Plant Physiology Preview
Published on March 16, 2007; 10.1104/pp.106.091579
OPEN ACCESS ARTICLE
Received October 19, 2006
Accepted March 2, 2007
MAM3 Catalyzes the Formation of All Aliphatic Glucosinolate Chain Lengths in Arabidopsis
Susanne Textor , Jan-Willem de Kraker , Bettina Hause , Jonathan Gershenzon *, and James G. Tokuhisa
Department of Biochemistry, Max Planck Institute for Chemical Ecology, Hans-Knöll Strasse 8, D-07745 Jena, Germany; Department of Secondary Metabolism, Leibniz Institute of Plant Biochemistry, D-06018 Halle, Germany
* Corresponding author; email: gershenzon{at}ice.mpg.de.
Chain elongated, methionine-derived glucosinolates are a major class of secondary metabolites in Arabidopsis thaliana. The key enzymatic step in determining the length of the chain is the condensation of acetyl-CoA with a series of  -methylthio-2-oxoalkanoic acids, catalyzed by methylthioalkylmalate (MAM) synthases. The existence of two MAM synthases has been previously reported in the A. thaliana ecotype Columbia: MAM1 and MAM3 (formerly known as MAM-L). Here we describe the biochemical properties of the MAM3 enzyme which is able to catalyze all six condensation reactions of methionine chain elongation that occur in A. thaliana. Underlining its broad substrate specificity, MAM3 also accepts a range of non-methionine-derived 2-oxoacids, e.g. converting pyruvate to citramalate and 2-oxoisovalerate to isopropylmalate, a step in leucine biosynthesis. To investigate its role in vivo, we identified plant lines with mutations in MAM3 that resulted in a complete lack or greatly reduced levels of long-chain glucosinolates. This phenotype could be complemented by reintroduction of a MAM3 expression construct. Analysis of MAM3 mutants demonstrated that MAM3 catalyzes the formation of all glucosinolate chain lengths in vivo as well as in vitro making this enzyme the major generator of glucosinolate chain length diversity in the plant. The localization of MAM3 in the chloroplast suggest that this organelle is the site of methionine chain elongation.
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