Demonstration of a metabolic grid at an early step in the streptonigrin biosynthetic pathway in streptomyces flocculus

The enzyme activities which catalyze the conversion of tryptophan to β-methyltryptophan by two different routes have been demonstrated in cell-free extracts of streptonigrin-producing Streptomyces flocculus. The first route involves direct methylation of tryptophan by a C-methyltransferase. The second involves transamination of tryptophan to indolepyruvate, methylation of indolepyruvate to β-methylindolepyruvate, followed by a reverse transamination reaction to yield β-methyltryptophan. The direct methylation route was confirmed by the fact that the methyltransferase activity is still present after the transaminase has been inactivated by hydroxylamine treatment. The L-tryptophan C-methyltransferase has been purified 30-fold by ammonium sulfate precipitation and a Sephadex G-150 column. The indolepyruvate C-methyltransferase activity copurified with the tryptophan C-methyltransferase activity, but the transaminase did not. These results show that a metabolic grid exists for the first antibiotic-committed step of the streptonigrin biosynthetic pathway.