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J. Biol. Chem., Vol. 283, Issue 41, 27776-27784, October 10, 2008

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Characterization of Three Homologs of the Large Subunit of the Magnesium Chelatase from Chlorobaculum tepidum and Interaction with the Magnesium Protoporphyrin IX Methyltransferase^*

From the Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, St. Paul, Minnesota 55108

Green bacteria synthesize several types of (bacterio)chlorophylls for the assembly of functional photosynthetic reaction centers and antenna complexes. A distinctive feature of green bacteria compared with other photosynthetic microbes is that their genomes contain multiple homologs of the large subunit (BchH) of the magnesium chelatase which is a three-subunit enzyme complex (BchH, BchD, and BchI) that inserts magnesium into protoporphyrin IX as the first committed step of (bacterio)chlorophyll biosynthesis. There is speculation that the additional BchH homologs may regulate the biosynthesis of each type of chlorophyll, although the biochemical properties of the different magnesium chelatase complexes from a single species of green bacteria have not yet been compared. In this study, we investigated the activities of all three chelatase complexes from the green sulfur bacterium Chlorobaculum tepidum and interactions with the next enzyme in the pathway, magnesium protoporphyrin IX methyltransferase (BchM). Although all three chelatase complexes insert magnesium into protoporphyrin IX, the activities range by a factor of 10⁵. Further, there are differences in the interactions between the BchH homologs and BchM; two of the subunits increase the methyltransferase activity by 30–60%, and the third decreases it by 30%. Expression of the chelatase complexes alone and together with BchM in Escherichia coli overproducing protoporphyrin IX suggests that the chelatase is the rate-limiting enzyme. We observed that BchM uses protoporphyrin IX without bound metal as a substrate. Our results conflict with expectations generated by previous gene inactivation studies and suggest a complex regulation of chlorophyll biosynthesis in green bacteria.

Received for publication, June 12, 2008 , and in revised form, August 7, 2008.

^* This research was supported by the David and Lucile Packard Foundation, the Institute on the Environment of the University of Minnesota, and National Science Foundation Grant CBET-0756296. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1–S5.

¹ To whom correspondence should be addressed: Dept. of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, 1479 Gortner Ave., St. Paul, MN 55108. Tel.: 612-625-5782; Fax: 612-625-5780; E-mail: schmi232{at}umn.edu.

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