Biosynthesis of the Paecilomyces marquandii conidial pigment saintopin.

Paecilomyces marquandii IBWF 003-21 produces vibrant purple pigmented conidia, the color of which can be attributed to the naphthacenedione natural product saintopin (1). The target compound was previously reported to exhibit potent topoisomerase-inhibitory activity, yet has not been extensively studied nor has the biosynthesis been elucidated. In an effort to elucidate the biosynthesis of 1, we mined the genome of Paecilomyces marquandii for non-reducing polyketide synthases (nrPKS), introduced them into the heterologous host Aspergillus oryzae OP12 and identified a prime candidate for the biosynthesis of 1 we termed stpA. Deletion of stpA in the native producer P. marquandii abolished production of 1, rendering conidia hyaline in color. stpA phylogenetically clusters with clade V nrPKS, canonically requiring trans-acting metallo-β-lactamase-like thioesterases (MβL) for product offloading, however, no MβL is encoded in the vicinity of stpA. Instead, a BLAST-search revealed a single MβL, stpB, encoded elsewhere in the P. marquandii genome, accompanied by a flavin-dependent monooxygenase (FMO), stpC, and an O-methyltransferase, stpD. Heterologous coexpression of stpA and stpC sufficed for reconstituting 1 biosynthesis in A. oryzae OP12 even without additional coexpression of stpB. Coexpression of stpC alongside the decaketide-synthase adaA involved in TAN-1612 biosynthesis also resulted in the production of 1, which implies that the formation of 1 proceeds via a decaketide precursor that is subsequently shortened. While the structure and biosynthesis of 1 are unique compared to other fungal naphthacenediones, further research is necessary to elucidate the elusive mechanism underlying the formation of 1.