A novel transcriptional repressor specifically regulates xylanase gene 1 in Trichoderma reesei.

Background: The well-known industrial fungus Trichoderma reesei has an excellent capability of secreting a large amount of cellulases and xylanases. The induced expression of cellulase and xylanase genes is tightly controlled at the transcriptional level. However, compared to the intensive studies on the intricate regulatory mechanism of cellulase genes, efforts to understand how xylanase genes are regulated are relatively limited, which impedes the further improvement of xylanase production by T. reesei via rational strain engineering.

Results: To identify transcription factors involved in regulating xylanase gene expression in T. reesei, yeast one-hybrid screen was performed based on the promoters of two major extracellular xylanase genes xyn1 and xyn2. A putative transcription factor named XTR1 showing significant binding capability to the xyn1 promoter but not that of xyn2, was successfully isolated. Deletion of xtr1 significantly increased the transcriptional level of xyn1, but only exerted a minor promoting effect on that of xyn2. The xylanase activity was increased by ~ 50% with XTR1 elimination but the cellulase activity was hardly affected. Subcellular localization analysis of XTR1 fused to a green fluorescence protein demonstrated that XTR1 is a nuclear protein. Further analyses revealed the precise binding site of XTR1 and nucleotides critical for the binding within the xyn1 promoter. Moreover, competitive EMSAs indicated that XTR1 competes with the essential transactivator XYR1 for binding to the xyn1 promoter.

Conclusions: XTR1 represents a new transcriptional repressor specific for controlling xylanase gene expression. Isolation and functional characterization of this new factor not only contribute to further understanding the stringent regulatory network of xylanase genes, but also provide important clues for boosting xylanase biosynthesis in T. reesei.