A heptaketide naphthaldehyde produced by a polyketide synthase from Nectria haematococca

Bostrycoidin and fusarubin are biologically active fungal polyketides produced by Nectria haematococca. This azaanthraquinone and naphthoquinone are thought to be biosynthesized via formation of a C ₁₄ heptaketide aldehyde as a common key intermediate. A BLAST search against the genome of N. haematococca revealed one candidate gene (NECHADRAFT-101778, NhPKS1), which encodes a multi-domain polyketide synthase (PKS) with a thiol reductase (TR) domain that would facilitate the reductive release of the intermediate to produce a free aldehyde. To investigate the possible involvement of NhPKS1 in the biosynthesis of bostrycoidin and fusarubin, NhPKS1 was heterologously expressed in Aspergillus oryzae, and shown to produce a heptaketide 3-acetonyl-1,6,8-trihydroxy-2-naphthaldehyde as a single product. Thus, NhPKS1 catalyzes a C-2/C-11 and C-4/C-9 aldol-type cyclization of a linear intermediate followed by a subsequent reductive product release to yield the naphthaldehyde. The results indicate NhPKS1 is the enzyme involved in the biosynthesis of bostrycoidin and fusarubin.