ATPase and Phosphate Exchange Activities in Mg-Chelatase Subunits of Chlorobium and Synechocystis

Magnesium protoporphyrin IX is produced as an intermediate in the synthesis of Chl/Bchls. Insertion of magnesium into protoporphyrin IX is a complex ATP-dependent reaction catalysed by the enzyme Mg-chelatase. In Bchl/Chl-synthesising organisms three separate Mg-chelatase subunit proteins, designated BchD/Ch1D, BchH/Ch1H and BchI/ChlI, are involved in the chelation reaction. In the reaction ATP is required for an activation step as well as the metal insertion step (1). Only the subunits D and I participate in the activation step (2). In Rhodobacter sphaeroides the BchH subunit and combination of the BchI and -D subunits were shown to catalyse ATP hydrolysis (3). In addition, the R. sphaeroides BchI subunit was shown to catalyse a transfer of PO4 from ATP to ADP (phosphate exchange activity). The ATPase activity of the combined BchI and -D was suggested to be involved in the activation step, whereas the ATPase activity of BchH and the exchange activity of BchI were thought to participate in the subsequent reactions leading to the insertion of Mg2+ into protoporphyrin IX (3). In the present study the Mg-chelatase encoding genes of the green sulfur bacterium Chlorobium vibrioforme (4,5) and of the cyanobacterium Synechocystis (6,7) were expressed separately in Escherichia coli.