Tocotrienols are vitamin E-related compounds whose occurrence in plants is limited primarily to the seeds and fruits of most monocot species (e.g., palm, wheat, rice and barley). Tocotrienols are structurally similar to tocopherols, including α-tocopherol or vitamin E, which occur ubiquitously in the plant kingdom as well as in photosynthetic microbes such as Synechocystis. Tocotrienols and tocopherols both contain a chromanol head group that is linked to a hydrocarbon side chain. The only structural difference between these molecules is the presence of three double bonds in the hydrocarbon side chain of tocotrienols. This difference is related to the biosynthetic origins of the side chains. Tocopherol side chains are derived from phytyl-pyrophosphate (PP), and the tocotrienol side chains are believed to be derived from geranylgeranyl-PP (Soll, J. et al. (1980) Arch. Biochem. Biophys. 204:544–550).
Four forms or molecular species of tocopherols and tocotrienols occur in nature: α, β, γ and δ. These molecular species contain different numbers of methyl groups that are bound to the aromatic portion of the chromanol head. Like tocopherols, tocotrienols are potent lipid-soluble antioxidants and therefore have considerable nutritive value in human and animal diets (Packer, L. et al. (2001) J. Nutr. 131:369S–373S). In addition, tocotrienols are believed to have therapeutic properties including a demonstrated ability to down regulate cholesterol biosynthesis (Theriault, A. et al. (1999) Clin. Biochem. 32:309–319; Qureshii, A. A. et al. (1986) J. Biol. Chem. 261:10544–10550).
It has been speculated that the first committed step in the biosynthesis of tocotrienols involves the condensation of geranylgeranyl-PP and homogentisate to form 2-methyl-6-geranylgeranylbenzoquinol (Soll, J. et al. (1980) Arch. Biochem. Biophys. 204:544–550). The enzyme that catalyzes this reaction can thus be functionally described as a homogentisate geranylgeranyl transferase (HGGT).
Functional identification of genes or cDNAs encoding HGGT polypeptides has yet to be reported. The lack of these nucleic acids limits efforts to manipulate the biosynthesis of the nutritionally important tocotrienols in plants and microbial hosts. The problem to be solved, therefore, is in identifying the nucleic acids that encode polypeptides required for tocotrienol biosynthesis in plants.