Delineating citrinin biosynthesis: Ctn-ORF3 dioxygenase-mediated multi-step methyl oxidation precedes a reduction-mediated pyran ring cyclization

[Display omitted] Citrinin (3) is a polyketide-derived mycotoxin, that is, produced by Monascus, Penicillium, and Aspergillus spp. and is a common contaminant in a number of agricultural products. ctPKS, a non-reducing type iterative polyketide synthase with a C-terminal reductive domain, is proposed to generate the polyketide backbone of 3. The targeted gene inactivation of ctn-orf1 or ctn-orf3 gene resulted in the accumulation of a benzaldehyde derivative 6, and the ectopic expression of ctPKS/ctnB in yeast produced 6, demonstrating that ctPKS generates 6 with the support of CtnB and suggesting that Ctn-ORF1/Ctn-ORF3 converts 6 into 3. The Δctn-orf1 mutant also produced a novel benzdialdehyde derivative 10. When either 6 or 10 was fed into a ΔctPKS mutant, 3 was readily detected, which confirms that both 6 and 10 are involved in the biosynthesis of 3. A bioconversion experiment of 6 in the ectopic expression system demonstrated that ctn-orf3 expression, but not ctn-orf1 expression, efficiently consumed 6. The resulting metabolite(s) of 6 could not be identified, however. A recombinant Ctn-ORF3 enzyme was demonstrated to convert 6 into 10 and a hypothetical carboxylic derivative 8, which substantiates that Ctn-ORF3 oxidizes the exocyclic methyl moiety of 6. Ctn-ORF1 is thus proposed to reduce 8 and the subsequent non-enzymatic reactions to complete the biosynthesis of 3. The present study delineates the biosynthetic route of 3, proposing the biochemical mechanism, that is, involved in producing the natural dihydropyranoquinone structure.