Lyase that catalyzes the formation of isoprene from dimethylallyl diphosphate. dimethylallyl diphosphate = diphosphate + isoprene recovered from purified chloroplasts of transgenic Arabidopsis plants. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Sufficient to confer the trait of isoprene emission when expressed in Arabidopsis. Belongs to the terpene synthase family. Tpsb subfamily.