A biosynthetic pathway for BE-7585A, a 2-thiosugar-containing angucycline-type natural product

Sulfur is an essential element found ubiquitously in living systems. However, there exist only a few sulfur-containing sugars in nature and their biosyntheses have not been studied. BE-7585A produced by Amycolatopsis orientalis subsp. vinearia BA-07585 has a 2-thiosugar and is a member of the angucycline class of compounds. We report herein the results of our initial efforts to study the biosynthesis of BE-7585A. Spectroscopic analyses verified the structure of BE-7585A, which is closely related to rhodonocardin A. Feeding experiments using ¹³C-labeled acetate were carried out to confirm that the angucycline core is indeed polyketide-derived. The results indicated an unusual manner of angular tetracyclic ring construction, perhaps via a Baeyer-Villiger type rearrangement. Subsequent cloning and sequencing led to the identification of the bex gene cluster spanning ∼30 kbp. A total of 28 open reading frames, which are likely involved in BE-7585A formation, were identified in the cluster. In view of the presence of a homologue of a thiazole synthase gene (thiG), bexX, in the bex cluster, the mechanism of sulfur incorporation into the 2-thiosugar moiety could resemble that found in thiamin biosynthesis. A glycosyltransferase homologue, BexG2, was heterologously expressed in Escherichia coli. The purified enzyme successfully catalyzed the coupling of 2-thioglucose 6-phoshate and UDP-glucose to produce 2-thiotrehalose 6-phosphate, which is the precursor of the disaccharide unit in BE-7585A. On the basis of these genetic and biochemical experiments, a biosynthetic pathway for BE-7585A can now be proposed. The combined results set the stage for future biochemical studies of 2-thiosugar biosynthesis and BE-7585A assembly.