Abstract: β-amyrin synthase（βAS）,the rate-limiting enzyme saponin of directional shunt oleanane and dammarane types,plays an important role in biosynthesis of triterpenoid saponin in medical plants. The primers were designed based on the transcript sequence of βAS from the transcriptome dataset of P. japonicus var. major. And the full-length cDNA sequence of βAS was cloned from the rhizome of P. japonicus var. major by utilizing RT-PCR method,named as pjβAS. The length of this gene was 2 655 base pairs containing a complete open reading frame of 2 286 base pairs and encoding 761 amino acids. The accession number in GenBank is KX254566. The bioinformatics analysis showed that the putative molecular weight and theory isoelectric point of protein encoded by pjβAS were 87.90 kD and 5.84,respectively. The protein contained no transmembrane domain,was a non-secretory protein with 38 phosphorylation sites,it played a physiological role in chloroplast and endoplasmic reticulum. The secondary structure of the protein indicated that α-helix accounted for 39.82,extended strand for 16.56%, β-turn for 10.38%,and random coil for 33.24%. Meanwhile,three feature conserved motifs of oxido squalene cyclase（OSC）were found in pjβAS,i.e.,DCTAE,QW（QXXXXXW）,and MWCRCY. The results of the sequence homology comparison concluded that the protein encoded by pjβAS was in 100% similarity with Panax japonicus C. A. Mey（AKN23431.1）,and pjβAS gene was classified in the same branch with Panax japonicus C. A. Mey（AKN23431.1）,Panax quinquefolius（AGG09939.1）,and Bupleurum chinense DC（ABY90140.2）in phylogenetic analysis. Real-time fluorescence quantitative PCR was used to analyze the relative expression quantity of pjβAS gene in different tissues of P. japonicus var. major,the results showed that there were different expression trends in flowers,leaves,stems and rhizomes,with the highest in leaves but the lowest in rhizomes.

Key words: Panax japonicus var. major, β-amyrin synthase, triterpenoid saponin, gene cloning, bioinformatics analysis