The ß-importin KAP8 (Pse1/Kap121) is required for nuclear import of the cellulase transcriptional regulator XYR1, asexual sporulation and stress resistance inTrichoderma reesei

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The ascomyceteTrichoderma reeseiis an industrial producer of cellulolytic and hemicellulolytic enzymes, and serves as a prime model for their genetic regulation. Most of its (hemi-)cellulolytic enzymes are obligatorily dependent on the transcriptional activator XYR1. Here, we investigated the nucleo-cytoplasmic shuttling mechanism that transports XYR1 across the nuclear pore complex. We identified 14 karyopherins inT. reesei, of which eight were predicted to be involved in nuclear import, and produced single gene-deletion mutants of all. We found KAP8, an ortholog ofAspergillus nidulansKapI, andSaccharomyces cerevisiaeKap121/Pse1, to be essential for nuclear recruitment of GFP-XYR1 and cellulase gene expression. Transformation with the native gene rescued this effect. Transcriptomic analyses of Δkap8revealed that under cellulase-inducing conditions 42 CAZymes, including all cellulases and hemicellulases known to be under XYR1 control, were significantly down-regulated. Δkap8strains were capable of forming fertile fruiting bodies but exhibited strongly reduced conidiation both in light and darkness, and showed enhanced sensitivity towards abiotic stress, including high osmotic pressure, low pH and high temperature. Together, these data underscore the significance of nuclear import of XYR1 in cellulase and hemicellulase gene regulation inT. reesei, and identify KAP8 as the major karyopherin required for this process.

Trichoderma reesei is an industrial producer of cellulolytic and hemicellulolytic enzymes, for which XYR1 is the main transcriptional regulator. Using a systematic gene deletion approach, in combination with RNAseq and live-cell imaging analysis, we identified the ß-importin gene kap8 to be essential for nuclear recruitment of XYR1 and the induced expression of the whole inventory of cellulase and hemicellulase genes in T. reesei.

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