The biosynthesis of brassicicolin A in the phytopathogen Alternaria brassicicola.

Alternaria brassicicola (Schwein.) Wiltshire is a phytopathogenic fungus that together with A. brassicae causes Alternaria black spot disease in Brassica species. Brassicicolin A is the major host-selective phytotoxin produced in cultures of A. brassicicola. Biosynthetic studies to establish the metabolic precursors of brassicicolin A were carried out with isotopically labeled compounds. Incorporation of D-[13C6]glucose, L-[15N]valine, or L-[2H8]valine into brassicicolin A was established using 1H, 13C, 15N NMR and INADEQUATE spectroscopy and HPLC-ESI-MS spectrometry. Based on analyses of the spectroscopic data, the labeling patterns of brassicicolin A isolated from cultures incubated with the labeled precursors are found to be consistent with both the glycolytic and the valine pathways. That is, the carbons of mannitol and acetyl units and the isocyanide carbon atoms are derived from D-[13C6]glucose whereas the hydroxyisopentanoyl and isocyanoisopentanoyl units are derived from L-valine, including the nitrogen atoms of both isocyanide groups.