Involved in chlorophyll biosynthesis. Catalyzes the insertion of magnesium ion into protoporphyrin IX to yield Mg-protoporphyrin IX. The magnesium-chelatase is a complex of three subunits, CHLI, CHLD and CHLH. The reaction takes place in two steps, with an ATP-dependent activation followed by an ATP-dependent chelation step. Possesses high affinity for ATP and may play a major role in chlorophyll biosynthesis. Does not bind abscisic acid (ABA), but is a positive regulator of ABA signaling. ATP + H2O + Mg(2+) + protoporphyrin IX = ADP + 3 H(+) + Mg-protoporphyrin IX + phosphate Redox regulation; active in reducing conditions, inactive in oxidizing conditions. Thioredoxins f and m mediate the reversible reductive activation of oxidized CHLI1. Porphyrin-containing compound metabolism; chlorophyll biosynthesis. The magnesium chelatase complex is a heterotrimer consisting of subunits CHLI, CHLD, AND CHLH. Interacts with CHLH and CHLD. Not regulated by light. Homozygous mutants are chlorotic lethal when grown on soil, but can grow on sucrose-containing medium. Belongs to the Mg-chelatase subunits D/I family.