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

摘要

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 .