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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Tyrosine and Phenylalanine Are Synthesized within the Plastids in Arabidopsis^1,[W]

Laboratoire de Physiologie Cellulaire Végétale, Institut National de la Recherche Agronomique, UMR 1200, F–38054 Grenoble, France; Centre de la Recherche Scientifique, UMR 5168, F–38054 Grenoble, France; Université Grenoble I, F–38054 Grenoble, France; and Institute of Life Sciences Research and Technologies, Commissariat à l'Energie Atomique, F–38054 Grenoble, France

While the presence of a complete shikimate pathway within plant plastids is definitively established, the existence of a cytosolic postchorismate portion of the pathway is still debated. This question is alimented by the presence of a chorismate mutase (CM) within the cytosol. Until now, the only known destiny of prephenate, the product of CM, is incorporation into tyrosine (Tyr) and/or phenylalanine (Phe). Therefore, the presence of a cytosolic CM suggests that enzymes involved downstream of CM in Tyr or Phe biosynthesis could be present within the cytosol of plant cells. It was thus of particular interest to clarify the subcellular localization of arogenate dehydrogenases (TYRAs) and arogenate dehydratases (ADTs), which catalyze the ultimate steps in Tyr and Phe biosynthesis, respectively. The aim of this study was to address this question in Arabidopsis (Arabidopsis thaliana) by analysis of the subcellular localization of the two TYRAAts and the six AtADTs. This article excludes the occurrence of a spliced TYRAAt1 transcript encoding a cytosolic TYRA protein. Transient expression analyses of TYRA- and ADT-green fluorescent protein fusions reveal that the two Arabidopsis TYRA proteins and the six ADT proteins are all targeted within the plastid. Accordingly, TYRA and ADT proteins were both immunodetected in the chloroplast soluble protein fraction (stroma) of Arabidopsis. No TYRA or ADT proteins were immunodetected in the cytosol of Arabidopsis cells. Taken together, all our data exclude the possibility of Tyr and/or Phe synthesis within the cytosol, at least in green leaves and Arabidopsis cultured cells.

² Present address: Université Pierre et Marie Curie-Paris 6, Centre National de la Recherche Scientifique, UMR 7180, PCMP, Ivry-sur-Seine F–94200, France.

The author responsible for the 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: Michel Matringe (michel.matringe{at}cea.fr).

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

www.plantphysiol.org/cgi/doi/10.1104/pp.108.130070

^* Corresponding author; e-mail michel.matringe{at}cea.fr.

Received September 19, 2008; accepted January 5, 2009; published January 9, 2009.

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