The d‐xylose reductase of Hypocrea jecorina is the major aldose reductase in pentose and d‐galactose catabolism and necessary for β‐galactosidase and cellulase induction by lactose

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Abstract
The Hypocrea jecorinad‐xylose reductase encoding gene xyl1 shows low basal transcript levels, and is induced by d‐xylose, l‐arabinose and l‐arabinitol and, to a lesser extent, by lactose, d‐galactose, galactitol and xylitol. The recombinantly expressed XYL1 catalyzes the NADPH‐dependent reduction of the pentoses d‐xylose and l‐arabinose and the hexose d‐galactose. Deletion of xyl1 slightly reduces growth on all carbon sources, but a significant decrease is found on d‐xylose, l‐arabinose and d‐galactose. Similar to pentose degradation, XYL1 reduces d‐galactose to galactitol in a recently identified second d‐galactose pathway. Strains impaired in both d‐galactose pathways are almost unable to grow on d‐galactose. Δxyl1 strains show reduced growth on lactose and are impaired in β‐galactosidase expression and induction of the major cellobiohydrolase gene cbh1. A strain deleted in the cellulase regulator XYR1 is even more severely impaired in growth and β‐galactosidase expression on lactose, and does not produce any cbh1 transcript at all. In this strain, only a low basal level of xyl1 transcription is found on lactose. Galactitol, but not d‐galactose is able to induce xyl1 transcription in a XYR1‐independent manner. Our results show that the role of the H. jecorina XYL1 is not restricted to d‐xylose catabolism and demonstrates its importance for induction of cellulases and β‐galactosidases.