array of volatile terpenes. Furthermore, we identified two gene clusters, deoxynivalenol and zearalenone, which encode for the production of mycotoxins. Linking the production of mycotoxins with in vitro bioassays, we found high virulence of F. culmorum PV on maize, barley and wheat. By using ultra-performance liquid chromatography-mass spectrometry, we confirmed several compounds important for the behaviour and lifestyle of F. culmorum. This research sets the basis for future studies in microbe-plant interactions.

Stereochemical investigations on the biosynthesis of achiral (Z)-γ-bisabolene in Cryptosporangium arvum

A newly identified bacterial (Z)-γ-bisabolene synthase was used for investigating the cyclisation mechanism of the sesquiterpene. Since the stereoinformation of both chiral putative intermediates, nerolidyl diphosphate (NPP) and the bisabolyl cation, is lost during formation of the achiral product, the intriguing question of their absolute configurations was addressed by incubating both enantiomers of NPP with the recombinant enzyme, which resolved in an exclusive cyclisation of (R)-NPP, while (S)-NPP that is non-natural to the (Z)-γ-bisabolene synthase was specifically converted into (E)-β-farnesene. A hypothetical enzyme mechanistic model that explains these observations is presented.

Addressing the Chemistry of Germacrene A by Isotope Labeling Experiments

Despite the central role of germacrene A in sesquiterpene biosynthesis and its widespread occurrence in nature, its complete NMR characterization is still pending. This problem was solved through enzymatic preparation of germacrene A isotopomers that allowed for a full signal assignment to all three conformers. The obtained materials gave insights into the stereochemical course of the Cope rearrangement to β-elemene and uncovered the Cope rearrangement as a new EI-MS fragmentation reaction.

Substrate promiscuity of enzymes from the sesquiterpene biosynthetic pathways from Artemisia annua and Tanacetum parthenium allows for novel combinatorial sesquiterpene production

The therapeutic properties of complex terpenes often depend on the stereochemistry of their functional groups. However, stereospecific chemical synthesis of terpenes is challenging. To overcome this challenge, metabolic engineering can be employed using enzymes with suitable stereospecific catalytic activity. Here we used a combinatorial metabolic engineering approach to explore the stereospecific modification activity of the Artemisia annua artemisinic aldehyde ∆11(13) double bond reductase2 (AaDBR2) on products of the feverfew sesquiterpene biosynthesis pathway (GAS, GAO, COS and PTS). This allowed us to produce dihydrocostunolide and dihydroparthenolide. For dihydroparthenolide we demonstrate that the preferred order of biosynthesis of dihydroparthenolide is by reduction of the exocyclic methylene of parthenolide, rather than through C4-C5 epoxidation of dihydrocostunolide. Moreover, we demonstrate a promiscuous activity of feverfew CYP71CB1 on dihydrocostunolide and dihydroparthenolide for the production of 3β-hydroxy-dihydrocostunolide and 3β-hydroxy-dihydroparthenolide, respectively. Combined, these results offer new opportunities for engineering novel sesquiterpene lactones with potentially improved medicinal value.

Characterisation of three terpene synthases for β-barbatene, β-araneosene and nephthenol from social amoebae

Three terpene synthases from social amoebae with new functions were discovered and their mechanisms were explored.

Mechanistic characterization of three sesquiterpene synthases from the termite-associated fungus Termitomyces

Three terpene synthases from the termite associated fungus Termitomyces were studied by isotopic labelling experiments and site-directed mutagenesis.

Volatiles from the hypoxylaceous fungi Hypoxylon griseobrunneum and Hypoxylon macrocarpum

The volatiles emitted by the ascomycetes Hypoxylon griseobrunneum and Hypoxylon macrocarpum (Hypoxylaceae, Xylariales) were collected by use of a closed-loop stripping apparatus (CLSA) and analysed by GC–MS. The main compound class of both species were polysubstituted benzene derivatives. Their structures could only be unambiguously determined by comparison to all isomers with different substitution patterns. The substitution pattern of the main compound from H. griseobrunneum, the new natural product 2,4,5-trimethylanisole, was explainable by a polyketide biosynthesis mechanism that was supported by a feeding experiment with (methyl-²H₃)methionine.

