Biosynthesis of the aurovertins B and D. The role of methionine and propionate in the simultaneous operation of two independent biosynthetic pathways

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Abstract
The complete assignment of the natural abundance 13C n.m.r. spectra of the aurovertins B (1) and D (2), toxic metabolites isolated from Calcarisporium arbuscula NRRL 3705, permitted a study of their biosynthetic origin using the following 13C-labelled precursors: [1-13C]-, [2-13C]-, [1,2-13C]-, and [2-13C, 2-2H3]-acetate, (2S)-[methyl-13C]methionine, [2-13C]malonate, and [1-13C]- and [3-13C]propionate. The results show that both the aurovertins B and D can be formed via two biosynthetic pathways which are distinguishable by the different origins of C(1)–C(3). The first pathway involves the C-methylation of a C20-polyketide precursor at C18, followed by the loss of the chain-initiating acetate unit, C19–C20; C(1) in the aurovertins is thus derived from methionine and C(2) and C(3) from malonate. The second pathway involves a C19-precursor, formed from a propionate chain-initiating unit and eight malonate units; C(1)–C(3) are derived from propionate. The simultaneous operation of two independent pathways in the biosynthesis of the aurovertins B and D is unique amongst fungal metabolites.