Summary: The disproportionate difficulty in obtaining compelling experimental evidence from C-radio-labelling that the indole moiety of the otherwise isoprenoid penitrem A is biosynthesized by directly from tryptophan has been explored, [-C]Tryptophan added to the broth beneath the mycelial mat of stationary liquid cultures labelled penitrem A with 1·4% incorporation, only threefold more than that determined for [-C]tryptophan or [U-C]tyrosine, incorporation of which could only have been indirect. In contrast, the substituted tryptophan-histidine diketopiperazine roquefortine, biosynthesized concurrently with penitrems by this organism, was labelled with compelling efficiency (23·4% incorporation of [-C]tryptophan). In submerged culture, concurrently biosynthesized an analogous pair of metabolites, 3-hydroxy-3-methylbutenyl paspalinine and lysergic acid α-hydroxyethylamide. This feature enabled experimental demonstratation of [-C]tryptophan incorporation to an extent more consistent with direct contribution of the indole moiety of the indole-diterpenoid paspalinine derivative. The same precursor applied to the sporing surface of stationary cultures also provided stronger evidence for a direct biosynthetic role in the formation of penitrem A. In the absence of competition from any other indolic secondary metabolite, a submerged culture of incorporated 5% of the [-C]tryptophan given during growth into the indole-diterpenoid paxilline. A double-labelling time-course experiment indicated temporal separation of steps in the biosynthesis of roquefortine. The inadequacy of classical precursor techniques for studying biosynthesis of indole-diterpenoids in is discussed. The more homogeneous submerged culture fermentation system is preferred for experimentation.


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