Ramona Riclea

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.

Algicidal lactones from the marine Roseobacter clade bacterium Ruegeria pomeroyi

Summary Volatiles released by the marine Roseobacter clade bacterium Rugeria pomeroyi were collected by use of a closed-loop stripping headspace apparatus (CLSA) and analysed by GC–MS. Several lactones were found for which structural proposals were derived from their mass spectra and unambiguously verified by the synthesis of reference compounds. An enantioselective synthesis of two exemplary lactones was performed to establish the enantiomeric compositions of the natural products by enantioselective GC–MS analyses. The lactones were subjected to biotests to investigate their activity against several bacteria, fungi, and algae. A specific algicidal activity was observed that may be important in the interaction between the bacteria and their algal hosts in fading algal blooms.

A single Sfp-type Phosphopantetheinyl transferase plays a major role in the biosynthesis of PKS and NRPS derived metabolites in Streptomyces ambofaciens ATCC23877

The phosphopantetheinyl transferases (PPTases) are responsible for the activation of the carrier protein domains of the polyketide synthases (PKS), non ribosomal peptide synthases (NRPS) and fatty acid synthases (FAS). The analysis of the Streptomyces ambofaciens ATCC23877 genome has revealed the presence of four putative PPTase encoding genes. One of these genes appears to be essential and is likely involved in fatty acid biosynthesis. Two other PPTase genes, samT0172 (alpN) and samL0372, are located within a type II PKS gene cluster responsible for the kinamycin production and an hybrid NRPS-PKS cluster involved in antimycin production, respectively, and their products were shown to be specifically involved in the biosynthesis of these secondary metabolites. Surprisingly, the fourth PPTase gene, which is not located within a secondary metabolite gene cluster, appears to play a pleiotropic role. Its product is likely involved in the activation of the acyl- and peptidyl-carrier protein domains within all the other PKS and NRPS complexes encoded by S. ambofaciens. Indeed, the deletion of this gene affects the production of the spiramycin and stambomycin macrolide antibiotics and of the grey spore pigment, all three being PKS-derived metabolites, as well as the production of the nonribosomally produced compounds, the hydroxamate siderophore coelichelin and the pyrrolamide antibiotic congocidine. In addition, this PPTase seems to act in concert with the product of samL0372 to activate the ACP and/or PCP domains of the antimycin biosynthesis cluster which is also responsible for the production of volatile lactones.

Volatile lactones from Streptomycetes arise via the Antimycin biosynthetic pathway

The volatiles released by several streptomycetes were collected by using a closed‐loop stripping apparatus (CLSA) and analysed by GC–MS. The obtained headspace extracts of various species contained blastmycinone, a known degradation product of the fungicidal antibiotic, antimycin A3b, and several unknown derivatives. The suggested structures of these compounds, based on their mass spectra and GC retention indices, were confirmed by comparison to synthetic reference samples. Additional compounds found in the headspace extracts were butenolides formed from the blastmycinones by elimination of the carboxylic acid moiety. Analysis of a gene knockout mutant in the antimycin biosynthetic gene cluster demonstrated that all blastmycinones and butenolides are formed via the antimycin biosynthetic pathway. The structural variation of the blastmycinones identified here is much larger than within the known antimycins, thus suggesting that several antimycin derivatives remain to be discovered.

Novel fatty acid methyl esters from the actinomycete Micromonospora aurantiaca

Summary The volatiles released by Micromonospora aurantiaca were collected by means of a closed-loop stripping apparatus (CLSA) and analysed by GC–MS. The headspace extracts contained more than 90 compounds from different classes. Fatty acid methyl esters (FAMEs) comprised the major compound class including saturated unbranched, monomethyl and dimethyl branched FAMEs in diverse structural variants: Unbranched, α-branched, γ-branched, (ω−1)-branched, (ω−2)-branched, α- and (ω−1)-branched, γ- and (ω−1)-branched, γ- and (ω−2)-branched, and γ- and (ω−3)-branched FAMEs. FAMEs of the last three types have not been described from natural sources before. The structures for all FAMEs have been suggested based on their mass spectra and on a retention index increment system and verified by the synthesis of key reference compounds. In addition, the structures of two FAMEs, methyl 4,8-dimethyldodecanoate and the ethyl-branched compound methyl 8-ethyl-4-methyldodecanoate were deduced from their mass spectra. Feeding experiments with isotopically labelled [2H10]leucine, [2H10]isoleucine, [2H8]valine, [2H5]sodium propionate, and [methyl-2H3]methionine demonstrated that the responsible fatty acid synthase (FAS) can use different branched and unbranched starter units and is able to incorporate methylmalonyl-CoA elongation units for internal methyl branches in various chain positions, while the methyl ester function is derived from S-adenosyl methionine (SAM).

