Abstract
In addition to being a vital component of proteins, phenylalanine is also a precursor of numerous aromatic primary and secondary metabolites with broad physiological functions. In plants phenylalanine is synthesized predominantly via the arogenate pathway in plastids. Here, we describe the structure, molecular players and subcellular localization of a microbial-like phenylpyruvate pathway for phenylalanine biosynthesis in plants. Using a reverse genetic approach and metabolic flux analysis, we provide evidence that the cytosolic chorismate mutase is responsible for directing carbon flux towards cytosolic phenylalanine production via the phenylpyruvate pathway. We also show that an alternative transcription start site of a known plastidial enzyme produces a functional cytosolic prephenate dehydratase that catalyzes the conversion of prephenate to phenylpyruvate, the intermediate step between chorismate mutase and phenylpyruvate aminotransferase. Thus, our results complete elucidation of phenylalanine biosynthesis via phenylpyruvate in plants, showing that this pathway splits from the known plastidial arogenate pathway at chorismate, instead of prephenate as previously thought, and the complete pathway is localized in the cytosol.
Highlights
In addition to being a vital component of proteins, phenylalanine is a precursor of numerous aromatic primary and secondary metabolites with broad physiological functions
Some of these arogenate dehydratases (ADTs) can use prephenate as substrate, the presence of prephenate aminotransferase (PPAAT) converting prephenate to arogenate in the same subcellular compartment with catalytic efficiency that is at least 50-fold (Arabidopsis) to 900-fold higher than that of respective ADTs with prephenate likely prevents formation of phenylpyruvate to support cytosolic phenylalanine biosynthesis (Fig. 1)[21]
We show that the intermediate step between chorismate mutase and phenylpyruvate aminotransferase is catalyzed by prephenate dehydratase, the cytosolic localization of which is the result of transcription from an alternative transcription start site of a known gene encoding a plastidial enzyme
Summary
In addition to being a vital component of proteins, phenylalanine is a precursor of numerous aromatic primary and secondary metabolites with broad physiological functions. The contribution of phenylpyruvate to phenylalanine biosynthesis has been demonstrated in Arabidopsis and petunia expressing a bacterial bifunctional chorismate mutase/prephenate dehydratase (CM/PDT) in plastids[14,15]. Six, nine, four, and three ADT genes have been identified in Arabidopsis, pine, rice and petunia, respectively[10,17,18,19], encoding enzymes that are all localized in plastids[10,19,20] Some of these ADTs can use prephenate as substrate, the presence of prephenate aminotransferase (PPAAT) converting prephenate to arogenate in the same subcellular compartment with catalytic efficiency that is at least 50-fold (Arabidopsis) to 900-fold (petunia) higher than that of respective ADTs with prephenate likely prevents formation of phenylpyruvate to support cytosolic phenylalanine biosynthesis (Fig. 1)[21]. Recent discovery of cytosolic PPY-AT16 and the widespread occurrence of cytosolic CM2s in plant species urge reconsideration of CM2 function and raise the prospect that plants contain a complete phenylpyruvate pathway for phenylalanine biosynthesis in the cytosol
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