Biosynthesis of the Polyoxins,Nucleoside Peptide Antibiotics: Formation of 5-Carboxyuracil Nucleosides by Streptomyces cacaoi

Biosynthetic origin of the 5-carboxyuracil base of the polyoxins and the octosyl acids is described. These unusual nucleosides are the metabolites of Streptomyces cacaoi var. asoensis. In vivo experiments show that the 5-carboxyuracil base of the polyoxins and the octosyl acids is biosynthesized from uracil and carbon-3 of serine. [2-¹⁴C]Thymine is incorporated into DNA-thymine but not into the 5-carboxyuracil base of these nucleosides. Exogenously supplied [2-¹⁴C]5-carboxyuracil is not taken up by cells. The biosynthesis of this base in the polyoxins is also shown to occur predominantly in the latest stage of the fermentation. Possible formation of this base through oxidation of thymine or 5-hydroxymethyluracil at the nucleoside level is discussed.