A Clade II-D Fungal Chimeric Diterpene Synthase from Colletotrichum gloeosporioides Produces Dolasta-1(15),8-diene

Based on a terpenoid overproduction platform in yeast for genome mining, a chimeric diterpene synthase from the endophytic fungus Colletotrichum gloeosporioides ES026 was characterized as the (5R,12R,14S)-dolasta-1(15),8-diene synthase. The absolute configuration was independently verified through the use of enantioselectively deuterated terpene precursors, which unequivocally established the predicted C1-III-IV cyclization mode for this first characterized clade II-D enzyme. Extensive isotopic labeling experiments and isolation of the intermediate (1R)-δ-araneosene supported the proposed cyclization mechanism.

Diversity and Functional Evolution of Terpene Synthases in Dictyostelid Social Amoebae

Dictyostelids, or social amoebae, have a unique life style in forming multicellular fruiting bodies from unicellular amoeboids upon starvation. Recently, dictyostelids were found to contain terpene synthase (TPS) genes, a gene type of secondary metabolism previously known to occur only in plants, fungi and bacteria. Here we report an evolutionary functional study of dictyostelid TPS genes. The number of TPS genes in six species of dictyostelids examined ranges from 1 to 19; and the model species Dictyostelium purpureum contains 12 genes. Using in vitro enzyme assays, the 12 TPS genes from D. purpureum were shown to encode functional enzymes with distinct product profiles. The expression of the 12 TPS genes in D. purpureum is developmentally regulated. During multicellular development, D. purpureum releases a mixture of volatile terpenes dominated by sesquiterpenes that are the in vitro products of a subset of the 12 TPS genes. The quality and quantity of the terpenes released from D. purpureum, however, bear little resemblance to those of D. discoideum, a closely related dictyostelid. Despite these variations, the conserved clade of dictyostelid TPSs, which have an evolutionary distance of more than 600 million years, has the same biochemical function, catalyzing the formation of a sesquiterpene protoillud-7-ene. Taken together, our results indicate that the dynamic evolution of dictyostelid TPS genes includes both purifying selection of an orthologous group and species-specific expansion with functional divergence. Consequently, the terpenes produced by these TPSs most likely have conserved as well as species-adaptive biological functions as chemical languages in dictyostelids.

Sesquiterpene Cyclisations Catalysed inside the Resorcinarene Capsule and Application in the Short Synthesis of Isolongifolene and Isolongifolenone

Terpenes constitute the largest class of natural products and serve as an important source for medicinal treatments. Despite constant progress in chemical synthesis, the construction of complex polycyclic sesqui- and diterpene scaffolds remains challenging. Natural cyclase enzymes, however, are able to form the whole variety of terpene structures from just a handful of linear precursors. Man-made catalysts able to mimic such natural enzymes are lacking. Here, we describe the examples of sesquiterpene cyclisations inside an enzyme-mimicking supramolecular catalyst. This strategy allowed the formation of the tricyclic sesquiterpene isolongifolene in only four steps. The mechanism of the catalysed cyclisation reaction was elucidated using ¹³C-labelling studies and DFT calculations.

Two Bacterial Diterpene Synthases from Allokutzneria albata Produce Bonnadiene, Phomopsene, and Allokutznerene

Two diterpene synthases from Allokutzneria albata were studied for their products, resulting in the identification of the new compound bonnadiene from the first enzyme. Although phylogenetically unrelated to fungal phomopsene synthase, the second enzyme produced a mixture of phomopsene and a biosynthetically linked new compound, allokutznerene, as well as spiroviolene. Both enzymes were subjected to in-depth mechanistic studies involving isotopic labelling experiments, metal-cofactor variation, and site-directed mutagenesis. Oxidation products of phomopsene and allokutznerene are also discussed.

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.

