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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

MAM3 Catalyzes the Formation of All Aliphatic Glucosinolate Chain Lengths in Arabidopsis^1,[W],[OA]

Department of Biochemistry, Max Planck Institute for Chemical Ecology, D–07745 Jena, Germany (S.T., J.-W.d.K., J.G., J.G.T.); and Department of Secondary Metabolism, Leibniz Institute of Plant Biochemistry, D–06018 Halle, Germany (B.H.)

Chain elongated, methionine (Met)-derived glucosinolates are a major class of secondary metabolites in Arabidopsis (Arabidopsis thaliana). The key enzymatic step in determining the length of the chain is the condensation of acetyl-coenzyme A 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 Arabidopsis 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 Met chain elongation that occur in Arabidopsis. Underlining its broad substrate specificity, MAM3 also accepts a range of non-Met-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 suggests that this organelle is the site of Met chain elongation.

² Present address: Department of Horticulture, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Jonathan Gershenzon (gershenzon{at}ice.mpg.de).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.106.091579

^* Corresponding author; e-mail gershenzon{at}ice.mpg.de; fax 49–3641–571302.

Received October 19, 2006; accepted March 2, 2007; published March 16, 2007.

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