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Plant Physiology Preview Published on December 21, 2007; 10.1104/pp.107.111534
Received October 23, 2007 Dual-targeting of Arabidopsis thaliana Holocarboxylase Synthetase 1: a small upstream open reading frame (uORF) regulates translation initiation and protein targeting
CNRS (UMR 5168)/CEA/Universite Joseph Fourier/INRA (UMR 1200), CEA-Grenoble, institut de Recherche en Technologies et Sciences pour le Vivant (iRTSV), Laboratoire de Physiologie Cellulaire Vegetale, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France * Corresponding author; email: ldenis{at}biologie.ens.fr.
Protein biotinylation is an original and very specific post-translational modification, compartmented, in plants, between mitochondria, plastids and the cytosol. This reaction modifies and activates few carboxylases committed in key metabolisms and is catalyzed by holocarboxylase synthetase (HCS). The molecular bases of this complex compartmentalization and the relative function of each of HCS genes, HCS1 and HCS2, identified in Arabidopsis thaliana are mainly unknown. Here, we showed by reverse genetics that HCS1 gene is essential for plant viability whereas disruption of HCS2 gene in Arabidopsis does not lead to any obvious phenotype when plants are grown under standard conditions. These findings suggest strongly that HCS1 is the only protein responsible for HCS activity in Arabidopsis cells, including the cytosolic, mitochondrial and plastidial compartments. A closer study of HCS1 gene expression enabled us to propose an original mechanism to account for this multiplicity of localizations. Located in HCS1 mRNA 5'-untranslated region, an upstream open reading frame (uORF) regulates the translation initiation of HCS1 and the subsequent targeting of HCS1 protein. Moreover, an exquisitely precise alternative splicing of HCS1 mRNA can regulate the presence and absence of this uORF. The existence of these complex and interdependent mechanisms creates a rich molecular platform where different parameters and factors could control HCS targeting and hence biotin metabolism.
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