Identification of Intermediates in the Biosynthesis of PR Toxin by Penicillium roqueforti

The sesquiterpenoid 7-epi-neopetasone was synthesized via the Wieland-Miescher ketone. The compound was identical to a previously tentatively identified headspace constituent of Penicillium roqueforti. Feeding experiments with (13) C-labeled mevalonolactone isotopomers demonstrated that oxidation at C12 and an isomerization of the C11C12 to a C7C11 double bond must occur independently and not via a C7-C11-C12 allyl radical in one step. Feeding with (11,12,13-(13) C3 )-7-epi-neopetasone resulted in labelling of the PR toxin, thus establishing this compound as a newly identified pathway intermediate.

Biosynthesis of acorane sesquiterpenes by Trichoderma

The volatiles of three Trichoderma strains have been collected by use of a closed-loop stripping apparatus (CLSA) and analysed by GC-MS. Several biosynthetically related sesquiterpenes have been identified in the complex headspace extracts. Additionally, the absolute configurations of some sesquiterpenes were determined by chiral GC-MS. The biosynthesis of the main compound tricho-acorenol was investigated by feeding of deuterated mevalonolactone isotopomers and contradicts a previously published pathway for Fusidium.

The Absolute Configuration of the Pyrrolosesquiterpenoid Glaciapyrrol A

The total syntheses of the structurally unique and moderately cytotoxic pyrrolosesquiterpenoid glaciapyrrol A that has been isolated from a marine streptomycete by Macherla et al. and of seven of its stereoisomers have been performed from geraniol or nerol, respectively, using a known diastereoselective Ru-catalysed approach for the synthesis of tetrahydrofurans previously reported by Stark and co-workers. Comparison of (1)H and (13)C NMR data unambiguously clarified the relative configuration of natural glaciapyrrol A that was previously only partly solved from the available NMR data. An enantioselective synthesis was carried out resulting in the unnatural enantiomer (11S,12R,15R)-(-)-glaciapyrrol A. These data establish the absolute configuration of the natural product as (11R,12S,15S)-(+)-glaciapyrrol A.

Biochemistry and Crystal Structure of Ectoine Synthase: A Metal-Containing Member of the Cupin Superfamily.

Ectoine is a compatible solute and chemical chaperone widely used by members of the Bacteria and a few Archaea to fend-off the detrimental effects of high external osmolarity on cellular physiology and growth. Ectoine synthase (EctC) catalyzes the last step in ectoine production and mediates the ring closure of the substrate N-gamma-acetyl-L-2,4-diaminobutyric acid through a water elimination reaction. However, the crystal structure of ectoine synthase is not known and a clear understanding of how its fold contributes to enzyme activity is thus lacking. Using the ectoine synthase from the cold-adapted marine bacterium Sphingopyxis alaskensis (Sa), we report here both a detailed biochemical characterization of the EctC enzyme and the high-resolution crystal structure of its apo-form. Structural analysis classified the (Sa)EctC protein as a member of the cupin superfamily. EctC forms a dimer with a head-to-tail arrangement, both in solution and in the crystal structure. The interface of the dimer assembly is shaped through backbone-contacts and weak hydrophobic interactions mediated by two beta-sheets within each monomer. We show for the first time that ectoine synthase harbors a catalytically important metal co-factor; metal depletion and reconstitution experiments suggest that EctC is probably an iron-dependent enzyme. We found that EctC not only effectively converts its natural substrate N-gamma-acetyl-L-2,4-diaminobutyric acid into ectoine through a cyclocondensation reaction, but that it can also use the isomer N-alpha-acetyl-L-2,4-diaminobutyric acid as its substrate, albeit with substantially reduced catalytic efficiency. Structure-guided site-directed mutagenesis experiments targeting amino acid residues that are evolutionarily highly conserved among the extended EctC protein family, including those forming the presumptive iron-binding site, were conducted to functionally analyze the properties of the resulting EctC variants. An assessment of enzyme activity and iron content of these mutants give important clues for understanding the architecture of the active site positioned within the core of the EctC cupin barrel.

Identification of isoafricanol and its terpene cyclase in Streptomyces violaceusniger using CLSA-NMR

The recently developed CLSA-NMR technique that is based on feeding experiments with (13)C-labelled precursors was applied in the identification of isoafricanol as the main volatile terpene emitted by Streptomyces violaceusniger. The isoafricanol synthase of this organism is presented, together with a recent phylogenetic analysis of bacterial terpene cyclases.