Ubiquitin pathway found to govern carbon catabolite repression in cellulase-producing fungus

A study published on 26 June 2026 in *PLOS Genetics* by Gen Xu, Yanli Cao, Yuxiao Xia and colleagues has identified a nuclear ubiquitin-conjugating enzyme, TrUbc4, and an F-box protein, TrFwd1, as components of a regulatory pathway that modifies the transcriptional repressor Cre1 in the cellulolytic fungus *Trichoderma reesei*.

Carbon catabolite repression (CCR) is a mechanism by which fungi prioritise readily available sugars — such as glucose — over the induction of enzymes needed to break down more complex carbohydrates, including cellulose. In *T. reesei*, this process is orchestrated largely by Cre1, which suppresses cellulase gene expression under high-glucose conditions. The molecular pathway governing how Cre1 activity is itself regulated had remained poorly understood.

The authors show that either repression of *Trubc4* or deletion of *Trfwd1* disrupts CCR and leads to altered cellulase gene expression. The findings suggest that targeted ubiquitin-mediated modification of Cre1 in the nucleus is required for the fungus to switch appropriately between repression and induction of cellulase genes depending on carbon source availability.

Because *T. reesei* is a widely used industrial organism for the production of cellulolytic enzymes relevant to lignocellulosic biofuel and biomass-processing industries, understanding the molecular switches governing CCR has practical interest for biotechnology research. The work also contributes to broader knowledge of how ubiquitin-proteasome pathways interface with transcriptional regulation in filamentous fungi.