Mg-chelatase of tobacco

Identification of a Chl D cDNA sequence encoding a third subunit, analysis of the interaction of the three subunits with the yeast two-hybrid system, and reconstitution of the enzyme activity by co-expression of recombinant CHL D, CHL H and CHL I

authored by
Jutta Papenbrock, Susanna Gräfe, Elisabeth Kruse, Frank Hänel, Bernhard Grimm

Mg-protoporphyrin IX chelatase catalyzes insertion of the magnesium ion into protoporphyrin IX, the last common intermediate precursor in chlorophyll and heme biosynthesis, to form Mg-protoporphyrin IX. In Rhodobacter sphaeroides, and Synechocystis, the three open reading frames bchD/chlD, bchH/chlH and bchl/chll encode proteins which are required for in vitro Mg-chelatase activity. In higher plants also, three proteins are necessary for the Mg chelation, and genes homologous to bchH and bchI have been isolated previously. In this study, a novel tobacco cDNA sequence homologous to bchD is isolated and initially characterized. Together with the tobacco clones encoding the other two subunits, full-length cDNAs are now available for the first time for all three subunits of one plant species. The CHL D polypeptide deduced from the open reading frame encodes a protein of 758 aa (82.9 kDa) with an amino terminal extension that resembles a plastid transit peptide. Sequence comparison of tobacco CHL D revealed similarities to the D subunit of Rhodobacter and Synechocystis of 44% and 75%. The amino terminal half of CHL D shows significant similarity (46%) to the entire CHL I peptide sequence, indicating a gene duplication from an ancestral gene. The carboxy terminal half seemed to be unique. Both parts of CHL D are linked with a glutamine/asparagine/proline-rich region flanked by a highly acid-rich segment. Protein-protein interaction among the three subunits CHL D, H and I was studied using the yeast two-hybrid system. Physical interaction was demonstrated between CHL D and CHL I indicating that CHL D is part of the Mg-chelatase. Heterodimer formation of CHL H with CHL I or CHL D could not be demonstrated by transactivation of the lacZ reporter gene. Homodimerization of the CHL D subunit was indicated in the more sensitive assay on X-Gal-containing agar plates. In vitro Mg2+ insertion into protoporphyrin IX was demonstrated in protein extracts of yeast strains expressing the three subunits of tobacco Mg-chelatase. The reconstitution of the recombinant enzyme activity required additional ATP.

Institute of Plant Genetics
External Organisation(s)
Leibniz Institute of Plant Genetics and Crop Plant Research (IPK)
Leibniz Institute for Natural Product Research and Infection Biology Hans Knöll Institute (HKI)
Plant Journal
No. of pages
Publication date
Publication status
Peer reviewed
ASJC Scopus subject areas
Genetics, Plant Science, Cell Biology
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