Dual Role of MtHAC-1 in Regulating Cellulase and Xylanase Production in Myceliophthora thermophila

Filamentous fungi produce large quantities of cellulase and xylanase as extracellular enzymes to degrade plant-derived polysaccharides. This process is controlled by a complex network of transcription factors (TFs). Here, we present the bZIP TF Mthac-1 exhibiting dual regulatory effects on the production of cellulase and xylanase in Myceliophthora thermophila. The deletion of Mthac-1 reduced the cellulase and xylanase activities and protein secretion during the early phase of cultivation but enhanced in the middle and late stages of cultivation, compared with the wild-type (WT) strain. It also led to fungal growth defects, characterised by few hyphal branching and reduced conidiation. Real-time quantitative reverse transcription PCR (RT-qPCR) analysis showed that Mthac-1 dynamically regulates the expression of major cellulase genes. Furthermore, electrophoretic mobility shift assays (EMSAs) demonstrated that Mthac-1 directly binds to the promoter regions of the β-glucosidase gene bgl1 (MYCTH_66804), cellobiohydrolase gene cbh1 (MYCTH_109566), endoglucanase gene egl2 (MYCTH_86753), xylanase gene xyn1 (MYCTH_112050) and the regulatory gene xyr1 (MYCTH_2310145), exhibiting higher binding affinity for xyn1 and xyr1. The comparative transcriptomic analysis indicated that Mthac-1 also plays an important role in the expression of 26S proteasome-encoding genes under cellulolytic conditions. This work provides new insights into the regulatory mechanisms underlying cellulase and xylanase gene expression with potential applications in fungal strain engineering in biorefinery industries.