Molecular cloning and characterization of one member of 3beta-hydroxy sterol glucosyltransferase gene family in Withania somnifera.

Sterol glycosides are constituents of plant cell membranes. Glucosylations of the sterols are catalyzed by sterol glucosyltransferases (SGTs), which are members of family 1 glycosyltransferases. We have identified the family of SGT genes expressed in the leaves of a medicinal plant Withania somnifera. One member (SGTL1) of this gene family was cloned. The full-length cDNA sequence of SGTL1 represents 2532 bp, comprising untranslated regions (UTRs) of 337 and 89 bp at the 5' and 3' ends, respectively. The amino acid sequence deduced from the 2103 bp open reading frame (ORF) showed homology (67-45%) to the reported plant SGTs. The presence of two putative transmembrane domains suggested the association of SGTL1 with membrane. The SGTL1 was expressed in Escherichia coli and recombinant enzyme from the supernatant was partially purified and biochemically characterized. The relative activity and kinetic properties of SGTL1 for different sterols were compared with a recombinant SGT (GenBank Accession No. Z83833) of Arabidopsis thaliana (AtSGT). Both the recombinant enzymes showed activity with 3-beta-OH sterols. The distribution of SGTL1 transcript in W. somnifera, as determined by quantitative PCR, showed higher expression in roots and mature leaves. Expression of the SGTL1 transcript in the leaves of W. somnifera was enhanced following the application of salicylic acid. In contrast, it decreased rapidly on exposure of the plants to heat shock, suggesting functional role of the enzyme in biotic and abiotic stresses.

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