¹Molecular Plant Pathology Laboratory, USDA-ARS, BARC-West, Beltsville, MD 20705, USA
²Plant Genetics Department, The Weizmann Institute of Science, Rehovot 76100, Israel
³Department of Field Crops, Vegetables and Genetics, Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot 76100, Israel


Summary

A cDNA (pWMT) comprising the complete coding region of a wheat mRNA that encodes a putative sterol C-24 methyltransferase has been cloned. The nucleotide sequence of pWMT consists of 1392 base pairs (bp) and codes for a 363-amino acid (aa) polypeptide that has significant homology with the Smt 1 class of sterol C-24 methyltransferases present in plants. Using pWMT as a probe, we have also isolated two genomic fragments which together encompass the complete open reading frame containing 11 exons and 10 introns. By aneuploid analysis, TA-MT has been localized on the distal portion of the long arm of group 4 chromosomes.

Key words: Reverse transcription, Library screening, Chromosome localization


Introduction

Vascular plants produce a large array of sterols. Among them, 24-methyl cholesterol, sitosterol and stigmasterol account for 70% of the total sterols (Benveniste 1986). The feature that makes the plant and fungal sterols to differ from the animal sterols is the presence of an extra alkyl group at C-24 (Benveniste 1986). The 24-alkyl sterols are the precursors for the biosynthesis of the brassinosteroid group of plant hormones (Ikekawa 1991). A putative scheme for the biosynthesis of sterols in plants has been proposed (Benveniste 1986). A key step in the formation of 24-alkyl sterols is the transmethylation that converts cycloartenol to 24-alkyl sterols catalyzed by the enzyme, S-adenosyl-L-methionine: Delta²⁴-sterol-C-methyltransferase (SMT). Therefore, studies on the structure and function of SMT, as well as the characterization of the expression of its gene are essential to understand how the biosynthesis of 24-alkyl sterols is regulated.

Recently the cDNAs of SMTs have been isolated from Arabidopsis (Husselstein et al. 1996), soybean (Shi et al. 1996), maize (Grebenok et al. 1997), rice and tobacco (Bouvier-Nave et al. 1997, 1998). Here we report the isolation, for the first time, of a genomic clone encoding SMT in wheat. We have also isolated the corresponding full-length cDNA clone.