**17. Other regulators involved in plant polysaccharide degradation in Aspergilli**

The enzymatic system responsible for plant polysaccharide degradation is induced and commonly amplified after releasing of the plant cell wall polymers components. Two impor‐ tant components of the plant cell wall are D-xylose and L-arabinose, present in the polymers arabinan, arabinogalactan, xyloglucan and xylan. In Aspergilli, D-xylose and L-arabinose are catabolized through the pentose catabolic pathway, PCP [120], consisting of a series of reversible reductase/dehydrogenase steps culminating with the formation of D-xylulose-5 phosphate, which enters the pentose phosphate pathway (PPP). In *A. niger*, the most of the genes involved in PCP have been characterized [110, 121 - 123]. In the presence of D-xylose, the transcriptional activator XlnR is known to regulate the expression of genes not only en‐ coding extracellular polysaccharide-degrading enzymes, but also regulates the expression of *xyrA*, the gene encoding D-xylose reductase, involved in the PCP [5]. L-arabinose induction of PCP is not mediated via XlnR, and the genes of the L-arabinose catabolic pathway are coregulated with genes encoding arabinolytic enzymes such as α-L-arabinofuranosidase and endoarabinanase [93, 124]. A L-arabinose catabolic pathway regulator, AraR, was identified in *A. niger*, and appeared to act as antagonist of the XlnR in the regulation of PCP [125]. AraR demonstrated to regulate the L-arabinose pathway specific genes, for instance, the Larabitol dehydrogenase encoding gene (*ladA*) [125]. In addition, the L-arabinose reductase and L-xylulose reductase encoding genes (*larA* and *lxrA*, respectively) appeared to be under the regulation of the arabinolytic system [125, 126]. AraR and XlnR both regulate the com‐ mon steps of L-arabinose and D-xylose catabolism, represented by xylitol dehydrogenaseand xylulose kinase-encoding genes, *xdhA* and *xkiA*, respectively [123, 125]. Comparative analysis have been shown that the regulation of PCP by AraR differs in *A. nidulans* and *A. niger*, whereas the regulation of the PCP by XlnR was similar in both species, suggesting dif‐ ferent evolutionary changes in these two species affecting pentose utilization [125]. These authors suggest that the differences in Aspergilli species implies that manipulating regulato‐ ry systems to improve the production of polysaccharide degrading enzymes may give dif‐ ferent results in different industrial fungi.

Furthermore, it was found that in *Aspergillus aculeatus*, a new XlnR-independent pathway for the regulation of cellulase- and hemicellulase-encoding genes exists [127]. This study suggests that cellobiose from Avicel (crystalline cellulose) degradation stimulates only the XlnR-independent signaling pathway for cellulase/hemicellulase production in *A. aculeatus*. In addition, sequential promoter truncation studies on *cbh1* gene demonstrated that a con‐ served sequence in the promoter region of *cbh1*, namely CeRe (required for *eglA* induction in *A. nidulans*; [128], plays a pivotal role on the expression regulated by the XlnR-independent signaling pathway triggered by cellulosic compounds in *A. aculeatus* [127].

Ace2 belongs to a zinc-binuclear cluster DNA-binding protein and appears to be an activa‐ tor of cellulase and hemicellulase genes in cellulose-induced cultures of *T. reesei*. Deletion of the *ace2* gene resulted in decreased expression of the cellulase genes *cbh1*, *cbh2*, *egl1*, *egl2*, and the xylanase gene *xyn2* upon *T. reesei* growth on cellulose as sole carbon source [119]. The same study has been demonstrated that the expression of these genes were not affected after *T. reesei* growing on sophorose, suggesting partially different mechanisms for hydro‐ lase-encoding genes expression upon different carbon sources [119]. Homologs of *ace2* in *A. nidulans*, *A. niger*, *N. crassa* and *Magnaporthe grisea* were not found to date, suggesting that different transcriptional regulators are used by these fungi to induce the expression of hy‐

226 Sustainable Degradation of Lignocellulosic Biomass - Techniques, Applications and Commercialization

**17. Other regulators involved in plant polysaccharide degradation in**

The enzymatic system responsible for plant polysaccharide degradation is induced and commonly amplified after releasing of the plant cell wall polymers components. Two impor‐ tant components of the plant cell wall are D-xylose and L-arabinose, present in the polymers arabinan, arabinogalactan, xyloglucan and xylan. In Aspergilli, D-xylose and L-arabinose are catabolized through the pentose catabolic pathway, PCP [120], consisting of a series of reversible reductase/dehydrogenase steps culminating with the formation of D-xylulose-5 phosphate, which enters the pentose phosphate pathway (PPP). In *A. niger*, the most of the genes involved in PCP have been characterized [110, 121 - 123]. In the presence of D-xylose, the transcriptional activator XlnR is known to regulate the expression of genes not only en‐ coding extracellular polysaccharide-degrading enzymes, but also regulates the expression of *xyrA*, the gene encoding D-xylose reductase, involved in the PCP [5]. L-arabinose induction of PCP is not mediated via XlnR, and the genes of the L-arabinose catabolic pathway are coregulated with genes encoding arabinolytic enzymes such as α-L-arabinofuranosidase and endoarabinanase [93, 124]. A L-arabinose catabolic pathway regulator, AraR, was identified in *A. niger*, and appeared to act as antagonist of the XlnR in the regulation of PCP [125]. AraR demonstrated to regulate the L-arabinose pathway specific genes, for instance, the Larabitol dehydrogenase encoding gene (*ladA*) [125]. In addition, the L-arabinose reductase and L-xylulose reductase encoding genes (*larA* and *lxrA*, respectively) appeared to be under the regulation of the arabinolytic system [125, 126]. AraR and XlnR both regulate the com‐ mon steps of L-arabinose and D-xylose catabolism, represented by xylitol dehydrogenaseand xylulose kinase-encoding genes, *xdhA* and *xkiA*, respectively [123, 125]. Comparative analysis have been shown that the regulation of PCP by AraR differs in *A. nidulans* and *A. niger*, whereas the regulation of the PCP by XlnR was similar in both species, suggesting dif‐ ferent evolutionary changes in these two species affecting pentose utilization [125]. These authors suggest that the differences in Aspergilli species implies that manipulating regulato‐ ry systems to improve the production of polysaccharide degrading enzymes may give dif‐

drolytic-encoding genes.

ferent results in different industrial fungi.

**Aspergilli**

AmyR is another transcriptional activator found in Aspergilli. AmyR was first described as a transcriptional regulator of genes encoding enzymes involved in starch and maltose hy‐ drolysis [129]. Nowadays, studies have been demonstrated a broader role for AmyR, which appears to regulate another gene expression systems. High levels of both α- and β-glucosi‐ dase as well as α- and β-galactosidase in the *amyR* multicopy strain of *A. niger* were found [130]. This study also demonstrated that AmyR-regulated genes in *A. niger* are induced dur‐ ing growth in low levels of D-glucose, as their expression increased during the cultivation. As D-glucose has been shown to act as a repressor through the carbon catabolite repressor protein CreA [75] (see next section), the authors suggested that repression levels of those genes are likely a balance between induction through AmyR and repression through CreA. Similar results were obtained for AmyR in *Aspergillus oryzae* [131].
