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J. Biol. Chem., Vol. 279, Issue 13, 12495-12502, March 26, 2004

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Dimorphecolic Acid Is Synthesized by the Coordinate Activities of Two Divergent ¹²-Oleic Acid Desaturases^*

Edgar B. Cahoon and Anthony J. Kinney¶

From the United States Department of Agriculture-Agricultural Research Service, Plant Genetics Research Unit, Donald Danforth Plant Science Center, St. Louis, Missouri 63132 and ^¶Crop Genetics Research, Pioneer Hi-Bred International, Inc., DuPont Experimental Station, Wilmington, Delaware 19880-0402

Dimorphecolic acid (9-OH-18:2^{10trans,12trans}) is the major fatty acid of seeds of Dimorphotheca species. This fatty acid contains structural features that are not typically found in plant fatty acids, including a C-9 hydroxyl group, ¹⁰,¹²-conjugated double bonds, and trans-¹² unsaturation. Expressed sequence tag analysis was conducted to determine the biosynthetic origin of dimorphecolic acid. cDNAs for two divergent forms of ¹²-oleic acid desaturase, designated DsFAD2-1 and Ds-FAD2-2, were identified among expressed sequence tags generated from developing Dimorphotheca sinuata seeds. Expression of DsFAD2-1 in Saccharomyces cerevisiae and soybean somatic embryos resulted in the accumulation of the trans-¹² isomer of linoleic acid (18: 2^9cis,12trans) rather than the more typical cis-¹² isomer. When co-expressed with DsFAD2-1 in soybean embryos or yeast, DsFAD2-2 converted 18:2^9cis,12trans into dimorphecolic acid. When DsFAD2-2 was expressed alone in soybean embryos or together with a typical cis-¹²-oleic acid desaturase in yeast, trace amounts of the cis-¹² isomer of dimorphecolic acid (9-OH-18:2^9cis,12cis) were formed from DsFAD2-2 activity with cis-¹²-linoleic acid. These results indicate that DsFAD2-2 catalyzes the conversion of the ⁹ double bond of linoleic acid into a C-9 hydroxyl group and ^10trans double bond and displays a substrate preference for the trans-¹², rather than the cis-¹², isomer of linoleic acid. Overall these data are consistent with a biosynthetic pathway of dimorphecolic acid involving the concerted activities of DsFAD2-1 and DsFAD2-2. The evolution of two divergent ¹²-oleic acid desaturases for the biosynthesis of an unusual fatty acid is unprecedented in plants.

Received for publication, December 31, 2003

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) AY494986 and AY494985.

^* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed: United States Department of Agriculture-Agricultural Research Service, Plant Genetics Research Unit, Donald Danforth Plant Science Center, 975 N. Warson Rd., St. Louis, MO 63132. Tel.: 314-587-1291; Fax: 314-587-1391; E-mail: ecahoon{at}danforthcenter.org.

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