Isolation and Characterization of Squalene Synthase Gene in Three Species of Achillea, a Rich Source of Saponins

Squalene synthase (SQS, EC.2.5.1.21) is a key enzyme involved in the biosynthesis pathway of triterpenoid and steroidal saponins. The present study aimed to collect molecular information about the SQS gene in Achillea species, the medicinal plants rich in saponins. For this reason, genomic DNA was isolated from leaves of three Achillea species, including A. millefolium, A. wilhelmsii, and A. vermicularis in Iran, then partial SQS gene was amplified through PCR and sequenced (NCBI accession numbers: AmSQS KX589055, AwSQS KX685330, and AvSQS KX685331). AmSQS was 800 bp, containing four exons and three introns; AwSQS and AvSQS were 510 bp and 500 bp, respectively, containing three exons and two introns. Phylogenetic analysis demonstrated that the isolated SQS sequences were significantly similar to each other, and to Artemisia annua, another species of the genus Achillea. Furthermore, in the phylogeny tree, the SQS gene sequences of dicots and monocots were located in separate clades. The deduced amino acid sequences obtained from the isolated SQS gene had also a high similarity to each other and other organisms SQSs ( 73% similarity to higher plants and more than 57% and 47% to the yeast and human). The deduced amino acid sequences included two regions overlapping with domains B and C of SQS, comprising an important motif of aspartate-rich (DYLED) for substrate binding via Mg^2+-bridge. Data resulting from this study was the first report of SQS gene isolation and characterization in Achillea species, which also showed the ability of this gene in taxonomic classification.