IDENTIFICATION AND CHARACTERIZATION OF AN S-ADENOSYL-L-METHIONINE - DELTA(24)-STEROL-C-METHYLTRANSFERASE CDNA FROM SOYBEAN

In plants, the dominant sterols are 24-alkyl sterols, which play multi ple roles in plant growth and development, i.e. as membrane constituen ts and as precursors to steroid growth regulators such as brassinoster oids. The initial step in the conversion of the phytosterol intermedia te cycloartenol to the 24-alkyl sterols is catalyzed by L-methionine:D elta(24)-sterol-C-methyl-transferase (SMT), a rate-limiting enzyme for phytosterol biosynthesis. A cDNA clone (SMT1) encoding soybean SMT wa s isolated from an etiolated hypocotyl cDNA library by immunoscreening using an anti-(plasma membrane) serum. The deduced amino acid sequenc e of the SMT1 cDNA contained three conserved regions found in S-adenos yl-L-methionine-dependent methyltransferases. The overall structure of the polypeptide encoded by the SMT1 cDNA is most similar to the predi cted amino acid sequence of the yeast ERG6 gene, the putative SMT stru ctural gene. The polypeptide encoded by the SMT1 cDNA was expressed as a fusion protein in Escherichia coli and shown to possess SMT activit y. The growing soybean vegetative tissues had higher levels of SMT tra nscript than mature vegetative tissues. Young pods and immature seeds had very low levels of the SMT transcript, The SMT transcript was high ly expressed in flowers. The expression of SMT transcript was suppress ed in soybean cell suspension cultures treated with yeast elicitor. Th e transcriptional regulation of SMT in phytosterol biosynthesis is dis cussed.