Prenylates umbelliferone in the presence of dimethylallyl diphosphate (DMAPP) at the 6 position to yield demethylsuberosin (DMS) as the main product, together with a minor amount of osthenol corresponding to 8-prenylated umbelliferone. No activity with other coumarine derivatives such as psoralen, bergapten and xanthotoxin, or with geranyl diphosphate or farnesyl diphosphate as prenyl donors. dimethylallyl diphosphate + umbelliferone = demethylsuberosin + diphosphate Inhibited by psoralen, a major downstream product. Optimum pH is 8.2. Expressed in roots, leaves and stems. Up-regulated upon UV-B irradiation. Belongs to the UbiA prenyltransferase family.