The Biosynthesis of the Nucleoside Antibiotics

The data presented suggest that GTP, but neither ATP nor ITP, is a substrate for the biosynthesis of the pyrrole ring of the pyrrolopyrimidine nucleoside antibiotics. GTP-8-formylhydrolase from Streptomyces rimosus has been purified 240-fold; the enzyme is not constitutive and chloramphenicol markedly inhibits the biosynthesis of the pyrrolopyrimidine nucleoside antibiotic, sangivamycin, when added to cells in the late log phase of growth. As sangivamycin production increases, the specific activity of GTP-8-formylhydrolase also increases. A streptomyces that produces the nucleoside antibiotic, showdomycin, does not have this enzyme. This enzyme catalyzes the release of carbon 8 of the imidazole ring of GTP as formic acid. The molecular weight is about 500,000. DEAE chromatography splits the enzyme into two active proteins when precipitated with ammonium sulfate and dialyzed against buffer without EDTA. The purified enzyme requires EDTA for full activity; oxygen, p-chloromercuribenzoate, and showdomycin are inhibitory; cysteine reverses this inhibition. These properties of the enzyme along with the in vivo studies and the correlation of enzyme activity in the streptomyces-producing pyrrolopyrimidine nucleoside antibiotics supply information that the ring-opened GTP is the branch point compound for the biosynthesis of folic acid, riboflavin, xanthopterin, toxoflavin, 5,6-dimethylbenzimidazole, and the pyrrolopyrimidine nucleoside antibiotics.