Radical S-Adenosylmethionine Sulfurtransferase MybB Catalyzed Formation of the 4-Thiazolidinone Core in Mycobacidin Represents an Intersection between Primary and Secondary Metabolism

Abstract

Mycobacidin is an antitubercular antibiotic structurally composed of a sulfur-containing 4-thiazolidinone ring, yet its biosynthesis including the mechanism of sulfur incorporation has remained an open question since its discovery in 1952. In this study, the mycobacidin biosynthetic gene cluster is identified from soil-dwelling Streptomyces, and the corresponding biosynthetic pathway starting with 7-oxoheptanoate is characterized. The radical SAM enzyme MybB catalyzes two sulfur insertion reactions, thereby bridging C₇ and C₁₀ to complete the 4-thiazolidinone heterocycle as the final step in mycobacidin maturation. MybB is a homologue of biotin synthase, and in both biosynthetic pathways, the bridging sulfur originates from the degradation of an enzymatic auxiliary [Fe₂S₂] cluster. Introduction of the two C–S bonds during 4-thiazolidinone cyclization is shown to take place in a fixed sequence with the terminal C₁₀–S bond generated first followed by the C₇–S bond. MybB thus represents a generalization of biotin synthase activity that contributes to the maturation of not only primary but also secondary metabolites via sequential sulfur insertion reactions to yield sulfur containing heterocycles.