**18. Transcriptional regulators of plant polysaccharide degradation genes in Neurospora crassa**

The filamentous ascomycete fungus *Neurospora crassa* has been commonly used as a model laboratory organism [132]. In nature, *N. crassa* can be found on burnt plant material, primar‐ ily grasses, including sugarcane and *Miscanthus* [133]. Previous studies have been demon‐ strated that *N. crassa* is able to express and secrete many plant cell wall degrading enzymes after grown on ground *Miscanthus* stems and crystalline cellulose [134]. Studies conducted with strains containing deletions of predicted transcription factors (TFs) in *N. crassa* demon‐ strated that a specific TF, named XLR1 (xylan degradation regulator-1), is essential for hemi‐ cellulose degradation in *N. crassa* [135]. The *xlr-1* gene is an ortholog to XlnR/Xyr1 found in *Aspergillus* and *Trichoderma* species, respectively. The results presented in this study have been shown that deletion of *xlr-1* in *N. crassa* abolished growth on xylan and xylose, but growth on cellulose and cellulolytic activities were not highly affected. The transcriptional profiling showed that *xlr-1* is required for induction of hemicellulase and xylose metabolism genes, and modulated the expression levels of few cellulase genes, but these genes do not require XLR-1 for induction [135]. These findings suggested that unknown TFs in *N. crassa* could be important for the induced expression of genes encoding cellulases in response to the presence of cellulose.

In fact, studies assessing a near-full genome deletion strain set in *N. crassa*, have been shown two transcription factors, named *clr-1* and *clr-2*, required for degradation of cellulose [136]. Homologs of *clr-1* and *clr-2* are present in the genomes of many filamentous ascomycete species capable of degrading plant-cell wall material, including *A. nidulans*. The *N. crassa* TFs *clr-1* and *clr-2* were able to induce all major cellulase and some hemicellulase genes, and functional CLR-1 was necessary for the expression of *clr-2* and efficient cellobiose utilization by the fungus. Besides, in *A. nidulans*, a deleted strain of the *clr-2* homolog (*clrB*) failed to induce cellulase gene expression and lacked cellulolytic activity on Avicel [136]. These au‐ thors reinforced the idea that further manipulation of the transcriptional regulation of cellu‐ lase/hemicellulase system may improve yields of cellulases for industrial applications, e.g., for biofuel production.

sources such as ethanol are simultaneously present with high amounts of a preferable car‐ bon source such as glucose, and the fungus is able to fine-tune the regulation in order to adapt to new nutrient. A second mechanism involves complete repression of the *alcR* gene, operating at high glucose concentration. Under these conditions, expression of the *alc* genes does not occur, and the fungus metabolizes only the rich carbon source (reviewed in [141]).

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A variety of studies have been demonstrated the mechanisms through which CreA re‐ presses some polysaccharide-degrading enzymatic systems in fungi. It was shown that CreA appears to repress *xlnA* transcription by the "double-lock" mechanism in *A. nidulans*, repres‐ sing directly the gene through its binding to the consensus *xlnA*.C1 site of the promoter, as well as indirect repression [144]. Studies on *A. nidulans xlnB* gene repression demonstrated that the four CreA target sites located in *xlnB* gene promoter region lack physiological rele‐ vance, suggesting that the repression exerted by CreA on *xlnB* is by an indirect mechanism [145]. The latter results suggested that an additional level of CreA repression via the xylano‐ lytic activator is present in *A. nidulans*. The authors suggested that this mechanism of regula‐ tion would be analogous to that described above for the *alc* regulon, where certain genes are under a double-lock mechanism of repression by CreA while others are not subject to direct repression, being regulated via CreA repression of the *alcR* regulatory gene. In fact, studies have been shown that the *xlnR* (the xylanolytic transcriptional activator) promoter is re‐ pressed by glucose via CreA in *A. nidulans*, and when this repression is eliminated, by pro‐ moter exchange, transcription of xylanolytic genes such as *xlnA*, *xlnB* and *xlnD* is derepressed [146]. These results demonstrated that a transcription factor cascade involving

F-box proteins are proteins containing at least one F-box domain in their structures. The Fbox domain is a protein structural motif of about 50 amino acids that mediates protein-pro‐ tein interactions [147]. Usually, F-box proteins mediate ubiquitination of proteins targeted for degradation by the proteasome, but these proteins have also been associated with cellu‐ lar functions such as signal transduction and regulation of cell-cycle [148]. A study that per‐ formed a screening of 42 *A. nidulans* F-box deletion mutants grown either on xylose or xylan as a sole carbon source in the presence of 2-deoxy-D-glucose was able to identify mutants with de-regulated xylanase induction [149]. In this study, a null mutant in a gene (*fbxA*) with decreased xylanase activity and reduced *xlnA* and *xlnD* mRNA accumulation was identi‐ fied. This mutant interacted genetically with *creA* mutants, emphasizing the importance of the CCR and ubiquitination in the *A. nidulans* xylanase induction. In addition, the identifica‐ tion of FbxA protein provides evidence for another level of regulatory network concerning

In summary, an intricate and fine-tuned regulation network exists in order to control the ex‐ pression of plant cell-wall degradation genes in fungi. A variety of transcriptional regulators are able to respond to different nutritional requirements of the fungus, depending on its life‐ style. In general, readily metabolizable carbon sources such as glucose represses the tran‐ scription of genes responsible for the poor carbon source catabolism, via different mechanisms. The carbon catabolite repression in fungi is a common mechanism of regula‐ tion through which the organism adapts to nutritional availability in their environment. For

CreA and XlnR regulates CCR of *A. nidulans* xylanolytic genes.

xylanase induction in filamentous fungi [149].
