[Cloning and functional characterization of two oxidosqualene cyclase genes from Siraitia grosvenorii]

Oxidosqualene cyclases(OSCs), belonging to a multigene family, can convert a common precursor 2,3-oxidosqualene into various types of triterpene skeletons. In this study, primers were designed according to the analysis of Siraitia grosvenorii transcriptome data, and two OSC genes SgAS1(GenBank No. QDO67189.1) and SgAS2(GenBank No. QDO67190.1) were cloned. The open reading frame(ORF) of SgAS1 was 2 262 bp, encoding 754 amino acids, and the ORF of SgAS2 was 2 289 bp, encoding 762 amino acids. Real-time quantitative PCR results demonstrated that the two SgOSCs genes showed different expression patterns in stems, leaves, and different stages of fruits. Phylogenetic analysis showed that both SgAS1 and SgAS2 were clustered with β-amyrin synthases into a branch, but further functional characterization using yeast heterologous expression found that SgAS1 was inactive and SgAS2 could produce β-amyrin as the sole product. Multiple sequence alignments revealed that SgAS2 had a conserved MWCYCR sequence related to β-amyrin biosynthesis, while SgAS1 had an unusual LFCYTR sequence, for which the authors performed site-directed mutagenesis analysis of this sequence and found that tryptophan residue(W) was the key amino acid residue that affected the function of SgOSCs. In addition, the authors transformed the monofunctional β-amyrin synthase SgAS2 into the chassis strain GH1, which was previously modified by the research group, and increased the yield of β-amyrin to 44.05 mg·L~(-1). This study first reported the monofunctional β-amyrin synthase SgAS2 from S. grosvenorii and conducted site-directed mutagenesis and synthetic biology investigation on it, providing a valuable resource for the directed biosynthesis of triterpenoids.