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Eukaryotic Cell, October 2005, p. 1620-1628, Vol. 4, No. 10
1535-9778/05/$08.00+0     doi:10.1128/EC.4.10.1620-1628.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Mg-Protoporphyrin IX and Heme Control HEMA, the Gene Encoding the First Specific Step of Tetrapyrrole Biosynthesis, in Chlamydomonas reinhardtii

Zinaida Vasileuskaya,1 Ulrike Oster,2 and Christoph F. Beck1*

University of Freiburg, Institute of Biology III, Schaenzlestrasse 1, D-79104 Freiburg, Germany,1 University of Muenchen, Department of Biology 1, Botany, Menzingerstrasse 67, D-80638 Muenchen, Germany2

Received 25 May 2005/ Accepted 28 July 2005

HEMA encodes glutamyl-tRNA reductase (GluTR), which catalyzes the first step specific for tetrapyrrole biosynthesis in plants, archaea, and most eubacteria. In higher plants, GluTR is feedback inhibited by heme and intermediates of chlorophyll biosynthesis. It plays a key role in controlling flux through the tetrapyrrole biosynthetic pathway. This enzyme, which in Chlamydomonas reinhardtii is encoded by a single gene (HEMA), exhibits homology to GluTRs of higher plants and cyanobacteria. HEMA mRNA accumulation was inducible not only by light but also by treatment of dark-adapted cells with Mg-protoporphyrin IX (MgProto) or hemin. The specificity of these tetrapyrroles as inducers was demonstrated by the absence of induction observed upon the feeding of protoporphyrin IX, the precursor of both heme and MgProto, or chlorophyllide. The HEMA mRNA accumulation following treatment of cells with light and hemin was accompanied by increased amounts of GluTR. However, the feeding of MgProto did not suggest a role for Mg-tetrapyrroles in posttranscriptional regulation. The induction by light but not that by the tetrapyrroles was prevented by inhibition of cytoplasmic protein synthesis. Since MgProto is synthesized exclusively in plastids and heme is synthesized in plastids and mitochondria, the data suggest a role of these compounds as organellar signals that control expression of the nuclear HEMA gene.

* Corresponding author. Mailing address: Institute of Biology III, University of Freiburg, Schaenzlestrasse 1, D-79104 Freiburg, Germany. Phone: 49-761-203 2713. Fax: 49-761-203 2745. E-mail: beck{at}uni-freiburg.de.

Eukaryotic Cell, October 2005, p. 1620-1628, Vol. 4, No. 10
1535-9778/05/$08.00+0     doi:10.1128/EC.4.10.1620-1628.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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