Cyclopropanol Warhead in Malleicyprol Confers Virulence of Human- and Animal-Pathogenic Burkholderia Species

Burkholderia species such as B. mallei and B. pseudomallei are bacterial pathogens causing fatal infections in humans and animals (glanders and melioidosis), yet knowledge on their virulence factors is limited. While pathogenic effects have been linked to a highly conserved gene locus (bur/mal) in the B. mallei group, the metabolite associated to the encoded polyketide synthase, burkholderic acid (syn. malleilactone), could not explain the observed phenotypes. By metabolic profiling and molecular network analyses of the model organism B. thailandensis, the primary products of the cryptic pathway were identified as unusual cyclopropanol-substituted polyketides. First, sulfomalleicyprols were identified as inactive precursors of burkholderic acid. Furthermore, a highly reactive upstream metabolite, malleicyprol, was discovered and obtained in two stabilized forms. Cell-based assays and a nematode infection model showed that the rare natural product confers cytotoxicity and virulence.

The novel globin protein fungoglobin is involved in low oxygen adaptation of Aspergillus fumigatus

The human pathogenic fungus Aspergillus fumigatus normally lives as a soil saprophyte. Its environment includes poorly oxygenated substrates that also occur during tissue invasive growth of the fungus in the human host. Up to now, few cellular factors have been identified that allow the fungus to efficiently adapt its energy metabolism to hypoxia. Here, we cultivated A. fumigatus in an O2 -controlled fermenter and analysed its responses to O2 limitation on a minute timescale. Transcriptome sequencing revealed several genes displaying a rapid and highly dynamic regulation. One of these genes was analysed in detail and found to encode fungoglobin, a previously uncharacterized member of the sensor globin protein family widely conserved in filamentous fungi. Besides low O2 , iron limitation also induced transcription, but regulation was not entirely dependent on the two major transcription factors involved in adaptation to iron starvation and hypoxia, HapX and SrbA respectively. The protein was identified as a functional haemoglobin, as binding of this cofactor was detected for the recombinant protein. Gene deletion in A. fumigatus confirmed that haem-binding fungoglobins are important for growth in microaerobic environments with O2 levels far lower than in hypoxic human tissue.

Biosynthesis of Diverse Antimicrobial and Antiproliferative Acyloins in Anaerobic Bacteria

Metabolic profiling and genome mining revealed that anaerobic bacteria have the potential to produce acyloin natural products. In addition to sattazolin A and B, three new sattazolin congeners and a novel acyloin named clostrocyloin were isolated from three strains of Clostridium beijerinckii, a bacterium used for industrial solvent production. Bioactivity profiling showed that the sattazolin derivatives possess antimicrobial activities against mycobacteria and pseudomonads with only low cytotoxicity. Clostrocyloin was found to be mainly active against fungi. The thiamine diphosphate (ThDP)-dependent sattazolin-producing synthase was identified in silico and characterized both in vivo and in in vitro enzyme assays. A related acyloin synthase from the clostrocyloin producer was shown to be responsible for the production of the acyloin core of clostrocyloin. The biotransformation experiments provided first insights into the substrate scope of the clostrocyloin synthase and revealed biosynthetic intermediates.

Oak-Associated Negativicute Equipped with Ancestral Aromatic Polyketide Synthase Produces Antimycobacterial Dendrubins

Anaerobic bacteria have only recently been recognized as a source of antibiotics; yet, the metabolic potential of Negativicutes (Gram-negative staining Firmicutes) such as the oak-associated Dendrosporobacter quercicolus has remained unknown. Genome mining of D. quercicolus and phylogenetic analyses revealed a gene cluster for a type II polyketide synthase (PKS) complex that belongs to the most ancestral enzyme systems of this type. Metabolic profiling, NMR analyses, and stable-isotope labeling led to the discovery of a new family of anthraquinone-type polyphenols, the dendrubins, which are diversified by acylation, methylation, and dimerization. Dendrubin A and B were identified as strong antibiotics against a range of clinically relevant, human-pathogenic mycobacteria.

Induced Production, Synthesis, and Immunomodulatory Action of Clostrisulfone, a Diarylsulfone from Clostridium acetobutylicum

The anaerobe Clostridium acetobutylicum belongs to the most important industrially used bacteria. Whereas genome mining points to a high potential for secondary metabolism in C. acetobutylicum, the functions of most biosynthetic gene clusters are cryptic. We report that the addition of supra-physiological concentrations of cysteine triggered the formation of a novel natural product, clostrisulfone (1). Its structure was fully elucidated by NMR, MS and the chemical synthesis of a reference compound. Clostrisulfone is the first reported natural product with a diphenylsulfone scaffold. A biomimetic synthesis suggests that pentamethylchromanol-derived radicals capture sulfur dioxide to form 1. In a cell-based assay using murine macrophages a biphasic and dose-dependent regulation of the LPS-induced release of nitric oxide was observed in the presence of 1.

Discovery and Biosynthesis of Celluxanthenes, Antibacterial Arylpolyene Alkaloids From Diverse Cellulose-Degrading Anaerobic Bacteria

Cellulose degradation by anaerobic bacteria plays an eminent role in the global carbon cycle and is a critical step in biofuel production. The anaerobic thermophile Clostridium thermocellum (now: Acetivibrio thermocellus) is particularly efficient at breaking down biomass and produces a "yellow affinity substance" (YAS), a pigment that has been implicated in signaling and conferring higher affinity of the cellulosome to YAS-loaded cellulose. However, the nature and biosynthetic origin of YAS have remained elusive. Here, we show by isolation and structure elucidation that YAS is a complex of unusual arylpolyene alkaloids (celluxanthenes). Stable isotope labeling experiments reveal all biosynthetic building blocks for celluxanthene assembly. Through a targeted gene deletion, we identify the celluxanthene (cex) biosynthesis gene cluster and propose a biosynthetic model in which an arylpolyene generated by an iterative type I polyketide synthase (PKS) undergoes a head-to-head fusion with a tryptophan-derived ketoacid to form a tetronate. Genome mining and metabolic profiling revealed that diverse cellulolytic anaerobes harbor cex gene loci and produce celluxanthene congeners. Celluxanthenes show antibiotic activity against Gram-positive bacteria including clinically relevant strains. This study solves the long-standing enigma surrounding the nature of YAS and lays the groundwork for elucidating the precise biological roles of these intricate pigments.