The enzyme dimethylallyltryptophan synthase catalyzes the "normal" prenylation of Tip at C-4 in the first step of ergot alkaloid biosynthesis. The Lys174Ala mutant is found to produce a hexahydropyrroloindole alkaloid that is "reverse-prenylated" at C-3 as its major product. This is interpreted as evidence in support of a mechanism that involves an initial "reverse-prenylation" at C-3, followed by a Cope rearrangement and rearomatization.