Equipped with 1-4, we evaluated their antibacterial, antifungal, antiparasitic, herbicidal and cytotoxic activities (Table and Table Considering the heterologous expression results, we propose the biosynthetic pathway to 1 outlined in Fig. . BueA, with the assistance of the trans-ER BueB, synthesises the backbone polyketide from malonyl-CoA and acetyl-CoA, with methylations derived from S-adenosylmethionine (SAM). A Diels-Alder reaction catalysed by BueA takes place at the heptaketide stage to form the trans-decalin ring from the acyclic substrate, in a mechanism resembling the cyclisation by LovB in lovastatin biosynthesis. However, LovB contains a condensation domain that plays a role in controlling the cyclisation, which is not present in BueA. Considering that the molecular basis of Diels-Alder formation within HR-PKS are still unknown, BueA serves as another example of PKS domain architecture that can perform decalin cyclisations.