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J. Biol. Chem., Vol. 279, Issue 48, 50366-50374, November 26, 2004

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The Nucleus-encoded Protein MOC1 Is Essential for Mitochondrial Light Acclimation in Chlamydomonas reinhardtii^*

Christine Schönfeld, Lutz Wobbe, Rüdiger Borgstädt, Alexandra Kienast, Peter J. Nixon, and Olaf Kruse¶

From the Molecular Cell Physiology Group, Department of Biology, University of Bielefeld, 33501 Bielefeld, Germany and the Department of Biological Sciences, Wolfson Biochemistry Building, Imperial College London, South Kensington Campus, London, SW7 2AZ, United Kingdom

Mitochondrial respiration plays an important role in optimizing photosynthetic efficiency in plants. As yet, the mechanisms by which plant mitochondria sense and respond to changes in the environment are unclear, particularly when exposed to light. Here we describe the characterization of the Chlamydomonas reinhardtii mutant stm6, which was identified on the basis of impaired state transitions, a mechanism that regulates light harvesting in the chloroplast. The gene disrupted in stm6, termed Moc1, encodes a homologue of the human mitochondrial transcription termination factor (mTERF). MOC1 is targeted to the mitochondrion, and its expression is up-regulated in response to light. Loss of MOC1 causes a high light-sensitive phenotype and disrupts the transcription and expression profiles of the mitochondrial respiratory complexes causing, as compared with wild type, light-mediated changes in the expression levels of nuclear and mitochondrial encoded cytochrome c oxidase subunits and ubiquinone-NAD subunits. The absence of MOC1 leads to a reduction in the levels of cytochrome c oxidase and of rotenone-insensitive external NADPH dehydrogenase activities of the mitochondrial respiratory electron transfer chain. Overall, we have identified a novel mitochondrial factor that regulates the composition of the mitochondrial respiratory chain in the light so that it can act as an effective sink for reductant produced by the chloroplast.

Received for publication, July 27, 2004 , and in revised form, September 14, 2004.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) AF531421.

^* This work was supported by Grant FOR387 the Deutsche Forschungsgemeinschaft for financial support. 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.

^¶ To whom correspondence should be addressed. Tel.: +49-521-1065611; Fax: +49-521-1066410; E-mail: olaf.kruse{at}uni-bielefeld.de.

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