Purification, cloning, and expression of a pathogen inducible UDP-glucose:Salicylic acid glucosyltransferase from tobacco

Salicylic acid (SA) plays an important role in plant disease resistance. In oculation of tobacco leaves with incompatible pathogens triggers the biosyn thesis of SA which accumulates primarily as the SA 8-O-beta-D-glucoside (SA G) and glucosyl salicylate (GS). The tobacco UDP-glucose:salicylic acid glu cosyltransferase (SA GTase) capable of forming both SAG and GS was purified , characterized, and partially sequenced. It has an apparent molecular mass of 48 kDa, a pH optimum of 7.0, and an isoelectric point at pH 4.4. UDP-gl ucose was the sole sugar donor for the enzyme. However, SA and several phen olics served as glucose accepters. The apparent K-m values for UDP-glucose and SA were 0.27 and 1-2 mar, respectively. Zn2+ and UDP inhibited its acti vity. The corresponding cDNA clone which encoded a protein of 459 amino aci ds was isolated from an SA-induced tobacco cDNA library and overexpressed i n Escherichia coli. The recombinant protein catalyzed the formation of SAG and GS, and exhibited a broad specificity to simple phenolics, similar to t hat of the purified enzyme. Northern blot analysis showed that the SA GTase mRNA was induced both by SA and incompatible pathogens. The rapid inductio n timing of the mRNA by SA indicates that it belongs to the early SA respon se genes.