Gerhard Höfle

New lessons for combinatorial biosynthesis from myxobacteria. The myxothiazol biosynthetic gene cluster of Stigmatella aurantiaca DW4/3-1.

The biosynthetic mta gene cluster responsible for myxothiazol formation from the fruiting body forming myxobacterium Stigmatella aurantiaca DW4/3-1 was sequenced and analyzed. Myxothiazol, an inhibitor of the electron transport via the bc 1-complex of the respiratory chain, is biosynthesized by a unique combination of several polyketide synthases (PKS) and nonribosomal peptide synthetases (NRPS), which are activated by the 4′-phosphopantetheinyl transferase MtaA. Genomic replacement of a fragment of mtaB and insertion of a kanamycin resistance gene into mtaA both impaired myxothiazol synthesis. Genes mtaC and mtaDencode the enzymes for bis-thiazol(ine) formation and chain extension on one pure NRPS (MtaC) and on a unique combination of PKS and NRPS (MtaD). The genes mtaE and mtaF encode PKSs including peptide fragments with homology to methyltransferases. These methyltransferase modules are assumed to be necessary for the formation of the proposed methoxy- and β-methoxy-acrylate intermediates of myxothiazol biosynthesis. The last gene of the cluster,mtaG, again resembles a NRPS and provides insight into the mechanism of the formation of the terminal amide of myxothiazol. The carbon backbone of an amino acid added to the myxothiazol-acid is assumed to be removed via an unprecedented module with homology to monooxygenases within MtaG.

The Biosynthesis of the Aromatic Myxobacterial Electron Transport Inhibitor Stigmatellin Is Directed by a Novel Type of Modular Polyketide Synthase

Deductions from the molecular analysis of the 65,000-bp stigmatellin biosynthetic gene cluster are reported. The biosynthetic genes (stiA–J) encode an unusual bacterial modular type I polyketide synthase (PKS) responsible for the formation of this aromatic electron transport inhibitor produced by the myxobacterium Stigmatella aurantiaca. Involvement of the PKS gene cluster in stigmatellin biosynthesis is shown using site-directed mutagenesis. One module of the PKS is assumed to be used iteratively during the biosynthetic process, which seems to involve an unusual transacylation of the biosynthetic intermediate from an acyl carrier protein domain back to the preceding ketosynthase domain. Finally, the polyketide chain which is presumably catalyzed by a novel C-terminal domain in StiJ that does not resemble thioesterases, is cyclized and aromatized. The presented results of feeding experiments are in good agreement with the proposed biosynthetic scheme. In contrast to all other PKS type I systems reported to date, each module of StiA–J is encoded on a separate gene. The gene cluster contains a “stand alone” O-methyltransferase and two unusualO-methyltransferase domains embedded in the PKS. In addition, inactivation of a cytochrome P450 monooxygenase-encoding gene involved in post-PKS hydroxylation of the aromatic ring leads to the formation of two novel stigmatellin derivatives.

Structure and Biosynthesis of Myxochromides S1–3 in Stigmatella aurantiaca: Evidence for an Iterative Bacterial Type I Polyketide Synthase and for Module Skipping in Nonribosomal Peptide Biosynthesis†

The myxobacterium Stigmatella aurantiaca DW4/3–1 harbours an astonishing variety of secondary metabolic gene clusters, at least two of which were found by gene inactivation experiments to be connected to the biosynthesis of previously unknown metabolites. In this study, we elucidate the structures of myxochromides S1–3, novel cyclic pentapeptide natural products possessing unsaturated polyketide side chains, and identify the corresponding biosynthetic gene locus, made up of six nonribosomal peptide synthetase modules. By analyzing the deduced substrate specificities of the adenylation domains, it is shown that module 4 is most probably skipped during the biosynthetic process. The polyketide synthase MchA harbours only one module and is presumably responsible for the formation of the variable complete polyketide side chains. These data indicate that MchA is responsible for an unusual iterative polyketide chain assembly.

