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.

Mo « ssbauer studies of the non-heme iron and cytochrome b 559 in a Chlamydomonas reinhardtii PSI Abstract Spin and valence states of the non-heme iron and the heme iron of cytochrome b 559 , as well as their interactions with K K-tocopherol quinone (K K-TQ) in photosystem II (PSII) thylakoid membranes prepared from the Chlamydomonas reinhardtii PSI 3 mutant have been studied using Mo «ssbauer spectroscopy. Both of the iron atoms are in low spin ferrous states. The Debye temperature of the non-heme is 194 K and of the heme iron is 182 K. The treatment of K K-TQ does not change the spin and the valence states of the non-heme iron but enhances the covalence of its bonds. K K-TQ oxidizes the heme iron into the high spin Fe 3+ state. A possible role of the non-heme iron and K K-TQ in electron £ow through the PSII is discussed.