text: Most photosynthetic, oxygen-producing organisms contain the positive chlorophyll biosynthesis regulator GENOMES UNCOUPLED 4 (GUN4). Research suggests that GUN4 regulates chlorophyll synthesis, by activating the enzyme Magnesium chelatase, which catalyzes the insertion of Mg into Protoporphyrin IX. Bilins noncovalently bind to CrGUN4, an algal GUN4 from Chlamydomonas reinhardtii, which has been shown to participate in retrograde signaling.