In plants and algae The GENOMES UNCOUPLED4 (GUN4) protein stimulates chlorophyll biosynthesis by increasing the activity of Mg-chelatase,the enzyme that inserts magnesium into protoporphyrin Ⅸ in the chlorophyll biosynthesis pathway.One of GUN4’s roles is in binding protoporphyrin and Mg protoporphyrin and appears to participate in plastid-to-nucleus signalling possibly through regulating Mg-Proto synthesis or trafficking.