ABSTRACTThe saprophytic fungusHypocrea jecorina(anamorph,Trichoderma reesei) is an important native producer of hydrolytic enzymes, including xylanases. Regarding principles of sustainability, cheap and renewable raw materials, such asd-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 ofd-xylose induce the highest expression of xylanase inH. jecorina. However, it was also reported that active metabolism ofd-xylose is necessary for induction. In this report, we demonstrate that xylitol, the next intermediate in the pentose pathway afterd-xylose, does not trigger transcription of xylanase-encoding genes inH. jecorinaQM9414. The highest level of transcription of xylanolytic enzyme-encoding genes occurred in anxdh1(encoding a xylitol dehydrogenase) deletion strain cultured in the presence of 0.5 mMd-xylose, suggesting that a metabolite upstream of xylitol is the inducer. The expression levels of xylanases in anxdh1-lad1double-deletion strain were lower than that of anxdh1deletion strain. This observation suggested thatl-xylulose is not an inducer and led to the hypothesis thatl-arabitol is the actual inducer of xylanase expression. A direct comparison of transcript levels following the incubation of theH. jecorinaparental strain with various metabolites of the pentose pathway confirmed this hypothesis. In addition, we demonstrate thatxyr1, 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 ofxyr1.