Monoterpene synthase (TPS) involved in the biosynthesis of monoterpene natural products used by traditional Chinese medicine to treat headache, inflammation and intoxication (Ref.1). Catalyzes the conversion of (2E)-geranyl diphosphate (GPP) into alpha-thujene (Ref.1). (2E)-geranyl diphosphate = alpha-thujene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Monomer. Expressed in developing and mature fruits (Ref.1). Barely detectable in leaves and shoots (Ref.1). The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family. Tpsb subfamily.