Mechanism of the incidental production of a melanin-like pigment during 6-demethylchlortetracycline production in Streptomyces aureofaciens

The secondary metabolite 6-demethychlortetracycline (6-DCT)? which is produ ced by Streptomyces aureofaciens, is used as a precursor of semisynthetic t etracyclines. Strains that produce 6-DCT also produce a melanin-like pigmen t (MP). The correlation between MP production and 6-DCT production was inve stigated qv using S. aureofaciens NRRL 3203, Production of both MP and 6-DC T was repressed by phosphate or ammonium ions, suggesting that syntheses of these compounds are controlled by the same regulators, Ten chlortetracycli ne-producing recombinants were derived from 6-DCT-producing mutant NRRL 320 3 by gene replacement. All of the recombinants produced chlortetracycline b ut not MP, indicating that MP production is the results of a defect in the 6-methylation step and suggesting that the polyketide nonaketideamide is a common intermediate leading to MP as well as 6-DCT. To further examine the possibility that MP might be synthesized via the 6-DCT-biosynthetic pathway , mutants defective in 6-DCT biosynthesis were derived from a 6-DCT produce r. Some of these mutants were able to produce AIP, while others, including mutants with mutations in the gene encoding anhydrotetracycline oxygenase, an enzyme catalyzing the penultimate step in the pathway, produced neither B-DCT nor MP. Production of 6-DCT and production of MP were restored simult aneously by integrative transformation with the corresponding 6-DCT-biosynt hetic genes, indicating that some of 6-DCT-biosynthetic enzymes are indispe nsable for MP production. These findings suggest that a defect in the 6 met hylation step results in redirection of carbon flux from a certain intermed iate in the 6-DCT-biosynthetic pathway to a shunt pathway and results in MP production.