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J Bacteriol, February 1998, p. 699-704, Vol. 180, No. 3
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Reconstitution of an Active Magnesium Chelatase Enzyme Complex from the bchI, -D, and -H Gene Products of the Green Sulfur Bacterium Chlorobium vibrioforme Expressed in Escherichia coli

Bent L. Petersen,¹ Poul Erik Jensen,² Lucien C. D. Gibson,² Bjarne M. Stummann,¹ C. Neil Hunter,² and Knud W. Henningsen^1,*

Department of Ecology and Molecular Biology, Royal Veterinary and Agricultural University, DK-1871 Frederiksberg C, Denmark,¹ and Krebs Institute for Biomolecular Research and Robert Hill Institute for Photosynthesis, Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S10 2TN, United Kingdom²

Received 2 July 1997/Accepted 21 November 1997

Magnesium-protoporphyrin chelatase, the first enzyme unique to the (bacterio)chlorophyll-specific branch of the porphyrin biosynthetic pathway, catalyzes the insertion of Mg²⁺ into protoporphyrin IX. Three genes, designated bchI, -D, and -H, from the strictly anaerobic and obligately phototrophic green sulfur bacterium Chlorobium vibrioforme show a significant level of homology to the magnesium chelatase-encoding genes bchI, -D, and -H and chlI, -D, and -H of Rhodobacter sphaeroides and Synechocystis strain PCC6803, respectively. These three genes were expressed in Escherichia coli; the subsequent purification of overproduced BchI and -H proteins on an Ni²⁺-agarose affinity column and denaturation of insoluble BchD protein in 6 M urea were required for reconstitution of Mg-chelatase activity in vitro. This work therefore establishes that the magnesium chelatase of C. vibrioforme is similar to the magnesium chelatases of the distantly related bacteria R. sphaeroides and Synechocystis strain PCC6803 with respect to number of subunits and ATP requirement. In addition, reconstitution of an active heterologous magnesium chelatase enzyme complex was obtained by combining the C. vibrioforme BchI and -D proteins and the Synechocystis strain PCC6803 ChlH protein. Furthermore, two versions, with respect to the N-terminal start of the bchI gene product, were expressed in E. coli, yielding ca. 38- and ca. 42-kDa versions of the BchI protein, both of which proved to be active. Western blot analysis of these proteins indicated that two forms of BchI, corresponding to the 38- and the 42-kDa expressed proteins, are also present in C. vibrioforme.

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^* Corresponding author. Mailing address: Department of Ecology and Molecular Biology, Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark. Phone: 45 35 28 26 08. Fax: 45 35 28 26 06. E-mail: blp{at}kvl.dk.

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