Substrate specificities of Fusarium biosynthetic enzymes explain the genetic basis of a mixed chemotype producing both deoxynivalenol and nivalenol-type trichothecenes

Abstract

Fusarium species are traditionally grouped into type A and type B trichothecene producers based on structural differences in the mycotoxin they synthesize. The type B trichothecene-producing Fusarium graminearum strains are further divided into 3-acetyldeoxynivalenol (3-ADON), 15-acetyldeoxynivalenol (15-ADON), and nivalenol (NIV) chemotypes. The former two chemotypes, collectively termed a deoxynivalenol (DON) chemotype, evolved from a NIV chemotype by inactivation of FgTri13, which encodes trichothecene C-4 hydroxylase, during the evolutionary process. Despite stable overexpression of FgTri13, however, both 3-acetylnivalenol (3-ANIV) and 3-ADON accumulate equally in shake flask culture of a transgenic 3-ADON chemotype. In this study, we investigated why the "3-ANIV chemotype" could not be obtained using this strategy. When analysis was extended to the transgenic NIV chemotype, in which FgTri7 C-4 acetylase gene was disrupted and FgTri8 deacetylase gene was replaced with the 3-ADON chemotype's orthologue, C-4 unoxygenated 3-ADON, as well as C-4 oxygenated 3-ANIV, accumulated as the end product. A feeding experiment with an ΔFgtri5ΔFgtri3 double gene disruptant, a trichothecene non-producing mutant unable to acetylate C-15 of the trichothecene ring, revealed the importance of the 15-O-acetyl group for efficient C-4 hydroxylation of DON-type trichothecenes. This implies that traditional DON and NIV chemotype diversification is not solely explained by FgTri13, but is also explained by the function of the FgTri8 trichothecene deacetylase gene. None of the crude cell extracts from existing chemotypes showed highly specific C-15 deacetylation activity against 3,15-diacetylnivalenol (3,15-diANIV) without deacetylating C-15 of the C-4 unoxygenated earlier intermediate, 3,15-diacetyldeoxynivalenol. Thus, an unnatural Fusarium trichothecene, 3-ANIV, could only be synthesized as part of a mixture with 3-ADON, unless the esterase encoded by FgTri8 evolves to act on the 15-O-acetyl of 3,15-diANIV with high specificity. We also explain why the transgenic "15-ANIV chemotype", which can be generated through functional inactivation of FgTri7, uses an engineered pathway via 3,15-diANIV, but not 15-ADON, to generate 15-ANIV. Tri genes appear to evolve continuously, and altered functions of trichothecene pathway enzymes result in the generation of new trichothecenes, such as NX-2 and NX-3, which have been recently discovered in field isolates of F. graminearum. As recombination of FgTri8 between existing F. graminearum isolates could give rise to a strain that produces mixtures of DON and NIV-type trichothecenes, it may also be noteworthy to monitor the emergence of a field isolate that invalidates traditional chemotype classification.

Keywords: 15-Acetyldeoxynivalenol (PubChem CID: 10382483); 15-Acetylnivalenol (PubChem CID: 132559605); 3-Acetyldeoxynivalenol (PubChem CID: 5458510); 3-Acetylnivalenol; 4-Acetylnivalenol (PubChem CID: 102515408); Artificial metabolic route; Cytochrome P450 monooxygenase; Deoxynivalenol (PubChem CID: 40024); Fusarium graminearum; Nivalenol (PubChem CID: 5284433); Trichothecene deacetylase; Type B trichothecene mycotoxin.