Purification and Characterization of Beta2-Tomatinase, an Enzyme Involved in the Degradation of alpha-Tomatine and Isolation of the Gene Encoding Beta2-Tomatinase fromSeptoria lycopersici
Lycopersicon species often contain the toxic glycoalkaloid alpha-tomatine, which is proposed to protect these plants from general microbial infection. however, fungal pathogens of tomato often are tolerant to alpha-tomatine and detoxification of alpha-tomatine may be how these pathogens avoid this potential barrier. As an initial step to evaluate this possibility, we have purfied to homogeneity a beta-1,2-D glucosidase from the tomato pathogen Septoria lycopersici that hydrolyzes the beta-1,2-D glucosyl bond on the tetrasaccharide moiety of alpha-tomatine to produce beta2-tomatine. The enzyme is a 110-kDa protein with a pI of 4.5 and a Km for alpha-tomatine of 62 microM. Little or no activity was detected on a variety of other glycosides. The gene encoding this protein was isolated and contains an open reading frame of 803 amino acids that shares sequence homology with several other beta-D-glucosidases. When S. lycopersici was incubated with alpha-tomatine, beta2-tomatinase mRNA accumulated, suggesting that the enzyme is substrate inducible. Aspergillus nidulans expressed ¿beta2-tomatinase¿ activity when transformed with this gene but transformants were still sensitive to alpha-tomatine.