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. Does not bind abscisic acid. ATP + H2O + Mg(2+) + protoporphyrin IX = ADP + 3 H(+) + Mg-protoporphyrin IX + phosphate Porphyrin-containing compound metabolism; chlorophyll biosynthesis. The magnesium chelatase complex is a heterotrimer consisting of subunits CHLI, CHLD, AND CHLH. Interacts with CHLI1 and CHLH. Not regulated by light. Pigment defective seeds and embryos. Over-expression of CHLD has no impact on abscisic acid sensitivity. Belongs to the Mg-chelatase subunits D/I family. Extended N-terminus.