Prenyltransferase involved in the biosynthesis of shikonin, a naphthoquinone secondary metabolite. Could accept only geranyl diphosphate and not dimethylallyl diphosphate, farnesyl diphosphate, or geranylgeranyl diphosphate as substrate. (2E)-geranyl diphosphate + 4-hydroxybenzoate = 3-geranyl-4-hydroxybenzoate + diphosphate Expressed only in roots. Up-regulated in the dark and by methyl jasmonate or oligogalacturonide. Down-regulated in the light and by ammonium or 2,4-dichlorophenoxyacetic acid (2,4-D). The N-terminus (1-130) is determinant for the chain length specificity. Region I (76-96) and region III (201-229) are involved in the recognition of both substrates in a coordinated manner. Belongs to the UbiA prenyltransferase family.