Protoporphyrinogen oxidase (Protox, EC 1.3.3.4) is the last enzyme in the common pathway of chlorophyll and heme biosynthesis (Beale and Weinstein, 1990). Protox catalyzes the oxidative O2-dependent aromatization of the colorless protoporphyrinogen IX (Protogen IX) to the highly conjugated protoporphyrin IX (Proto IX). Protox is also the target enzyme of phthalimide-type herbicides such as S23142