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J. Biol. Chem., Vol. 282, Issue 2, 976-985, January 12, 2007

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Knock-out of the Plastid-encoded PetL Subunit Results in Reduced Stability and Accelerated Leaf Age-dependent Loss of the Cytochrome b₆f Complex^*

From the Max-Planck-Institut für Molekulare Pflanzenphysiologie, Am Mühlenberg 1, D-14476 Potsdam-Golm, Germany

The cytochrome-b₆ f complex, a key component of the photosynthetic electron transport chain, contains a number of very small protein subunits whose functions are not well defined. Here we have investigated the function of the 31-amino acid PetL subunit encoded in the chloroplast genome in all higher plants. Chloroplast-transformed petL knock-out tobacco plants display no obvious phenotype, suggesting that PetL is not essential for cytochrome b₆ f complex biogenesis and function (Fiebig, A., Stegemann, S., and Bock, R. (2004) Nucleic Acids Res. 32, 3615–3622). We show here that, whereas young mutant leaves accumulate comparable amounts of cytochrome b₆ f complex and have an identical assimilation capacity as wild type leaves, both cytochrome b₆ f complex contents and assimilation capacities of mature and old leaves are strongly reduced in the mutant, indicating that the cytochrome b₆ f complex is less stable than in the wild type. Reduced complex stability was also confirmed by in vitro treatments of isolated thylakoids with chaotropic reagents. Adaptive responses observed in the knockout mutants, such as delayed down-regulation of plastocyanin contents, indicate that plants can sense the restricted electron flux to photosystem I yet cannot compensate the reduced stability of the cytochrome b₆ f complex by adaptive up-regulation of complex synthesis. We propose that efficient cytochrome b₆ f complex biogenesis occurs only in young leaves and that the capacity for de novo synthesis of the complex is very low in mature and aging leaves. Gene expression analysis indicates that the ontogenetic down-regulation of cytochrome b₆ f complex biogenesis occurs at the post-transcriptional level.

Received for publication, July 6, 2006 , and in revised form, November 17, 2006.

^* This work was supported by Deutsche Forschungsgemeinschaft Grant SFB 429, project A12 (to M.-A. S. and R. B.). 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.

¹ To whom correspondence should be addressed. Tel.: 49-331-567-8700; Fax: 49-331-567-8701; E-mail: schoettler{at}mpimp-golm.mpg.de.

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