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Title: CHARACTERIZATION OF TOCOPHEROL CYCLASES FROM HIGHER PLANTS AND CYANOBACTERIA, EVOLUTIONARY IMPLICATIONS FOR TOCOPHEROL SYNTHESIS AND FUNCTION

Author
item SATTLER, SCOTT - MICHIGAN STATE UNIV
item Cahoon, Edgar
item COUGHLAN, SEAN - DUPONT CROP GENETICS
item DELLAPENNA, DEAN - MICHIGAN STATE UNIV

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2003
Publication Date: 8/1/2003
Citation: SATTLER, S.E., CAHOON, E.B., COUGHLAN, S.J., DELLAPENNA, D. CHARACTERIZATION OF TOCOPHEROL CYCLASES FROM HIGHER PLANTS AND CYANOBACTERIA, EVOLUTIONARY IMPLICATIONS FOR TOCOPHEROL SYNTHESIS AND FUNCTION. PLANT PHYSIOLOGY. 2003. v. 132. p. 2184-2195.

Interpretive Summary: Vitamin E is a fat-soluble antioxidant and an important nutrient in human diets and animal feeds. Dietary intake of Vitamin E typically does not meet the recommended daily allowance, due in part to its low content in food products. In this study, the gene for an essential enzyme in the Vitamin E biosynthetic pathway was identified and characterized from plants and blue-green algae. Mutation of this gene was shown to abolish Vitamin E synthesis. The findings of this study may be useful for molecular biologists attempting to genetically enhance the Vitamin E content of horticultural and agronomic crop plants. This research will ultimately result in increased nutritive value of foods for consumers and animal feeds for U.S. farmers.

Technical Abstract: Tocopherols are lipophilic antioxidants synthesized exclusively by photosynthetic organisms, and collectively constitute Vitamin E, an essential nutrient for both humans and animals. Tocopherol cyclase, a key enzyme in tocopherol synthesis, catalyzes the conversion of various phytyl quinol pathway intermediates into their corresponding tocopherols through the formation of the chromanol ring. In Arabidopsis, two mutants, vitamin e 1-1 (vte1-1) and vitamin e 1-2 (vte1-2), were isolated that were deficient in tocopherols and accumulated 2,3-dimethyl-6-phytyl-1,4-benzoquinone (DMPBQ), a substrate for the tocopherol cyclase. A map-based cloning approach identified the VTE1 locus as At4g32770. The protein corresponding to At4g32770 shares 48% and 79% similarity, respectively, to ORF slr1737 from Synechocystis and the product of the Sucrose eXport Deficient 1 (SXD1) locus in maize. Like vte1, mutations in SXD1 or the slr1737 gene resulted both in tocopherol deficiency and the accumulation of DMPBQ. Consistent with the activity of tocopherol cyclase, recombinant SXD1 and At4g32770 proteins converted DMPBQ to gamma-tocopherol in vitro. In addition, expression of maize SXD1 in the Synechocystis slr1737mutant restored tocopherol biosynthetic capability, indicating that tocopherol cyclase activity is evolutionarily conserved between plants and cyanobacteria. The sxd1 phenotype suggests that in addition to their presumed antioxidant activities, tocopherols or tocopherol breakdown products also function as signal transduction molecules or that the DMPBQ intermediate that accumulates in sxd1 disrupts signaling required for efficient sucrose export in maize.