Redox enzymes play a central role in generating structural complexity during natural product biosynthesis. In the postassembly tailoring steps, redox cascades can transform nascent chemical scaffolds into structurally complex final products. Chaetoglobosin A (1) is biosynthesized by a hybrid polyketide synthase-nonribosomal peptide synthetase. It belongs to the chaetoglobosin family of natural products, comprising many analogs having different degrees of oxidation introduced during their biosynthesis. We report here the determination of the complete biosynthetic steps leading to the formation of 1 from prochaetoglobosin I (2). Each oxidation step was elucidated using Chaetomium globosum strains carrying various combinations of deletion of the three redox enzymes, one FAD-dependent monooxygenase, and two cytochrome P450 oxygenases, and in vivo biotransformation of intermediates by heterologous expression of the three genes in Saccharomyces cerevisiae. Five analogs were identified in this study as intermediates formed during oxidization of 2 to 1 by those redox enzymes. Furthermore, a stereochemical course of each oxidation step was clearly revealed with the absolute configurations of five intermediates determined from X-ray crystal structure. This approach allowed us to quickly determine the biosynthetic intermediates and the enzymes responsible for their formation. Moreover, by addressing the redox enzymes, we were able to discover that promiscuity of the redox enzymes allowed the formation of a network of pathways that results in a combinatorial formation of multiple intermediate compounds during the formation of 1 from 2. Our approach should expedite elucidation of pathways for other natural products biosynthesized by many uncharacterized enzymes of this fungus.

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Fungal genome sequencing has revealed many genes coding for biosynthetic enzymes, including polyketide synthases and nonribosomal peptide synthetases. However, characterizing these enzymes and identifying the compounds they synthesize remains a challenge, whether the genes are expressed in their original hosts or in more tractable heterologous hosts, such as yeast. Here, we developed a streamlined method for isolating biosynthetic genes from fungal sources and producing bioactive molecules in an engineered Saccharomyces cerevisiae host strain. We used overlap extension PCR and yeast homologous recombination to clone desired fungal polyketide synthase or a nonribosomal peptide synthetase genes (5–20 kb) into a yeast expression vector quickly and efficiently. This approach was used successfully to clone five polyketide synthases and one nonribosomal peptide synthetase, from various fungal species. Subsequent detailed chemical characterizations of the resulting natural products identified six polyketide and two nonribosomal peptide products, one of which was a new compound. Our system should facilitate investigating uncharacterized fungal biosynthetic genes, identifying novel natural products, and rationally engineering biosynthetic pathways for the production of enzyme analogues possessing modified bioactivity.

Postgenomic analysis revealed that many microorganisms carry numerous secondary metabolite biosynthetic genes on their genome. However, activities of those putative genes are not clearly reflected in the metabolic profile of the microorganisms, especially in fungi. A recent genome mining effort is promising in discovering new natural products. However, many fungi and other organisms are not amenable to molecular genetics manipulations, making the study difficult. Here we report successful engineering of Chaetomium globosum, a known producer of various valuable natural products, that allows its genetic manipulation via targeted homologous recombination. This strain permitted us to abolish transcriptional regulators associated with epigenetic silencing of secondary metabolite biosynthetic pathways, leading to the identification of the products generated by different gene clusters and isolation of novel secondary metabolites. We were able to identify six gene clusters that are responsible for the biosynthesis of 11 natural products previously known to be produced by C. globosum, including one cytochalasan and six azaphilone-type compounds. In addition, we isolated two new compounds, mollipilin A and B, that were only recently identified in a related Chaetomium species. Furthermore, our investigation into the mechanism of biosynthesis of those natural products in C. globosum also led to the discovery of a secondary metabolite, aureonitol, that acts like a transcriptional regulator for the biosynthesis of other secondary metabolites. Similar approaches should facilitate exploration of the untapped potential of fungal biosynthetic capability and identification of various unique biological functions that those secondary metabolites possess.

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