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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Elucidation of the Final Reactions of DIMBOA-Glucoside Biosynthesis in Maize: Characterization of Bx6 and Bx7^1,[W],[OA]

Technische Universität München, Lehrstuhl für Genetik, 85354 Freising, Germany (R.J., H.S., A.O., A.F., K.S., M.F., A.G.); Technische Universität München, Department Chemie, Lehrstuhl Biotechnologie, 85747 Garching, Germany (M.H.); Institut for Organic Chemistry (D.S.) and Institut für Analytische Chemie (D.H.), Universität Leipzig, 04103 Leipzig, Germany (D.S.); and Pioneer Hi-Bred International, Inc., Johnston, Iowa 50131 (N.Y., C.S.)

Benzoxazinoids were identified in the early 1960s as secondary metabolites of the grasses that function as natural pesticides and exhibit allelopathic properties. Benzoxazinoids are synthesized in seedlings and stored as glucosides (glcs); the main aglucone moieties are 2,4-dihydroxy-2H-1,4-benzoxazin-3(4H)-one (DIBOA) and 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one (DIMBOA). The genes of DIBOA-glc biosynthesis have previously been isolated and the enzymatic functions characterized. Here, the enzymes for conversion of DIBOA-glc to DIMBOA-glc are identified. DIBOA-glc is the substrate of the dioxygenase BENZOXAZINLESS6 (BX6) and the produced 2,4,7-trihydroxy-2H-1,4-benzoxazin-3-(4H)-one-glc is metabolized by the methyltransferase BX7 to yield DIMBOA-glc. Both enzymes exhibit moderate K_m values (below 0.4 mM) and k_cat values of 2.10 s^–1 and 0.25 s^–1, respectively. Although BX6 uses a glucosylated substrate, our localization studies indicate a cytoplasmic localization of the dioxygenase. Bx6 and Bx7 are highest expressed in seedling tissue, a feature shared with the other Bx genes. At present, Bx6 and Bx7 have no close relatives among the members of their respective gene families. Bx6 and Bx7 map to the cluster of Bx genes on the short arm of chromosome 4.

² Present address: School of Sciences, Oxford Brookes University, Oxford OX3 0BP, UK.

³ Present address: Forschungszentrum Jülich, ZCH/BioSpec, Leo-Brandt-Str., 52428 Jülich, Germany.

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: Monika Frey (monika.frey{at}wzw.tum.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.107.111237

^* Corresponding author; e-mail monika.frey{at}wzw.tum.de.

Received October 18, 2007; accepted January 8, 2008; published January 11, 2008.