Stable production of thermotolerant xylanase B of Clostridium stercorarium in transgenic tobacco and rice.

The xylanase B gene encoding a thermostable family 10 xylanase of Clostridium stercorarium was expressed in plants under the control of a constitutive promoter. Two forms of the xylanase B gene, the xynB gene encoding the full length of the xylanase B gene including the bacterial signal sequence and the xynBM gene without the signal sequence region, were introduced into tobacco BY-2 cells and tobacco plants respectively under the control of the cauliflower mosaic virus 35S promoter. Transgenic BY-2 cells and tobacco plants showed xylanase activity and normal growth. The recombinant enzyme produced in transgenic BY-2 cells harboring the xynB gene was secreted into the culture supernatant, and the recombinant enzyme produced in transgenic BY-2 cells harboring the xynBM gene was localized in the cells. In contrast to tobacco plants, expression of the xynB gene under the control of the rice actin promoter in rice plants was toxic to host cells. However, the recombinant XynBM accumulated in leaf cells, and no phenotypic effect of expression of the xynBM gene was observed. Enzyme activity was maintained in cell-free extracts of transgenic rice leaves at 60 degrees C for 72 h, and the recombinant XynBM degraded hemicellulosic polymers in cell-free extracts of transgenic rice leaves.