Characterization of UDP-glucosyltransferase from Indigofera tinctoria

Indican is a secondary metabolite in Indigofera tinctoria; its synthesis from indoxyl and UDP-glucose is catalyzed by a UDP-glucosyltransferase (UGT). In this study, we partially purified UGT extracted from I. tinctoria leaves and analyzed the protein by peptide mass fingerprinting. We identified two fragments that were homologous to UGT after comparison with the transcriptomic data of I. tinctoria leaves. The fragments were named itUgt1 and itUgt2 and were amplified using rapid amplification of cDNA ends polymerase chain reaction to obtain full-length cDNAs. The resultant nucleotide sequences of itUgt1 and itUgt2 encoded peptides of 477 and 475 amino acids, respectively. The primary structure of itUGT1 was 89% identical to that of itUGT2 and contained an important plant secondary product glycosyltransferase (PSPG) box sequence and a UGT motif. The recombinant proteins expressed in Escherichia coli were found to possess high indican synthesis activity. Although the properties of the two proteins itUGT1 and itUGT2 were very similar, itUGT2 was more stable at high temperatures than itUGT1. Expression levels of itUGT mRNA and protein in plant tissues were examined by UGT assay, immunoblotting, and semi-quantitative reverse transcription polymerase chain reaction. So far, we presume that itUGT1, but not itUGT2, primarily catalyzes indican synthesis in I. tinctoria leaves.