Biosynthesis of mycotoxin tenuazonic acid by novel type of fungal PKS/NRPS hybrid enzyme

The recent sequencing of fungal genomes has shown that fungi are richer in secondary metabolism-related genes than previously believed. However, the majority of these genes are silent or quiescent under laboratory culture conditions. During the past decade, successful approaches such as epigenetic regulation, promoter exchange, and heterologous expression have been applied to activate these silent gene clusters in fungi. In this study, we activated the tenuazonic acid (TeA) biosynthetic gene in Magnaporthe oryzae (Rice blast fungus) via disruption of OSM1, which is involved in the response to environmental signals in M. oryzae. Additionally, while screening compounds that induce the production of secondary metabolites, we found that 1% DMSO also induced TeA production in M. oryzae under static culture conditions. TeA is a well-known mycotoxin produced by various plant pathogenic fungi. However, its biosynthetic gene has been unknown to date. Thus, we undertook this study to identify the biosynthetic gene of TeA in M. oryzae. We identified the TeA biosynthetic gene from M. oryzae by finding two TeA-inducing conditions of a low-producing strain using DNA micro array. We demonstrated that TeA was synthesized from isoleucine and acetoacetyl-coenzyme A by TeA synthetase 1 (TAS1) using a recombinant protein. TAS1 is a unique non-ribosomal peptide synthetase and polyketide synthase (NRPS-PKS) hybrid enzyme that begins with an NRPS module and this report is the first to describe this type of fungal enzyme. In contrast to other PKS/NRPS hybrid enzymes, the PKS portion of TAS1 has only a ketosynthase (KS) domain and we demonstrate that this KS domain is indispensable for TeA biosynthesis through domain deletion and an in vitro enzyme reaction. Phylogenetic analysis classifies this KS domain as an independent clade close to type I PKS KS domain. We demonstrated that the TAS1 KS domain was involved in the final cyclization step for TeA release (Fig. 1). These results indicate that TAS1 is a unique type of NRPS-PKS hybrid enzyme [1].

Fig. 1: Proposed model for tenuazonic acid biosynthesis.

Keywords: secondary metabolite, tenuazonic acid, Magnaporthe oryzae, NRPS-PKS.

References:

[1] Yun C-S, Motoyama T, Osada H. Biosynthesis of the mycotoxin tenuazonic acid by a fungal NRPS-PKS hybrid enzyme. Nat Commun 2015; 6: 8758