Dual targeting of Arabidopsis holocarboxylase synthetase1: a small upstream open reading frame regulates translation initiation and protein targeting.

Protein biotinylation is an original and very specific posttranslational 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 the HCS genes, HCS1 and HCS2, identified in Arabidopsis (Arabidopsis thaliana) are mainly unknown. Here, we showed by reverse genetics that the HCS1 gene is essential for plant viability, whereas disruption of the HCS2 gene in Arabidopsis does not lead to any obvious phenotype when plants are grown under standard conditions. These findings strongly suggest 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 the HCS1 messenger RNA 5'-untranslated region, an upstream open reading frame regulates the translation initiation of HCS1 and the subsequent targeting of HCS1 protein. Moreover, an exquisitely precise alternative splicing of HCS1 messenger RNA can regulate the presence and absence of this upstream open reading frame. 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.