Use of carbon-13 in biosynthetic studies: origin of the malonyl coenzyme A incorporated into tetracycline by Streptomyces aureofaciens.

The proton noise decoupled 13C nuclear magnetic resonance spectrum of tetracycline hydrochloride prepared from Streptomyces aureofaciens cultures supplemented with [1-13C]acetate and [2-13C]acetate showed enrichment of nine alternating ring carbons. In addition, a small enrichment of the carboxamide carbon by [1-13C]acetate was observed. The labelling patterns clearly demonstrated the polyketide origin of the tetracyclic nucleus. The 13C nuclear magnetic resonance spectrum of tetracycline hydrochloride derived from [1,2-13C]acetate showed all 18 ring carbons as doublets with coupling constants appropriate for the incorporation of nine intact two-carbon precursors, confirming that head-to-tail condensation of C2 units had occurred. Absence of bond scission within the C2 units and a low level of uncoupled 13C in the carboxamide substituent indicated that when the organism is supplemented with acetate, malonyl coenzyme A used for tetracycline biosynthesis is formed by direct carboxylation of acetyl coenzyme A.