Distribution of ATPase and ATP-to-ADP phosphate exchange activities in magnesium chelatase subunits of Chlorobium vibrioforme and Synechocystis PCC6803

Insertion of magnesium into protoporphyrin IX is a complex ATP-dependent re action catalysed by the enzyme Mg-chelatase. Three separate proteins (Mg-ch elatase subunits), designated as D, II and I, are involved in the chelation reaction. The genes encoding the Mg-chelatase subunits of the green sulfur bacterium Chlorobium vibrioforme and of the cyanobacterium Synechocystis s train PCC6803 were expressed in Escherichia coli. The recombinant proteins were purified, tested for ATPase and phosphate exchange activities, and com pared with the activities of the corresponding subunits of Rhodobacter spha eroides. The Synechocystis strain PCC6803 I subunit and the C. vibrioforme E-l and I subunits hydrolysed ATP at the rates of 2.0, 1.8 and 0.16 nmol (m g protein)(-1) min(-1), respectively. The ATPase activity of the C. vibriof orme H subunit was similar to that reported for the R. sphaeroides H subuni t. The Synechocystis strain PCC6803 H subunit failed to hydrolyse ATP. The I subunit of Synechocystis strain PCC6803 and C.vibrioforme catalysed a tra nsfer of PO4 from ATP to ADP (exchange activity) at die rate of 1.75 +/- 0. 15 nmol (mg protein)(-1) min(-1). This exchange rate was 300-fold lower tha n that reported for the R. sphaeroides I subunit. The PO4 exchange activiti es were correlated with the presence of the sequence GXRGTGKSTXVRALA in the primary structure of the three I subunits. Mg-chelatase: activity was reco nstituted by combining the three subunits of the same bacterium [rates of 4 1-89 pmol Mg-deuteroporphyrin (mg protein)(-1) min(-1)], Heterologous subun it combinations resulted in low or no Mg-chelatase activity.