Rearrangement of the skeleton is crucial for improving the structural complexity and diversity of type II polyketide natural products. In this study, we investigated the rearrangement process from a planar aromatic tetracyclic intermediate to the caged lactones, which is managed by five oxidoreductases. We chemically synthesized the proposed linear tetracyclic substrate, validated the transformation process through in vivo and in vitro experiments, and elucidated the enzyme-catalyzed mechanism using isotope labeling. Significantly, a short-chain dehydrogenase TjhD5 was discovered to play a multifunctional role for multistep reactions.