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SYSTEMS BIOLOGY, MOLECULAR BIOLOGY, AND GENE REGULATION

CIA2 Coordinately Up-Regulates Protein Import and Synthesis in Leaf Chloroplasts^1,[W],[OA]

Department of Life Sciences, National Taiwan Normal University, Taipei 116, Taiwan

Plastid biogenesis and maintenance depend on the coordinated assembly of proteins imported from the cytosol with proteins translated within plastids. Chloroplasts in leaf cells have a greater need for protein import and protein synthesis than plastids in other organs due to the large amount of proteins required for photosynthesis. We previously reported that the Arabidopsis (Arabidopsis thaliana) transcription factor CIA2 specifically up-regulates leaf expression of genes encoding protein translocons Toc33 and Toc75, which are essential for protein import into chloroplasts. Protein import efficiency was therefore reduced in cia2 mutant chloroplasts. To further understand the function of CIA2, gene expression profiles of the wild type and a cia2 mutant were compared by microarray analysis. Interestingly, in addition to genes encoding protein translocon components, other genes down-regulated in cia2 almost exclusively encode chloroplast ribosomal proteins. Isolated cia2 mutant chloroplasts showed reduced translation efficiency and steady-state accumulation of plastid-encoded proteins. When CIA2 was ectopically expressed in roots, expression of both the protein translocon and ribosomal protein genes increased. Further analyses in vivo revealed that CIA2 up-regulated these genes by binding directly to their promoter regions. We propose that CIA2 is an important factor responsible for fulfilling the higher protein demands of leaf chloroplasts by coordinately increasing both protein import and protein translation efficiencies.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Chih-Wen Sun (cwsun{at}ntnu.edu.tw).

^[W] The online version of this article contains Web-only data.

^[OA] Open access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.109.137240

^* Corresponding author; e-mail cwsun{at}ntnu.edu.tw.

Received February 18, 2009; accepted April 14, 2009; published April 22, 2009.