Growth of in the presence of valine led to a greater production of spiramycin and excretion of short-chain fatty acids compared with growth on ammonium ions as nitrogen source. We determined the activities of enzymes that lead to the formation of malonyl-CoA, a key intermediate in the biosynthesis of spiramycin. In valine-grown cultures, the activities of acetyl-CoA carboxylase and pyruvate dehydrogenase were increased during the antibiotic production phase. During this idiophase, oxaloacetate dehydrogenase activity was much higher in cultures growing on ammonium compared with valine. Two pathways are proposed concerning the mode of malonyl-CoA formation: a route via oxaloacetate dehydrogenase following growth on ammonium, and a route via acetyl-CoA carboxylase after growth on valine.


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