Conserved and essential transcription factors for cellulase gene expression in ascomycete fungi

Rational engineering of filamentous fungi for improved cellulase production is hampered by our incomplete knowledge of transcriptional regulatory networks. We therefore used the model filamentous fungus Neurospora crassa to search for uncharacterized transcription factors associated with cellulose deconstruction. A screen of a N. crassa transcription factor deletion collection identified two uncharacterized zinc binuclear cluster transcription factors (clr-1 and clr-2) that were required for growth and enzymatic activity on cellulose, but were not required for growth or hemicellulase activity on xylan. Transcriptional profiling with next-generation sequencing methods refined our understanding of the N. crassa transcriptional response to cellulose and demonstrated that clr-1 and clr-2 were required for the bulk of that response, including induction of all major cellulase and some major hemicellulase genes. Functional CLR-1 was necessary for expression of clr-2 and efficient cellobiose utilization. Phylogenetic analyses showed that CLR-1 and CLR-2 are conserved in the genomes of most filamentous ascomycete fungi capable of degrading cellulose. In Aspergillus nidulans, a strain carrying a deletion of the clr-2 homolog (clrB) failed to induce cellulase gene expression and lacked cellulolytic activity on Avicel. Further manipulation of this control system in industrial production strains may significantly improve yields of cellulases for cellulosic biofuel production.