**12. Induction of hemicellulases and pectinases**

Hemicellulase expression has been studied mostly in Aspergilli and *T. reesei*. However, a comparison of the genome sequences of *T. reesei* [87] and *Aspergillus niger* [88] demonstrated that *A. niger* is more versatile in the range of hemicellulases, and therefore this specie has become a very useful model fungus for basic studies on CWDEs in recent years. It is known that the presence of the hemicelluloses xylan, xyloglucan, arabinan and mannan usually in‐ duces a high production of hemicellulases, though the mechanism of sensing is still lacking, as assigned for cellulase induction. Usually, small hemicellulose-derived molecules are able to induce the expression of a wide range of hemicellulases.

ethanol factories, was able to induce a variety of cell-wall degrading enzyme genes in *A. ni‐ ger* [98]. This is especially important for the development of second-generation ethanol (cel‐ lulosic ethanol) technology in countries producing ethanol from sugarcane, which could be

Microbial Degradation of Lignocellulosic Biomass

http://dx.doi.org/10.5772/54325

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The ligninolytic system of many fungi appears to be induced under nutrient deprivation, main‐ ly nitrogen, carbon and sulphur. Therefore, the expression of most of the ligninolytic genes is regarded as a stress response to nutrient deprivation. Also, the presence of Mn(II) is required for induction of manganese peroxidase (MnPs) genes in the white-rot fungi *P. chrysosporium* [99 - 101]. Besides nutrient depletion and the presence of Mn(II), MnP genes were found to be expressed under heat shock in nitrogen limited cultures [102], and in the presence of H2O2, chemical stress or molecular oxygen [103]. Similarly, lignin peroxidase (LiP) genes are also ex‐

Laccases are multicopper oxidase proteins, and therefore can be induced by copper, al‐ though other metals can induce the expression of laccase genes as well, such as manganese and cadmium [105, 106]. Many natural and xenobiotic aromatic compounds, which are often related to lignin or humic substances, were shown to induce genes related to laccases [107]. In general, it has been postulated that laccases are the first enzymes degrading lignin, and possible further degradation products released from the polymer could act as inducers to

amplify laccase expression, and subsequently induce other ligninolytic genes [105].

**14. Transcription factors involved in the expression of cellulase and**

regulators involved in plant-polysaccharide degradation found in fungi are presented.

A number of genes encoding plant cell wall degrading enzymes appears to present in their pro‐ moter regions regulatory elements for binding of transcriptional activators. The filamentous fungi *A. niger* and *T. reesei* have been the subject of many studies regarding the mechanism of transcriptional regulation of cellulase- and hemicellulase-encoding genes. Also, genes respon‐ sible for the expression of transcription factors found in *A. niger* and *T. reesei* have their homo‐ logs in other *Aspergilli* species and *Neurospora crassa*. In this section, the main transcription

Complementation by transformation of an *A. niger* mutant lacking xylanolytic activity led to the isolation of *xlnR* gene, a gene encoding the first known transcriptional activator controlling the expression of genes for xylanolytic and cellulolytic enzymes in filamentous fungi [108]. Ini‐

able to use a very cheap raw material to produce the biofuel.

pressed under carbon and nitrogen limitation in *P. chrysosporium* [104].

**13. Induction of ligninases**

**hemicellulase-encoding genes**

**15. The transcriptional activator XlnR**

The monosaccharide D-xylose is a well-known inducer of xylanolytic enzymes in *Aspergillus* species. In *A. niger*, D-xylose appears to induce other hydrolase genes rather than xylanase genes, such as the accessory enzymes α-glucuronidase (*aguA*), acetylxylan esterase (*axeA*) and feruloyl esterase (*faeA*) [89, 90]. Some results have been demonstrated that xylose can act as a repressor carbon source of hemicellulase induction at high concentrations [91], whereas other studies demonstrated that utilization of a high D-xylose concentration was beneficial for the induction of hemicellulase-encoding genes [92]. In addition to xylose, xylo‐ biose and D-glucose-β-1,2-D-xylose have been demonstrated to induce expression of xylano‐ lytic genes in *A. terreus* [80].

The genes encoding enzymes responsible for the degradation of arabinoxylan in *A. niger* were induced by arabinose and L-arabitol. These genes encode enzymes such as arabinofuranosi‐ dases (*afbA* and *afbB*) and arabinoxylan arabinofuranosidases (*axhA*) [6]. High intracellular ac‐ cumulation of L-arabitol in *A. nidulans* mutant strains was able to induce higher amounts of arabinofuranosidases and endoarabinases than in the wild type strain [93]. The arabinolytic system appears to be independent from xylanolytic system in *A. niger*, as demonstrated by the isolation of *ara* mutants of *A. niger* defective for the induction of the genes encoding arabinolyt‐ ic enzymes, but not for the induction of enzymes belonging to xylanolytic system [94].

Regarding to pectinolytic enzymes, D-galacturonic acid, polygalacturonate and sugar beet pectin have been shown to induce virtually all the genes encoding for pectin degradation enzymes in *A. niger*, suggesting that these genes are under the control of a general pectino‐ lytic regulatory system responding to D-galacturonic acid or a metabolic product derived from it in *A. niger* [48, 90].

In *T. ressei*, the induction of hemicellulases was observed during growth in the presence of cellulose, xylan, sophorose, xylobiose and L-arabitol [6]. A xylanase gene (*xyn2*) was in‐ duced by sophorose and xylobiose [95, 96]. Xylobiose was able to induce genes involved in xylan degradation in *T. reesei*, such as the xylanase genes *xyn1* and *xyn2* and the β-xilosidase gene *bxl1*. In addition, sophorose induced some genes encoding enzymes that cleave the side chains of xylan such as *agl1* and *agl2* (α-galactosidase genes), and *glr1*, encoding a αglucuronidase gene [97].

Complex mixtures of polysaccharides have been shown to induce a wide range of cellulases/ hemicellulases genes in *A. niger*. For instance, sugarcane bagasse, a by-product of sugar/ ethanol factories, was able to induce a variety of cell-wall degrading enzyme genes in *A. ni‐ ger* [98]. This is especially important for the development of second-generation ethanol (cel‐ lulosic ethanol) technology in countries producing ethanol from sugarcane, which could be able to use a very cheap raw material to produce the biofuel.