Acyl-group specificity of AHL synthases involved in quorum-sensing in Roseobacter group bacteria

N-Acylhomoserine lactones (AHLs) are important bacterial messengers, mediating different bacterial traits by quorum sensing in a cell-density dependent manner. AHLs are also produced by many bacteria of the marine Roseobacter group, which constitutes a large group within the marine microbiome. Often, specific mixtures of AHLs differing in chain length and oxidation status are produced by bacteria, but how the biosynthetic enzymes, LuxI homologs, are selecting the correct acyl precursors is largely unknown. We have analyzed the AHL production in Dinoroseobacter shibae and three Phaeobacter inhibens strains, revealing strain-specific mixtures. Although large differences were present between the species, the fatty acid profiles, the pool for the acyl precursors for AHL biosynthesis, were very similar. To test the acyl-chain selectivity, the three enzymes LuxI₁ and LuxI₂ from D. shibae DFL-12 as well as PgaI₂ from P. inhibens DSM 17395 were heterologously expressed in E. coli and the enzymes isolated for in vitro incubation experiments. The enzymes readily accepted shortened acyl coenzyme A analogs, N-pantothenoylcysteamine thioesters of fatty acids (PCEs). Fifteen PCEs were synthesized, varying in chain length from C₄ to C₂₀, the degree of unsaturation and also including unusual acid esters, e.g., 2E,11Z-C18:2-PCE. The latter served as a precursor of the major AHL of D. shibae DFL-12 LuxI₁, 2E,11Z-C18:2-homoserine lactone (HSL). Incubation experiments revealed that PgaI₂ accepts all substrates except C₄ and C₂₀-PCE. Competition experiments demonstrated a preference of this enzyme for C₁₀ and C₁₂ PCEs. In contrast, the LuxI enzymes of D. shibae are more selective. While 2E,11Z-C18:2-PCE is preferentially accepted by LuxI₁, all other PCEs were not, except for the shorter, saturated C₁₀–C₁₄-PCEs. The AHL synthase LuxI₂ accepted only C₁₄ PCE and 3-hydroxydecanoyl-PCE. In summary, chain-length selectivity in AHLs can vary between different AHL enzymes. Both, a broad substrate acceptance and tuned specificity occur in the investigated enzymes.

Volatiles from the fungal microbiome of the marine sponge Callyspongia cf. flammea

The volatiles emitted by five fungal strains previously isolated from the marine sponge Callyspongia cf. flammea were captured with a closed-loop stripping apparatus (CLSA) and analyzed by GC-MS. Besides several widespread compounds, a series of metabolites with interesting bioactivities were found, including the quorum sensing inhibitor protoanemonin, the fungal phytotoxin 3,4-dimethylpentan-4-olide, and the insect attractant 1,2,4-trimethoxybenzene. In addition, the aromatic polyketides isotorquatone and chartabomone that are both known from Eucalyptus and a new O-desmethyl derivative were identified. The biosynthesis of isotorquatone was studied by feeding experiments with isotopically labeled precursors and its absolute configuration was determined by enantioselective synthesis of a reference compound. Bioactivity testings showed algicidal activity for some of the identified compounds, suggesting a potential ecological function in sponge defence.

Chemical differentiation of three DMSP lyases from the marine Roseobacter group

Dimethylsulfoniopropionate (DMSP) catabolism of marine bacteria plays an important role in marine and global ecology. The genome of Ruegeria pomeroyi DSS-3, a model organism from the Roseobacter group, harbours no less than three genes for different DMSP lyases (DddW, DddP and DddQ) that catalyse the degradation of DMSP to dimethyl sulfide (DMS) and acrylate. Despite their apparent similar function these enzymes show no significant overall sequence identity. In this work DddQ and DddW from R. pomeroyi and the DddP homolog from Phaeobacter inhibens DSM 17395 were functionally characterised and their substrate scope was tested using several synthetic DMSP analogues. Comparative kinetic assays revealed differences in the conversion of DMSP and its analogues in terms of selectivity and overall velocity, giving additional insights into the molecular mechanisms of DMSP lyases and into their putatively different biological functions.

