D-Xylose as a Repressor or Inducer of Xylanase Expression in Hypocrea jecorina (Trichoderma reesei)

For Hypocrea jecorina (anamorph Trichoderma reesei), a filamentous fungus used for hydrolase production in different industries, it has been a long-term practice to use D-xylose as an inducing substance. We demonstrate in this study that the degree of xylanase-encoding gene induction strictly depends on the concentration of D-xylose, which was found to be optimal from 0.5 to 1 mM for 3 h of cultivation. At higher concentrations of D-xylose, a reduced level of xylanase gene expression was observed. In the present study, we also provide evidence that the D-xylose concentration-dependent induction is antagonized by carbon catabolite repressor 1. This repressor mediates its influence on D-xylose indirectly, by reducing the expression of xylanase regulator 1, the main activator of most hydrolase-encoding genes. Additionally, a direct influence of the repressor on xylanase 1 expression in the presence of D-xylose was found. Furthermore, we show that D-xylose reductase 1 is needed to metabolize D-xylose to achieve full induction of xylanase expression. Finally, a strain which expresses xylanase regulator 1 at a constant level was used to partially overcome the negative influence exerted by carbon catabolite repressor 1 on D-xylose.