Biologically active secondary metabolites from myxobacteria

New chemical structures with proven biological activity still are badly needed for a host of applications and are intensively screened for. Suitable compounds may be used as such, or in the form of their derivatives or, equally important, may serve as lead compounds for designing synthetic analogs. One way to new compounds is the exploitation of new producer organisms. During the past 15 years the myxobacteria have been shown in our laboratories to be a rich source of novel secondary metabolites, many of the compounds showing interesting and sometimes unique mechanisms of action. About 50 basic structures and nearly 300 structural variants have been elucidated, and almost all of them turned out to be new compounds. Several myxobacterial substances may have a good chance of an application.

Myxobacteria: a source of new antibiotics

Abstract Myxobacteria form highly colored macroscopic fruiting bodies on rotting wood and other substrates. The organisms can move by gliding or creeping, for example, across glass and agar surfaces. They also produce a large number of unusual secondary metabolites some of which have considerable potential as antibiotics. The large-scale cultivation of myxobacteria has also, therefore, become of great interest.

The mode of action of stigmatellin, a new inhibitor of the cytochrome b-c1 segment of the respiratory chain☆

The new antibiotic stigmatellin, obtained from the myxobacterium Stigmatella aurantiaca, was found to block the electron flow in the respiratory chain of bovine heart submitochondrial particles at the site of the cytochrome b-c1 segment. Its inhibitory potency was identical with that of antimycin and myxothiazol, and like these antibiotics, stigmatellin caused a shift in the spectrum of reduced cytochrome b. Difference spectroscopic studies with the three inhibitors in various combinations indicated that the binding site of stigmatellin was different from that of antimycin, but more or less identical with that of myxothiazol. Experiments with 14 synthesized derivatives of stigmatellin showed that good inhibitory activity can be expected only if the side chain was kept relatively lipophilic, and the keto and the hydroxy groups of the chromone system were left intact.

Isolation and spectroscopic structure elucidation of sorangicin a, a new type of macrolide-polyether antibiotic from gliding bacteria - XXX.

Abstract The broad spectrum antibiotic sorangicin A (1) has been isolated from Sorangium cellulosum , and its structure was determined spectroscopically as a 31-membered macrocyclic hydroxy-lactone carboxylic acid, containing four ether rings.

13C-NMR-spektroskopie chinoider verbindungen—II: Substituierte 1,4-naphthochinone und anthrachinone

Zusammenfassung Von 18 1,4-Naphtho- und 13 Anthrachinonen mit Sauerstoff-, Stickstoff und Halogensubstituenten wurden 1H-gekoppelte und 1H-breitbandentkoppelte 13C-NMR-spektren aufgenommen. Die weitgehende Zuordnung der Signale erfolgte mit Hilfe der geminalen und vicinalen 13C-1H-Kopplungen und den Substituenteneffekten.

Stigmatellin. A dual type inhibitor of photosynthetic electron transport

Abstract Stigmatellin, an antibiotic produced by the myxobacterium Stigmatella aurantiaca, is a powerful inhibitor of photosynthetic electron transport. In the photosynthetic electron-transport chain, it has two different inhibition sites. One is located at the reducing side of Photosystem II (I50 value, 52.5 nM) and identical to the inhibition site of inhibitors of the 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea type. The second one is located at the cytochrome b 6 f- complex (I50 value, 59.0 nM) and corresponds to the inhibition site of 2.5-dibromo-3-methyl-6-isopropyl-1,4-benzoquinone. Stigmatellin is the most potent inhibitor of spinach cytochrome b 6 f complex known so far.

Aurafuron A and B, new bioactive polyketides from Stigmatella aurantiaca and Archangium gephyra (Myxobacteria). Fermentation, isolation, physico-chemical properties, structure and biological activity.

New antibiotic polyketides, named aurafuron A (1) and B (2) were isolated from culture extracts of myxobacteria of the species Stigmatella aurantiaca and Archangium gephyra, strain Ar 10844. By multi-step chromatography 1 and 2 were separated from a variety of other non-related co-metabolites, and their structures elucidated by spectroscopic methods as new 5-alkenyl-3 3(2H)-furanones. Aurafurons inhibited the growth of some filamentous fungi and additionally, aurafuron B was weakly active against few Gram-positive bacteria. Both compounds also showed cytotoxic activity against the mouse fibroblast cell line L929.