Five Glutamic Acid Residues in the C-Terminal Domain of the ChlD Subunit Play a Major Role in Conferring Mg2+ Cooperativity upon Magnesium Chelatase

Magnesium chelatase catalyzes the first committed step in chlorophyll biosynthesis by inserting a Mg2+ ion into protoporphyrin IX in an ATP-dependent manner. The cyanobacterial (Synechocystis) and higher-plant chelatases exhibit a complex cooperative response to free magnesium, while the chelatases from Thermosynechococcus elongatus and photosynthetic bacteria do not. To investigate the basis for this cooperativity, we constructed a series of chimeric ChlD proteins using N-terminal, central, and C-terminal domains from Synechocystis and Thermosynechococcus. We show that five glutamic acid residues in the C-terminal domain play a major role in this process.