L-Arabitol Is the Actual Inducer of Xylanase Expression in Hypocrea jecorina (Trichoderma reesei)

The saprophytic fungus Hypocrea jecorina (anamorph, Trichoderma reesei) is an important native producer of hydrolytic enzymes, including xylanases. Regarding principles of sustainability, cheap and renewable raw materials, such as D-xylose (the backbone monomer of xylan), have been receiving increasing attention from industries. Recently, it was demonstrated that small (0.5 to 1 mM) amounts of D-xylose induce the highest expression of xylanase in H. jecorina. However, it was also reported that active metabolism of D-xylose is necessary for induction. In this report, we demonstrate that xylitol, the next intermediate in the pentose pathway after D-xylose, does not trigger transcription of xylanase-encoding genes in H. jecorina QM9414. The highest level of transcription of xylanolytic enzyme-encoding genes occurred in an xdh1 (encoding a xylitol dehydrogenase) deletion strain cultured in the presence of 0.5 mM D-xylose, suggesting that a metabolite upstream of xylitol is the inducer. The expression levels of xylanases in an xdh1-lad1 double-deletion strain were lower than that of an xdh1 deletion strain. This observation suggested that L-xylulose is not an inducer and led to the hypothesis that L-arabitol is the actual inducer of xylanase expression. A direct comparison of transcript levels following the incubation of the H. jecorina parental strain with various metabolites of the pentose pathway confirmed this hypothesis. In addition, we demonstrate that xyr1, the activator gene, is not induced in the presence of pentose sugars and polyols, regardless of the concentration used; instead, we observed low constitutive expression of xyr1.