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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Biosynthesis of Phytosterol Esters: Identification of a Sterol O-Acyltransferase in Arabidopsis^1,[OA]

Plant Biotechnology Institute, National Research Council Canada, Saskatoon, Saskatchewan, Canada S7N 0W9

Fatty acyl esters of phytosterols are a major form of sterol conjugates distributed in many parts of plants. In this study we report an Arabidopsis (Arabidopsis thaliana) gene, AtSAT1 (At3g51970), which encodes for a novel sterol O-acyltransferase. When expressed in yeast (Saccharomyces cerevisiae), AtSAT1 mediated production of sterol esters enriched with lanosterol. Enzyme property assessment using cell-free lysate of yeast expressing AtSAT1 suggested the enzyme preferred cycloartenol as acyl acceptor and saturated fatty acyl-Coenyzme A as acyl donor. Taking a transgenic approach, we showed that Arabidopsis seeds overexpressing AtSAT1 accumulated fatty acyl esters of cycloartenol, accompanied by substantial decreases in ester content of campesterol and -sitosterol. Furthermore, fatty acid components of sterol esters from the transgenic lines were enriched with saturated and long-chain fatty acids. The enhanced AtSAT1 expression resulted in decreased level of free sterols, but the total sterol content in the transgenic seeds increased by up to 60% compared to that in wild type. We conclude that AtSAT1 mediates phytosterol ester biosynthesis, alternative to the route previously described for phospholipid:sterol acyltransferase, and provides the molecular basis for modification of phytosterol ester level in seeds.

² Present address: Department of Biology, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5E2.

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: Jitao Zou (jitao.zou{at}nrc-cnrc.gc.ca).

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www.plantphysiol.org/cgi/doi/10.1104/pp.107.106278

^* Corresponding author; e-mail jitao.zou{at}nrc-cnrc.gc.ca.

Received July 25, 2007; accepted September 6, 2007; published September 20, 2007.