Volatiles from three genome sequenced fungi from the genus Aspergillus

The volatiles emitted by agar plate cultures of three genome sequenced fungal strains from the genus Aspergillus were analysed by GC-MS. All three strains produced terpenes for which a biosynthetic relationship is discussed. The obtained data were also correlated to genetic information about the encoded terpene synthases for each strain. Besides terpenes, a series of aromatic compounds and volatiles derived from fatty acid and branched amino acid metabolism were identified. Some of these compounds have not been described as fungal metabolites before. For the compound ethyl (E)-hept-4-enoate known from cantaloupe a structural revision to the Z stereoisomer is proposed. Ethyl (Z)-hept-4-enoate also occurs in Aspergillus clavatus and was identified by synthesis of an authentic standard.

Volatiles from the xylarialean fungus Hypoxylon invadens

The volatiles emitted by agar plate cultures of the xylarialean fungus Hypoxylon invadens were investigated by use of a closed loop stripping apparatus in combination with GC-MS. Several aromatic compounds were found that could only be identified by comparison to all possible constitutional isomers with different ring substitution patterns. For the set of identified compounds a plausible biosynthetic scheme was suggested that gives further support for the assigned structures.

Metabolism of 2,3-dihydroxypropane-1-sulfonate by marine bacteria

Both enantiomers of the sulfoquinovose breakdown product 2,3-dihydroxypropane-1-sulfonate, an important sulfur metabolite produced by marine algae, were synthesised in a ³⁴S-labelled form and used in feeding experiments with marine bacteria. The labelling was efficiently incorporated into the sulfur-containing antibiotic tropodithietic acid and sulfur volatiles by the algal symbiont Phaeobacter inhibens, but not into sulfur volatiles released by marine bacteria associated with crustaceans. The ecological implications and the relevance of these findings for the global sulfur cycle are discussed.

Volatiles from the tropical ascomycete Daldinia clavata (Hypoxylaceae, Xylariales)

The volatiles from the fungus Daldinia clavata were collected by use of a closed-loop stripping apparatus and analysed by GC–MS. A few compounds were readily identified by comparison of measured to library mass spectra and of retention indices to published data, while for other compounds a synthesis of references was required. For one of the main compounds, 5-hydroxy-4,6-dimethyloctan-3-one, the relative and absolute configuration was determined by synthesis of all eight stereoisomers and gas chromatographic analysis using a homochiral stationary phase. Another identified new natural product is 6-nonyl-2H-pyran-2-one. The antimicrobial and cytotoxic effects of the synthetic volatiles are also reported.

The absolute configuration of isochamigrene: new insights into the cyclisation mechanism of trichodiene synthase

The enantioselective synthesis of isochamigrene clarified its absolute configuration and biosynthetic relation to trichodiene.

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 ¹³ C- and ² H-labeled synthetic mevalonolactone isotopologues, followed by analysis of the incorporation patterns by ¹³ C NMR spectroscopy and GC/MS. The structure of harzianone was further supported from a ¹³ C,¹³ C COSY experiment of the in-vivo-generated fully ¹³ C-labeled diterpene.

Characterisation of the l-Cystine β-Lyase PatB from Phaeobacter inhibens: An Enzyme Involved in the Biosynthesis of the Marine Antibiotic Tropodithietic Acid

The l-cystine β-lyase from Phaeobacter inhibens is involved in the biosynthesis of the sulfur-containing antibiotic tropodithietic acid. The recombinant enzyme was obtained by heterologous expression in Escherichia coli and biochemically characterised by unambiguous chemical identification of the products formed from the substrate l-cystine, investigation of the substrate spectrum, determination of the enzyme kinetics, sequence alignment with closely related homologues and site-directed mutagenesis to identify a highly conserved lysine residue that is critical for functionality. PatB from P. inhibens is a new member of the small group of characterised l-cystine β-lyases and the first example of an enzyme with such an activity that is required for the biosynthesis of an antibiotic. A comparison of PatB to previously reported enzymes with l-cystine β-lyase activity from bacteria and plants is given.