Daniel Heine

Vinylogous chain branching catalysed by a dedicated polyketide synthase module

Bacteria use modular polyketide synthases (PKSs) to assemble complex polyketides, many of which are leads for the development of clinical drugs, in particular anti-infectives and anti-tumoral agents. Because these multifarious compounds are notoriously difficult to synthesize, they are usually produced by microbial fermentation. During the past two decades, an impressive body of knowledge on modular PKSs has been gathered that not only provides detailed insight into the biosynthetic pathways but also allows the rational engineering of enzymatic processing lines to yield structural analogues. Notably, a hallmark of all PKS modules studied so far is the head-to-tail fusion of acyl and malonyl building blocks, which leads to linear backbones. Yet, structural diversity is limited by this uniform assembly mode. Here we demonstrate a new type of PKS module from the endofungal bacterium Burkholderia rhizoxinica that catalyses a Michael-type acetyl addition to generate a branch in the carbon chain. In vitro reconstitution of the entire PKS module, X-ray structures of a ketosynthase-branching didomain and mutagenesis experiments revealed a crucial role of the ketosynthase domain in branching the carbon chain. We present a trapped intermediary state in which acyl carrier protein and ketosynthase are covalently linked by the branched polyketide and suggest a new mechanism for chain alkylation, which is functionally distinct from terpenoid-like β-branching. For the rice seedling blight toxin rhizoxin, one of the strongest known anti-mitotic agents, the non-canonical polyketide modification is indispensable for phytotoxic and anti-tumoral activities. We propose that the formation of related pharmacophoric groups follows the same general scheme and infer a unifying vinylogous branching reaction for PKS modules with a ketosynthase-branching–acyl-carrier-protein architecture. This study unveils the structure and function of a new PKS module that broadens the biosynthetic scope of polyketide biosynthesis and sets the stage for rationally creating structural diversity.

Phytotoxin production in Aspergillus terreus is regulated by independent environmental signals

Secondary metabolites have a great potential as pharmaceuticals, but there are only a few examples where regulation of gene cluster expression has been correlated with ecological and physiological relevance for the producer. Here, signals, mediators, and biological effects of terrein production were studied in the fungus Aspergillus terreus to elucidate the contribution of terrein to ecological competition. Terrein causes fruit surface lesions and inhibits plant seed germination. Additionally, terrein is moderately antifungal and reduces ferric iron, thereby supporting growth of A. terreus under iron starvation. In accordance, the lack of nitrogen or iron or elevated methionine levels induced terrein production and was dependent on either the nitrogen response regulators AreA and AtfA or the iron response regulator HapX. Independent signal transduction allows complex sensing of the environment and, combined with its broad spectrum of biological activities, terrein provides a prominent example of adapted secondary metabolite production in response to environmental competition. DOI: http://dx.doi.org/10.7554/eLife.07861.001

Three Redundant Synthetases Secure Redox-Active Pigment Production in the Basidiomycete Paxillus involutus

The symbiotic fungus Paxillus involutus serves a critical role in maintaining forest ecosystems, which are carbon sinks of global importance. P. involutus produces involutin and other 2,5-diarylcyclopentenone pigments that presumably assist in the oxidative degradation of lignocellulose via Fenton chemistry. Their precise biosynthetic pathways, however, remain obscure. Using a combination of biochemical, genetic, and transcriptomic analyses, in addition to stable-isotope labeling with synthetic precursors, we show that atromentin is the key intermediate. Atromentin is made by tridomain synthetases of high similarity: InvA1, InvA2, and InvA5. An inactive atromentin synthetase, InvA3, gained activity after a domain swap that replaced its native thioesterase domain with that of InvA5. The found degree of multiplex biosynthetic capacity is unprecedented with fungi, and highlights the great importance of the metabolite for the producer.

Enzymatic Polyketide Chain Branching To Give Substituted Lactone, Lactam, and Glutarimide Heterocycles†

Polyketides typically result from head-to-tail condensation of acyl thioesters to produce highly functionalized linear chains. The biosynthesis of the phytotoxin rhizoxin, however, involves a polyketide synthase (PKS) module that introduces a δ-lactone chain branch through Michael addition of a malonyl extender to an α,β-unsaturated intermediate unit. To evaluate the scope of the branching module, polyketide mimics were synthesized and their biotransformation by the reconstituted PKS module from the Rhizopus symbiont Burkholderia rhizoxinica was monitored in vitro. The impact of the type and configuration of the δ-substituents was probed and it was found that amino-substituted surrogates yield the corresponding lactams. A carboxamide analogue was transformed into a glutarimide unit, which can be found in many natural products. Our findings illuminate the biosynthesis of glutarimide-bearing polyketides and also demonstrate the utility of this branching module for synthetic biology.

Genomics-Guided Discovery of Endophenazines from Kitasatospora sp. HKI 714

In this study we report on the genomics-guided exploration of the metabolic potential of the newly discovered strain Kitasatospora sp. HKI 714. The bioinformatics analysis of the whole genome sequence revealed the presence of a biosynthetic gene cluster presumably responsible for the biosynthesis of formerly unknown endophenazine derivatives. A 200 L cultivation combined with bioactivity-guided isolation techniques revealed four new natural products belonging to the endophenazines and the 5,10-dihydrophenazines. Detailed descriptions of their biological effects, mainly focused on antimicrobial properties against several mycobacteria, are given.

A Fivefold Parallelized Biosynthetic Process Secures Chlorination of Armillaria mellea (Honey Mushroom) Toxins

ABSTRACT The basidiomycetous tree pathogen Armillaria mellea (honey mushroom) produces a large variety of structurally related antibiotically active and phytotoxic natural products, referred to as the melleolides. During their biosynthesis, some members of the melleolide family of compounds undergo monochlorination of the aromatic moiety, whose biochemical and genetic basis was not known previously. This first study on basidiomycete halogenases presents the biochemical in vitro characterization of five flavin-dependent A. mellea enzymes (ArmH1 to ArmH5) that were heterologously produced in Escherichia coli. We demonstrate that all five enzymes transfer a single chlorine atom to the melleolide backbone. A 5-fold, secured biosynthetic step during natural product assembly is unprecedented. Typically, flavin-dependent halogenases are categorized into enzymes acting on free compounds as opposed to those requiring a carrier-protein-bound acceptor substrate. The enzymes characterized in this study clearly turned over free substrates. Phylogenetic clades of halogenases suggest that all fungal enzymes share an ancestor and reflect a clear divergence between ascomycetes and basidiomycetes.

Polyketide synthase chimeras reveal key role of ketosynthase domain in chain branching

Biosynthesis of rhizoxin in Burkholderia rhizoxinica affords an unusual polyketide synthase module with ketosynthase and branching domains that install the δ-lactone, conferring antimitotic activity. To investigate their functions in chain branching, we designed chimeric modules with structurally similar domains from a glutarimide-forming module and a dehydratase. Biochemical, kinetic and mutational analyses reveal a structural role of the accessory domains and multifarious catalytic actions of the ketosynthase.