Identification of the cis-acting elements involved in regulation of xylanase III gene expression in Trichoderma reesei PC-3-7

The xylanase III gene (xyn3) from the filamentous fungus Trichoderma reesei PC-3-7 is only induced by cellulose, its derivatives and L-sorbose, but not by xylan. In this study, we defined three cis-acting elements within the xyn3 upstream region by using detailed deletion and mutation analysis. In addition to the Xyr1/ACEII-binding motif (5'-GGCTAA-3'), the analogous motifs 5'-GGCTAT-3' and 5'-GGCAAA-3' presented as an inverted repeat spaced by 16-bp internal sequences were identified as essential elements for xyn3 expression. Electrophoretic mobility shift assay using heterologously expressed Xyr1 demonstrates that all the identified cis-acting elements are able to interact with Xyr1. Furthermore, no xyn3 transcripts were formed in the xyr1-knockout strain upon induction by sophorose and L-sorbose. These results indicate that xyn3 expression is transcriptionally regulated by Xyr1, and suggest that the 5'-GGCTAT-3' and 5'-GGCAAA-3' motifs play roles in Xyr1-mediated cellulase and xylanase gene expression in T. reesei.