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The transcriptional activator XlnR from Aspergillus niger is a zinc binuclear cluster transcription factor that belongs to the GAL4 superfamily. Several putative structural domains in XlnR were predicted using database and protein sequence analysis. Thus far, only the functionality of the N-terminal DNA-binding domain has been determined experimentally. Deletion mutants of the xlnR gene were constructed to localize the functional regions of the protein. The results showed that a putative C-terminal coiled-coil region is involved in nuclear import of XlnR. After deletion of the C-terminus, including the coiled-coil region, XlnR was found in the cytoplasm, while deletion of the C-terminus downstream of the coiled-coil region resulted in nuclear import of XlnR. The latter mutant also showed increased xylanase activity, indicating the presence of a region with an inhibitory function in XlnR-controlled transcription. Previous findings had already shown that a mutation in the XlnR C-terminal region resulted in transcription of the structural genes under non-inducing conditions. A regulatory model of XlnR is presented in which the C-terminus responds to repressing signals, resulting in an inactive state of the protein.

Screening of an Aspergillus niger differential cDNA library, constructed by subtracting cDNA fragments of a xlnR loss‐of‐function mutant from wild‐type cDNA fragments, resulted in the cloning of the gene encoding d‐xylose reductase (xyrA). Northern blot analysis using an A. niger wild‐type strain, a xlnR multiple‐copy strain and a xlnR loss‐of‐function mutant confirmed that the xyrA gene is regulated by XlnR, the transcriptional activator of the xylanolytic enzyme system in A. niger. d‐xylose reductase catalyses the NADPH‐dependent reduction of d‐xylose to xylitol, which is the first step in d‐xylose catabolism in fungi. Until now, XlnR was shown to control the transcription of genes encoding extracellular hydrolytic enzymes involved in cellulose and xylan degradation. In the present study, we show that A. niger is able to harmonize its sugar metabolism and extracellular xylan degradation via XlnR by regulating the expression of XyrA.

A novel gene, eglC, encoding an endoglucanase, was cloned from Aspergillus niger. Transcription of eglC is regulated by XlnR, a transcriptional activator that controls the degradation of polysaccharides in plant cell walls. EglC is an 858-amino-acid protein and contains a conserved C-terminal cellulose-binding domain. EglC can be classified in glycoside hydrolase family 74. No homology to any of the endoglucanases from Trichoderma reesei was found. In the plant cell wall xyloglucan is closely linked to cellulose fibrils. We hypothesize that the EglC cellulose-binding domain anchors the enzyme to the cellulose chains while it is cleaving the xyloglucan backbone. By this action it may contribute to the degradation of the plant cell wall structure together with other enzymes, including hemicellulases and cellulases. EglC is most active towards xyloglucan and therefore is functionally different from the other two endoglucanases from A. niger, EglA and EglB, which exhibit the greatest activity towards ␤-glucan. Although the mode of action of EglC is not known, this enzyme represents a new enzyme function involved in plant cell wall polysaccharide degradation by A. niger.Plant cell walls are composed predominantly of structural polysaccharides, which are associated in a matrix of cellulose microfibrils, hemicellulose polymers, and pectin. Bacteria and filamentous fungi, including Aspergillus and Trichoderma species, can degrade plant cell wall polysaccharides efficiently by producing a mixture of extracellular hydrolytic enzymes.The major component of plant cell walls is the ␤-1,4-glucan cellulose. Cellulose can be degraded by the coordinated action of cellulolytic enzymes, such as endoglucanase, cellobiohydrolase, and ␤-glucosidase. Most cellulolytic enzymes consist of a catalytic domain linked to a cellulose-binding domain (CBD) by a Pro/Ser/Thr-rich linker peptide. The cellulolytic enzyme system of Trichoderma reesei is the best-studied fungal example. This system contains five genes encoding endoglucanases, egl1 to egl5 (14), two genes encoding cellobiohydrolases, cbh1 and cbh2 (2, 19), and two ␤-glucosidase-encoding genes, bgl1 and bgl2 (1, 21). Endoglucanases cleave internal ␤-1,4-glucosidic bonds, while cellobiohydrolases cleave the disaccharide cellobiose from either the nonreducing or reducing end of the cellulose polymer chain (22). ␤-Glucosidases hydrolyze cellooligosaccharides and cellobiose to D-glucose. Expression of cellulases is controlled at the transcriptional level in both T. reesei and Aspergillus niger. In the presence of D-glucose transcription is repressed, while in the absence of D-glucose and in the presence of cellulose certain oligosaccharide and/or disaccharide (e.g., sophorose) transcription is strongly induced. In T. reesei D-glucose repression of transcription is mediated by Cre1, which also mediates repression of genes coding for enzymes involved in the degradation of hemicellulose (10, 12).In A. niger two genes encoding endoglucanases, eglA and eglB (25), and two cellobiohydrolase-en...

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