Cloning and characterization of a β-amyrin synthase gene from the medicinal tree Aralia elata (Araliaceae)

Aralia elata is an important medicinal plant in China; it produces large amounts of oleanane type triterpene saponins. A full-length cDNA encoding β-amyrin synthase (designated as AeAS) was isolated from young leaves of A. elata by reverse transcription-PCR. The full-length cDNA of AeAS was found to have a 2292-bp open reading frame, encoding a protein with 763 amino acid residues. The deduced amino acid sequence of AeAS showed the highest identity (97%) to Panax ginseng β-amyrin synthase. When AeAS cDNA was expressed in Escherichia coli, an 87.8-kDa recombinant protein was detected by SDS-PAGE and Western blotting. The sequence was also heterologously expressed in the yeast Pichia pastoris, and production of β-amyrin was detected by HPLC. Tissue expression pattern analysis by real-time reverse transcription-PCR revealed that AeAS is strongly expressed in leaves and stems, and weakly expressed in roots and flowers.