Cloning and expression of a cDNA encoding betanidin 5-O-glucosyltransferase, a betanidin- and flavonoid-specific enzyme with high homology to inducible glucosyltransferases from the Solanaceae

Based on protein sequence data and RT-PCR, a full length cDNA encoding beta nidin 5-O-glucosyltransferase (5-GT) was obtained from a cDNA library of Do rotheanthus bellidiformis (Burm,f.) N.E.Br. (Aizoaceae). 5-GT catalyses the transfer of glucose from UDP-glucose to the 5-hydroxyl group of the chromo genic betanidin. Betanidin and its conjugates, referred to as betacyanins, are characteristic fruit and flower pigments in most members of the Caryoph yllales, which fail to synthesise anthocyanins. The 5-GT cDNA displayed hom ology to previously published glucosyltransferase sequences and exhibited h igh identity to sequences of several inducible glucosyltransferases of toba cco and tomato (Solanaceae). The open reading frame encodes a polypeptide o f 489 amino acids with a calculated molecular mass of 55.24kDa. The corresp onding cDNA was expressed in Escherichia coli. The recombinant protein disp layed identical substrate specificity compared to the native enzyme purifie d from D. bellidiformis cell suspension cultures. In addition to the natura l substrate betanidin, ortho-dihydroxylated flavonols and flavones were gly cosylated preferentially at the B-ring 4'-hydroxyl group. 5-GT is the first enzyme of betalain biosynthesis in plants, of which the corresponding cDNA has been cloned and expressed. The results are discussed in relation to mo lecular evolution of plant glucosyltransferases.