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Published on December 27, 2007; 10.1104/pp.107.111609

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Received October 24, 2007
Accepted December 12, 2007

Arabidopsis thaliana branched-chain aminotransferase 3 (BCAT3) functions in bothamino acid and glucosinolate biosynthesis

Tanja Knill , Joachim Schuster , Michael Reichelt , Jonathan Gershenzon , and Stefan Binder *

Institut Molekulare Botanik, Universitat Ulm, Albert-Einstein-Allee 11, 89069 Ulm, Germany; Max Planck Institut fur Chemische Okologie, Biochemie, Hans-Knoll-Straße 8, Beutenberg Campus, 07745 Jena, Germany

* Corresponding author; email: stefan.binder{at}uni-ulm.de.

In Arabidopsis thaliana, transamination steps in the Leu biosynthetic and catabolic pathways and the Met chain elongation cycle of aliphatic glucosinolate formation are catalyzed by branched-chain aminotransferases (BCATs), which are encoded by a small gene family of six members. One member of this family, the plastid-located BCAT3, was shown to participate in both amino acid and glucosinolate metabolism. In vitro activity tests with the recombinant protein identified highest activities with the 2-oxo acids of Leu, Ile and Val, but also revealed substantial conversion of intermediates of the Met chain elongation pathway. Metabolite profiling of bcat3-1 single and bcat3-1/bcat4-2 double knockout mutants showed significant alterations in the profiles of both amino acids and glucosinolates The changes in glucosinolate proportions suggest that BCAT3 most likely catalyzes the terminal steps in the chain elongation process leading to short-chain glucosinolates: the conversion of 5-methylthiopentyl-2-oxo and 6-methylthiohexyl-2-oxo acids to their respective Met derivatives, homomethionine and dihomomethionine, respectively. The enzyme can also at least partially compensate for the loss of BCAT4, which catalyzes the initial step of Met chain elongation by converting Met to 4-methylthio-2-oxobutanoate (MTOB). Our results show the interdependence of amino acid and glucosinolate metabolism and demonstrate that a single enzyme plays a role in both processes.






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