Proposed mechanism of A-74528 biosynthesis. For transformations in which multiple enzymes are bracketed and denoted with *, the precise enzyme(s) involved in the polyketide tailoring step is unclear. (For example, SanJ, SanK, SanL, SanP, and SanQ are all possible candidates for the monooxygenase responsible for the C28–29 epoxide formation.) For all other steps, enzyme function is assigned based on previously characterized functions of paralogs in other type II PKS gene clusters. For details, see Discussion section.