The Aspergillus niger transcriptional activator XlnR, which is involved inthe degradation of the polysaccharides xylan and cellulose, also regulatesD-xylose reductase gene expression

Screening of an Aspergillus niger differential cDNA library, constructed by subtracting cDNA fragments of a xlnR loss-of-function mutant from wild-typ e cDNA fragments, resulted in the cloning of the gene encoding d-xylose red uctase (xyrA). Northern blot analysis using an A. niger wild-type strain, a xlnR multiple-copy strain and a xlnR loss-of-function mutant confirmed tha t the xyrA gene is regulated by XlnR, the transcriptional activator of the xylanolytic enzyme system in A. niger. d-xylose reductase catalyses the NAD PH-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 trans cription of genes encoding extracellular hydrolytic enzymes involved in cel lulose and xylan degradation. In the present study, we show that A. niger i s able to harmonize its sugar metabolism and extracellular xylan degradatio n via XlnR by regulating the expression of XyrA.